Adapted from 3rd edition, Encyclopaedia of Occupational Health and Safety

Before a new hazardous substance is received for storage, information concerning its correct handling should be provided to all users. Planning and maintaining of storage areas are necessary to avoid material losses, accidents and disasters. Good housekeeping is essential, and special attention should be paid to incompatible substances, suitable location of products and climatic conditions.

Written instructions of storage practices should be provided, and the chemicals’ material safety data sheets (MSDSs) should be available in storage areas. Locations of the different classes of chemicals should be illustrated in a storage map and in a chemical register. The register should contain the maximum allowed quantity of all chemical products and the maximum allowed quantity of all chemical products per class. All substances should be received at a central location for distribution to the storerooms, stockrooms and laboratories. A central receiving area is also helpful in monitoring substances that may eventually enter the waste-disposal system. An inventory of substances contained in the storerooms and stockrooms will give an indication of the quantity and nature of substances targeted for future disposal.

Stored chemicals should be examined periodically, at least annually. Chemicals with expired shelf lives and deteriorated or leaking containers should be disposed of safely. A “first in, first out” system of keeping stock should be used.

The storage of dangerous substances should be supervised by a competent, trained person. All workers required to enter storage areas should be fully trained in appropriate safe work practices, and a periodic inspection of all storage areas should be carried out by a safety officer. A fire alarm should be situated in or near the outside of the storage premises. It is recommended that persons should not work alone in a storage area containing toxic substances. Chemical storage areas should be located away from process areas, occupied buildings and other storage areas. In addition, they should not be in proximity of fixed sources of ignition.

Labelling and Relabelling Requirements

The label is the key to organizing chemical products for storage. Tanks and containers should be identified with signs indicating the name of the chemical product. No containers or cylinders of compressed gases should be accepted without the following identifying labels:

• identification of contents
• description of principal hazard (e.g., flammable liquid)
• precautions to minimize hazards and prevent accidents
• correct first aid procedures
• correct procedures for cleaning up spills
• special instructions to medical personnel in case of an accident.

The label may also offer precautions for correct storage, such as “Keep in a cool place” or “Keep container dry”. When certain dangerous products are delivered in tankers, barrels or bags and repackaged at the workplace, each new container should be relabelled so that the user will be able to identify the chemical and recognize the risks immediately.

Explosive Substances

Explosive substances include all chemicals, pyrotechnics and matches which are explosives per se and also those substances such as sensitive metallic salts which, by themselves or in certain mixtures or when subject to certain conditions of temperature, shock, friction or chemical action, may transform and undergo an explosive reaction. In the case of explosives, most countries have stringent regulations regarding safe storage requirements and precautions to be taken in order to prevent theft for use in criminal activities.

The storage places should be situated far away from other buildings and structures so as to minimize damage in case of an explosion. Manufacturers of explosives issue instructions as to the most suitable type of storage. The storerooms should be of solid construction and kept securely locked when not in use. No store should be near a building containing oil, grease, waste combustible material or flammable material, open fire or flame.

In some countries there is a legal requirement that magazines should be situated at least 60 m from any power plant, tunnel, mine shaft, dam, highway or building. Advantage should be taken of any protection offered by natural features such as hills, hollows, dense woods or forests. Artificial barriers of earth or stone walls are sometimes placed around such storage places.

The storage place should be well ventilated and free from dampness. Natural lighting or portable electric lamps should be used, or lighting provided from outside the storehouse. Floors should be constructed of wood or other non-sparking material. The area surrounding the storage place should be kept free of dry grass, rubbish or any other material likely to burn. Black powder and explosives should be stored in separate storehouses, and no detonators, tools or other materials should be kept in an explosive store. Non-ferrous tools should be used for opening cases of explosives.

Oxidizing Substances

Oxidizing substances provide sources of oxygen, and thus are capable of supporting combustion and intensifying the violence of any fire. Some of these oxygen suppliers give off oxygen at storage-room temperature, but others require the application of heat. If containers of oxidizing materials are damaged, the contents may mix with other combustible materials and start a fire. This risk can be avoided by storing oxidizing materials in a separate storage place. However, this practice may not always be available, as, for example, in dock warehouses for goods in transit.

It is dangerous to store powerful oxidizing substances near liquids that even have a low flash point or even slightly flammable materials. It is safer to keep all flammable materials away from a place where oxidizing substances are stored. The storage area should be cool, well ventilated and of fire-resisting construction.

Flammable Substances

A gas is deemed to be flammable if it burns in the presence of air or oxygen. Hydrogen, propane, butane, ethylene, acetylene, hydrogen sulphide and coal gas are among the most common flammable gases. Some gases such as hydrogen cyanide and cyanogen are both flammable and poisonous. Flammable materials should be stored in places which are cool enough to prevent accidental ignition if the vapours mix with the air.

Vapours of flammable solvents may be heavier than air and may move along the floor to a distant ignition source. Flammable vapours from spilled chemicals have been known to descend into stairwells and elevator shafts and ignite at a lower storey. It is therefore essential that smoking and open flames be prohibited where these solvents are handled or stored.

Portable, approved safety cans are the safest vessels for storing flammables. Quantities of flammable liquids greater than 1 litre should be stored in metal containers. Two-hundred-litre drums are commonly used to ship flammables, but are not intended as long-term storage containers. The stopper should be removed carefully and replaced by an approved pressure-relief vent to avoid increased internal pressure from heat, fire or exposure to sunlight. When transferring flammables from metal equipment, the worker should use an enclosed transfer system or have adequate exhaust ventilation.

The storage area should be situated away from any source of heat or fire hazard. Highly flammable substances should be kept apart from powerful oxidizing agents or from materials which are susceptible to spontaneous combustion. When highly volatile liquids are stored, any electric light fittings or apparatus should be of certified flameproof construction, and no open flames should be permitted in or near the storage place. Fire extinguishers and absorbent inert materials, such as dry sand and earth, should be available for emergency situations.

The walls, ceilings and floors of the storage room should consist of materials with at least a 2-hour fire resistance. The room should be fitted with self-closing fire doors. The storage-room installations should be electrically grounded and periodically inspected, or equipped with automatic smoke- or fire-detection devices. Control valves on storage vessels containing flammable liquids should be clearly labelled, and pipelines should be painted with distinctive safety colours to indicate the type of liquid and the direction of flow. Tanks containing flammable substances should be situated on ground sloping away from the main buildings and plant installations. If they are on level ground, protection against fire spread can be obtained by adequate spacing and the provision of dykes. The dyke capacity should preferably be 1.5 times that of the storage tank, as a flammable liquid may be likely to boil over. Provision should be made for venting facilities and flame arrestors on such storage tanks. Adequate fire extinguishers, either automatic or manual, should be available. No smoking should be allowed.

Toxic Substances

Toxic chemicals should be stored in cool, well ventilated areas out of contact with heat, acids, moisture and oxidizing substances. Volatile compounds should be stored in spark-free freezers (–20 °C) to avoid evaporation. Because containers may develop leaks, storerooms should be equipped with exhaust hoods or equivalent local ventilation devices. Open containers should be closed with tape or other sealant before being returned to the storeroom. Substances which can react chemically with each other should be kept in separate stores.

Corrosive Substances

Corrosive substances include strong acids, alkalis and other substances which will cause burns or irritation of the skin, mucous membranes or eyes, or which will damage most materials. Typical examples of these substances include hydrofluoric acid, hydrochloric acid, sulphuric acid, nitric acid, formic acid and perchloric acid. Such materials may cause damage to their containers and leak into the atmosphere of the storage area; some are volatile and others react violently with moisture, organic matter or other chemicals. Acid mists or fumes may corrode structural materials and equipment and have a toxic action on personnel. Such materials should be kept cool but well above their freezing point, since a substance such as acetic acid may freeze at a relatively high temperature, rupture its container and then escape when the temperature rises again above its freezing point.

Some corrosive substances also have other dangerous properties; for example, perchloric acid, in addition to being highly corrosive, is also a powerful oxidizing agent which can cause fire and explosions. Aqua regia has three dangerous properties: (1) it displays the corrosive properties of its two components, hydrochloric acid and nitric acid; (2) it is a very powerful oxidizing agent; and (3) application of only a small amount of heat will result in the formation of nitrosyl chloride, a highly toxic gas.

Storage areas for corrosive substances should be isolated from the rest of the plant or warehouses by impervious walls and floor, with provision for the safe disposal of spillage. The floors should be made of cinder blocks, concrete that has been treated to reduce its solubility, or other resistant material. The storage area should be well ventilated. No store should be used for the simultaneous storage of nitric acid mixtures and sulphuric acid mixtures. Sometimes it is necessary to store corrosive and poisonous liquids in special types of containers; for example, hydrofluoric acid should be kept in leaden, gutta percha or ceresin bottles. Since hydrofluoric acid interacts with glass, it should not be stored near glass or earthenware carboys containing other acids.

Carboys containing corrosive acids should be packed with kieselguhr (infusorial earth) or other effective inorganic insulating material. Any necessary first-aid equipment such as emergency showers and eyewash bottles should be provided in or immediately close to the storage place.

Water-reactive Chemicals

Some chemicals, such as sodium and potassium metals, react with water to produce heat and flammable or explosive gases. Certain polymerization catalysts, such as alkyl aluminium compounds, react and burn violently on contact with water. Storage facilities for water-reactive chemicals should not have water in the storage area. Non-water automatic sprinkler systems should be employed.

Legislation

Detailed legislation has been drawn up in many countries to regulate the manner in which various dangerous substances may be stored; this legislation includes the following specifications:

• type of building, its location, the maximum amounts of various substances that may be stored in one place
• type of ventilation required
• precautions to be taken against fire, explosion and the release of dangerous substances
• type of lighting (e.g., flameproof electrical equipment and light fixtures when explosive or flammable materials are stored)
• number and location of fire exits
• security measures against entry by unauthorized persons and against theft
• labelling and marking of storage vessels and pipelines
• warning notices to workers as to the precautions to be observed.

In many countries there is no central authority concerned with the supervision of the safety precautions for the storage of all dangerous substances, but a number of separate authorities exist. Examples include mine and factory inspectorates, dock authorities, transport authorities, police, fire services, national boards and local authorities, each of which deals with a limited range of dangerous substances under various legislative powers. It is usually necessary to obtain a licence or permit from one of these authorities for the storage of certain types of dangerous substances such as petroleum, explosives, cellulose and cellulose solutions. The licensure procedures require that storage facilities comply with specified safety standards.

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Adapted from 3rd edition, Encyclopaedia of Occupational Health and Safety

Gases in their compressed state, and particularly compressed air, are almost indispensable to modern industry, and are also used widely for medical purposes, for the manufacture of mineral waters, for underwater diving and in connection with motor vehicles.

For purposes of the present article, compressed gases and air are defined as being those with a gauge pressure exceeding 1.47 bar or as liquids having a vapour pressure exceeding 2.94 bar. Thus, consideration is not given to such cases as natural gas distribution, which is dealt with elsewhere in this Encyclopaedia.

Table 1 shows the gases commonly encountered in compressed cylinders.

Table 1. Gases often found in compressed form

*These gases are flammable.

All the above gases present either an irritant, asphyxiant or highly toxic respiratory hazard and may also be flammable and an explosive when compressed. Most countries provide for a standard colour-coding system whereby different coloured bands or labels are applied to the gas cylinders to indicate the type of hazard to be expected. Particularly toxic gases, such as hydrogen cyanide, are also given special markings.

All compressed gas containers are so constructed that they are safe for the purposes for which they are intended when first put into service. However, serious accidents may result from their misuse, abuse or mishandling, and the greatest care should be exercised in the handling, transport, storage and even in the disposal of such cylinders or containers.

Characteristics and Production

Depending on the characteristics of the gas, it may be introduced into the container or cylinder in liquid form or simply as a gas under high pressure. In order to liquefy a gas, it is necessary to cool it to below its critical temperature and to subject it to an appropriate pressure. The lower the temperature is reduced below the critical temperature, the less the pressure required.

Certain of the gases listed in table 1 have properties against which precautions must be taken. For example, acetylene can react dangerously with copper and should not be in contact with alloys containing more than 66% of this metal. It is usually delivered in steel containers at about 14.7 to 16.8 bar. Another gas that has a highly corrosive action on copper is ammonia, which must also be kept out of contact with this metal, use being made of steel cylinders and authorized alloys. In the case of chlorine, no reaction takes place with copper or steel except in the presence of water, and for this reason all storage vessels or other containers must be kept free from contact with moisture at all times. Fluorine gas, on the other hand, although reacting readily with most metals, will tend to form a protective coating, as, for example, in the case of copper, where a layer of copper fluoride over the metal protects it from further attack by the gas.

Among the gases listed, carbon dioxide is one of the most readily liquefied, this taking place at a temperature of 15 °C and a pressure of about 14.7 bar. It has many commercial applications and may be kept in steel cylinders.

The hydrocarbon gases, of which liquefied petroleum gas (LPG) is a mixture formed mainly of butane (about 62%) and propane (about 36%), are not corrosive and are generally delivered in steel cylinders or other containers at pressures of up to 14.7 to 19.6 bar. Methane is another highly flammable gas that is also generally delivered in steel cylinders at a pressure of 14.7 to 19.6 bar.

Hazards

Storage and transport

When a filling, storage and dispatch depot is being selected, consideration must be given to the safety of both the site and the environment. Pump rooms, filling machinery and so on must be located in fire-resistant buildings with roofs of light construction. Doors and other closures should open outwards from the building. The premises should be adequately ventilated, and a system of lighting with flameproof electrical switches should be installed. Measures should be taken to ensure free movement in the premises for filling, checking and dispatch purposes, and safety exits should be provided.

Compressed gases may be stored in the open only if they are adequately protected from the weather and direct sunlight. Storage areas should be located at a safe distance from occupied premises and neighbouring dwellings.

During the transport and distribution of containers, care must be taken to ensure that valves and connections are not damaged. Adequate precautions should be taken to prevent cylinders from falling off the vehicle and from being subjected to rough usage, excessive shocks or local stress, and to prevent excessive movement of liquids in large tanks. Every vehicle should be equipped with a fire extinguisher and an electrically conductive strip for earthing static electricity, and should be clearly marked “Flammable liquids”. Exhaust pipes should have a flame-control device, and engines should be halted during loading and unloading. The maximum speed of these vehicles should be rigorously limited.

Use

The main dangers in the use of compressed gases arise from their pressure and from their toxic and/or flammable properties. The principal precautions are to ensure that equipment is used only with those gases for which it was designed, and that no compressed gases are used for any purpose other than that for which their use has been authorized.

All hoses and other equipment should be of good quality and should be examined frequently. The use of non-return valves should be enforced wherever necessary. All hose connections should be in good condition and no joints should be made by forcing together threads that do not exactly correspond. In the case of acetylene and combustible gases, a red hose should be used; for oxygen the hose should be black. It is recommended that for all flammable gases, the connection-screw thread shall be left-handed, and for all other gases, it shall be right-handed. Hoses should never be interchanged.

Oxygen and some anaesthetic gases are often transported in large cylinders. The transfer of these compressed gases to small cylinders is a hazardous operation, which should be done under competent supervision, making use of the correct equipment in a correct installation.

Compressed air is widely used in many branches of industry, and care should be taken in the installation of pipelines and their protection from damage. Hoses and fittings should be maintained in good condition and subjected to regular examinations. The application of a compressed air hose or jet to an open cut or wound through which air can enter the tissues or the bloodstream is particularly dangerous; precautions should also be taken against all forms of irresponsible behaviour which could result in a compressed air jet coming in contact with any openings in the body (the result of which can be fatal). A further hazard exists when compressed air jets are used to clean machined components or workplaces: flying particles have been known to cause injury or blindness, and precautions against such dangers should be enforced.

Labelling and marking

4.1.1.  The competent authority, or a body approved or recognized by the competent authority, should establish requirements for the marking and labelling of chemicals to enable persons handling or using chemicals to recognize and distinguish between them, both when receiving and when using them, so that they may be used safely (see paragraph 2.1.8 (criteria and requirements)). Existing criteria for marking and labelling established by other competent authorities may be followed where they are consistent with the provisions of this paragraph and are encouraged where this may assist uniformity of approach. 

4.1.2.  Suppliers of chemicals should ensure that chemicals are marked and hazardous chemicals are labelled, and that revised labels are prepared and provided to employers whenever new relevant safety and health information becomes available (see paragraphs 2.4.1 (suppliers’ responsibilities) and 2.4.2 (classification)). 

4.1.3.  Employers receiving chemicals that have not been labelled or marked should not use them until the relevant information is obtained from the supplier or from other reasonably available sources. Information should be obtained primarily from the supplier but may be obtained from other sources listed in paragraph 3.3.1 (sources of information), with a view to marking and labelling in accordance with the requirements of the national competent authority, prior to use. ...

4.3.2.   The purpose of the label is to give essential information on:

1. (a) the classification of the chemical;
2. (b) its hazards;
3. (c) the precautions to be observed.

The information should refer to both acute and chronic exposure hazards.

4.3.3.     Labelling requirements, which should be in conformity with national requirements, should cover:

(a) the information to be given on the label, including as appropriate:

1. trade names;
2. identity of the chemical;
3. name, address and telephone number of the supplier;
4. hazard symbols;
5. nature of the special risks associated with the use of the chemical;
6. safety precautions;
7. identification of the batch;
8. the statement that a chemical safety data sheet giving additional information is available from the employer;
9. the classification assigned under the system established by the competent authority;

(b)     the legibility, durability and size of the label;

(c)     the uniformity of labels and symbols, including colours.

Source: ILO 1993, Chapter 4.

Labelling and marking should be in accordance with standard practice in the country or region in question. The use of one gas for another by mistake, or the filling of a container with a gas different from that which it previously contained, without the necessary cleaning and decontamination procedures, may cause serious accidents. Colour marking is the best method of avoiding such errors, painting specific areas of containers or piping systems in accordance with the colour code stipulated in national standards or recommended by the national safety organization.

Gas Cylinders

For convenience in handling, transportation and storage, gases are commonly compressed in metal gas cylinders at pressures that range from a few atmospheres overpressure to 200 bar or even more. Alloy steel is the material most commonly used for the cylinders, but aluminium is also widely used for many purposes—for example, for fire extinguishers.

The hazards met with in handling and using compressed gases are:

• normal hazards entailed in handling heavy objects
• hazards connected with pressure (i.e., the amount of stored energy in the gases)
• hazards from the special properties of the gas content, which may be flammable, poisonous, oxidizing and so on.

Cylinder manufacture. Steel cylinders may be seamless or welded. The seamless cylinders are made from high-quality alloy steels and carefully heat-treated in order to obtain the desired combination of strength and toughness for high-pressure service. They may be forged and hot-drawn from steel billets or hot-formed from seamless tubes. Welded cylinders are made from sheet material. The pressed top and bottom parts are welded to a cylindrical seamless or welded tube section and heat-treated to relieve material stresses. Welded cylinders are extensively used in low-pressure service for liquefiable gases and for dissolved gases such as acetylene.

Aluminium cylinders are extruded in large presses from special alloys that are heat-treated to give the desired strength.

Gas cylinders must be designed, produced and tested according to strict norms or standards. Every batch of cylinders should be checked for material quality and heat treatment, and a certain number of cylinders tested for mechanical strength. Inspection is often aided by sophisticated instruments, but in all cases the cylinders should be inspected and hydraulically tested to a given test pressure by an approved inspector. Identification data and the inspector’s mark should be permanently stamped on the cylinder neck or another suitable place.

Periodic inspection. Gas cylinders in use may be affected by rough treatment, corrosion from inside and outside, fire and so on. National or international codes therefore require that they shall not be filled unless they are inspected and tested at certain intervals, which mostly range between two and ten years, depending on the service. Internal and external visual inspection together with a hydraulic pressure test is the basis for the approval of the cylinder for a new period in a given service. The test date (month and year) is stamped on the cylinder.

Disposal. A large number of cylinders are scrapped every year for various reasons. It is equally important that these cylinders be disposed of in such a way that they will not find their way back into use through uncontrolled channels. The cylinders should therefore be made completely unserviceable by cutting, crushing or a similar safe procedure.

Valves. The valve and any safety attachment must be regarded as a part of the cylinder, which must be kept in good working condition. Neck and outlet threads should be intact, and the valve should close tight without the use of undue force. Shut-off valves are often equipped with a pressure-relief device. This may be in the form of a resetting safety valve, bursting disc, fuse plug (melt plug) or a combination of bursting disc and fuse plug. The practice varies from country to country, but cylinders for low-pressure liquefied gases are always equipped with safety valves connected to the gas phase.

Hazards

Different transport codes classify gases as compressed, liquefied or dissolved under pressure. For the purpose of this article, it is useful to use the type of hazard as a classification.

High pressure. If cylinders or equipment burst, damage and injuries may be caused by flying debris or by the gas pressure. The more a gas is compressed, the higher is the stored energy. This hazard is always present with compressed gases and will increase with temperature if the cylinders are heated. Hence:

• Mechanical damage to the cylinder (dents, cuts and so on) should be avoided.
• Cylinders should be stored away from heat and not in direct sun.
• Cylinders should be removed from fires.
• Cylinders should only be connected to equipment suitable for the intended use.
• The cylinder valve should be protected with the cap during transport.
• Cylinders should be secured in use against falling, which may knock off the valve.
• Tampering with safety devices should be avoided.
• Cylinders should be handled with care to avoid mechanical shocks in very cold climates, since steel may become brittle at low temperature.
• Corrosion, which reduces the strength of the shell, should be avoided.

Low temperature. Most liquefied gases will evaporate rapidly under atmospheric pressure, and may reach very low temperatures. A person whose skin is exposed to such liquid may sustain injuries in the form of “cold burns”. (Liquid CO₂ will form snow particles when expanded.) Correct protective equipment (e.g., gloves, goggles) should therefore be used.

Oxidation. The hazard of oxidation is most evident with oxygen, which is one of the most important compressed gases. Oxygen will not burn on its own, but is necessary for combustion. Normal air contains 21% oxygen by volume.

All combustible materials will ignite more easily and burn more vigorously when the oxygen concentration is increased. This is noticeable with even a slight increase in oxygen concentration, and utmost care must be taken to avoid oxygen enrichment in the working atmosphere. In confined spaces small oxygen leaks may lead to dangerous enrichment.

The danger with oxygen increases with increasing pressure to the point where many metals will burn vigorously. Finely divided materials may burn in oxygen with explosive force. Clothing that is saturated with oxygen will burn very rapidly and be difficult to extinguish.

Oil and grease have always been regarded as dangerous in combination with oxygen. The reason is that they react readily with oxygen, their existence is common, the ignition temperature is low and the developed heat may start a fire in the underlying metal. In high-pressure oxygen equipment the necessary ignition temperature may easily be reached by the compression shock that may result from rapid valve opening (adiabatic compression).

Therefore:

• Valves should be operated slowly.
• All oxygen equipment should be kept clean and free from oil and dirt.
• Only materials that are proven to be safe with oxygen should be used.
• Workers should refrain from lubricating oxygen equipment.
• Entering confined spaces where oxygen may exist in higher concentration should be avoided.
• The atmosphere should be checked and the use of oxygen instead of compressed air or some other gas should be strictly avoided.

Flammability. The flammable gases have flashpoints below room temperature and will form explosive mixtures with air (or oxygen) within certain limits known as the lower and upper explosion limits.

Escaping gas (also from safety valves) may ignite and burn with a shorter or longer flame depending on the pressure and amount of gas. The flames may again heat nearby equipment, which may burn, melt or explode. Hydrogen burns with an almost invisible flame.

Even small leaks may cause explosive mixtures in confined spaces. Some gases, such as liquefied petroleum gases, mostly propane and butane, are heavier than air and are difficult to vent away, as they will concentrate in the lower parts of buildings and “float” through channels from one room to another. Sooner or later, the gas may reach an ignition source and explode.

Ignition may be caused by hot sources, but also by electrical sparks, even very small ones.

Acetylene takes a special place among the combustible gases because of its properties and wide use. If heated, the gas may start to decompose with the development of heat even without the presence of air. If allowed to proceed, this may lead to cylinder explosion.

Acetylene cylinders are, for safety reasons, filled with a highly porous mass which also contains a solvent for the gas. Outside heating from a fire or welding torch, or in certain cases internal ignition by strong backfires from welding equipment, may start a decomposition within the cylinder. In such cases:

• The valve should be closed (using protective gloves if necessary) and the cylinder should be removed from fire.
• If part of the cylinder becomes hotter, it should be put into a river, canal or the like to cool down or cooled with water sprays.
• If the cylinder is too hot to be handled, it should be sprayed with water from a safe distance.
• Cooling should continue until the cylinder stays cool by itself.
• The valve should be kept closed, because gas flow will accelerate decomposition.

Acetylene cylinders in several countries are equipped with fuse (melting) plugs. These will release the gas pressure when they melt (usually at about 100 °C) and prevent cylinder explosion. At the same time there is a risk that the released gas may ignite and explode.

Common precautions to observe in respect of combustible gases are as follows:

• Cylinders should be stored separately from other gases in a well ventilated area above ground level.
• Leaking cylinders or equipment should not be used.
• Liquid gas cylinders should be stored and used in an upright position. Larger quantities of gas will come out if liquid is expelled through the safety valves instead of gas. The pressure will be reduced more slowly. Very long flame will result if the gas ignites.
• In case of leaks, any possible ignition source should be avoided.
• Smoking where flammable gases are stored or used should be prohibited.
• The safest way of extinguishing a fire is usually to stop the supply of gas. Merely extinguishing the flame may cause the formation of an explosive cloud, which may re-ignite in contact with a hot object.

Toxicity. Certain gases, if not the most common, may be toxic. At the same time, they may be irritating or corrosive to the skin or eyes.

Persons who handle these gases should be well trained and aware of the danger involved and the necessary precautions. The cylinders should be stored in a well ventilated area. No leaks should be tolerated. Suitable protective equipment (gas masks or breathing equipment) should be used.

Inert gases. Gases such as argon, carbon dioxide, helium and nitrogen are widely used as protective atmospheres to prevent unwanted reactions in welding, chemical plants, steel works and so on. These gases are not labelled as being hazardous, and serious accidents may happen because only oxygen can support life.

When any gas or gas mixture displaces the air so that the breathing atmosphere becomes deficient in oxygen, there is a danger of asphyxiation. Unconsciousness or death may result very rapidly when there is little or no oxygen, and there is no warning effect.

Confined spaces where the breathing atmosphere is deficient in oxygen must be ventilated before entering. When breathing equipment is used, the person entering must be supervised. Breathing equipment must be used even in rescuing operations. Normal gas masks give no protection against oxygen deficiency. The same precaution must be observed with large, permanent firefighting installations, which are often automatic, and those who may be present in such areas should be warned of the danger.

Cylinder filling. Cylinder filling involves the operation of high-pressure compressors or liquid pumps. The pumps may operate with cryogenic (very low-temperature) liquids. The filling stations may also incorporate large storage tanks of liquid gases in a pressurized and/or deeply refrigerated state.

The gas filler should check that the cylinders are in acceptable condition for filling, and should fill the correct gas in not more than the approved amount or pressure. The filling equipment should be designed and tested for the given pressure and type of gas, and protected by safety valves. Cleanliness and material requirements for oxygen service must be observed strictly. When filling flammable or toxic gases, special attention should be given to the safety of the operators. The primary requirement is good ventilation combined with correct equipment and technique.

Cylinders which are contaminated with other gases or liquids by the customers constitute a special hazard. Cylinders with no residual pressure may be purged or evacuated before filling. Special care should be taken to ensure that medical gas cylinders are free from any harmful matter.

Transport. Local transport tends to become more mechanized through the use of fork-lift trucks and so on. Cylinders should be transported only with the caps on and secured against falling from the vehicles. Cylinders must not be dropped from trucks directly onto the ground. For hoisting with cranes, suitable lifting cradles should be used. Magnetic lifting devices or caps with uncertain threads should not be used for lifting cylinders.

When cylinders are manifolded into larger packages, great care should be taken to avoid strain on the connections. Any hazard will be increased because of the greater amount of gas involved. It is good practice to divide larger units into sections and to place shut-off valves where they can be operated in any emergency.

The most frequently occurring accidents in cylinder handling and transport are injuries caused by the hard, heavy and difficult-to-handle cylinders. Safety shoes should be worn. Trolleys should be provided for longer transport of single cylinders.

In international transport codes, compressed gases are classified as dangerous goods. These codes give details about which gases may be transported, cylinder requirements, allowed pressure, marking and so on.

Identification of content. The most important requirement for safe handling of compressed gases is the correct identification of the gas content. Stamping, labelling, stencilling and colour marking are the means that are used for this purpose. Certain requirements for marking are covered in International Organization for Standardization (ISO) standards. The colour marking of medical gas cylinders follows the ISO standards in most countries. Standardized colours are also used in many countries for other gases, but this is not a sufficient identification. In the end only the written word can be regarded as a proof of the cylinder content.

Standardized valve outlets. The use of a standardized valve outlet for a certain gas or group of gases strongly reduces the chance of connecting cylinders and equipment made for different gases. Adapters should therefore not be used, as this sets aside the safety measures. Only normal tools and no excessive force should be used when making connections.

Safe Practice for Users

The safe use of compressed gases entails applying the safety principles outlined in this chapter and the ILO Code of Practice Safety in the Use of Chemicals at Work (ILO 1993). This is not possible unless the user has some basic knowledge of the gas and the equipment that he or she is handling. In addition the user should take the following precautions:

• Gas cylinders should only be used for the purpose for which they are intended and not as rollers or work supports.
• The cylinders should be stored and handled in such a way that their mechanical strength is not reduced (e.g., by severe corrosion, sharp dents, cuts and so on).
• The cylinders should be removed from fires or excessive heat.
• Only the necessary number of gas cylinders should be kept in working areas or occupied buildings. It is preferable for them to be kept near doors and not in emergency escape routes or difficult-to-reach areas.
• Any cylinders that have been exposed to fires should be clearly marked and returned to the filler (owner), since the cylinders may have become brittle or lost their strength.
• Cylinders should be stored in a well-ventilated place, away from rain or snow and any combustible storage.
• Cylinders in use should be secured against falling.
• Gas content should be positively identified before use.
• Labels and instructions should be carefully read.
• Cylinders should only be connected to equipment meant for the particular service.
• Connections should be kept clean and in good order; their condition should be checked at regular intervals.
• Good tools (e.g., normal length, fixed wrenches) should be used.
• Loose valve keys should be left in place when the cylinder is in use.
• Valves should be kept closed when cylinders are not in use.
• Cylinders or connected equipment should be removed from confined spaces when not in use (even during short breaks).
• The atmosphere should be checked for oxygen content and, if possible, for flammable gases before confined spaces are entered and during prolonged working periods.
• It should be kept in mind that heavy gases may concentrate in lower areas and that they may be difficult to remove by ventilation.
• Cylinders should be protected against contamination from pressurized equipment, since backflow of other gases may lead to serious accidents. Proper non-return valves, block-and-bleed arrangements and the like should be used.
• Empty cylinders should be returned to the filler with the valves closed and the caps in place. A little residual pressure should always be left in the cylinder to prevent contamination with air and moisture.
• The filler should be notified of any faulty cylinders.
• Acetylene should only be used at a correctly reduced pressure.
• Flame arrestors should only be used in acetylene lines where acetylene is used with compressed air or oxygen.
• Fire extinguishers and heat-protecting gloves should be available with gas welding equipment.
• Liquid gas cylinders should be stored and used in an upright position.
• Poisonous and irritating gases, such as chlorine, should be handled only by well-informed operators with personal safety equipment.
• Unidentified cylinders should not be kept in stock. Fixed installations, with the gas cylinders connected in separate gas centrals, are safest where gases are used regularly.

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Occupational exposure to hazardous chemicals in laboratories 1990 OSHA Laboratory Standard 29 CFR 1910.1450

The following description of a laboratory chemical hygiene plan corresponds with Section (e:1-4), Chemical hygiene plan-General, of the 1990 OSHA Laboratory Standard. This plan should be made readily available to employees and employee representatives.The chemical hygiene plan shall include each of the following elements and shall indicate specific measures that the employer will take to ensure laboratory employee protection:

1. Stand operating procedures relevant to safety and health considerations to be followed when laboratory work involves the use of hazardous chemicals;
2. Criteria that the employer will use to determine and implement control measures to reduce employee exposure to hazardous chemicals, including engineering controls, the use of personal protective equipment and hygiene practices; particular attention shall be given to the selection of control measures for chemicals that are known to be extremely hazardous;
3. A requirement that fume hoods and other protective equipment are functioning properly, and specific measures that shall be taken to ensure proper and adequate performance of such equipment;
4. Provisions for employee information and training as prescribed [elsewhere in this plan];
5. The circumstances under which a particular laboratory operation, procedure or activity shall require prior approval from the employer or the employer’s designee before implementation;
6. Provisions for medical consultation and medical examinations...;
7. Designation of personnel responsible for implementation of the chemical hygiene plan, including the assignment of a chemical hygiene officer and, if appropriate, establishment of a chemical hygiene committee; and
8. Provision for additional employee protection for work with particularly hazardous substances. These include “select carcinogens”, reproductive toxins and substances which have a high degree of acute toxicity. Specific consideration shall be given to the following provisions, which shall be included where appropriate:

 (a)     establishment of a designated area;

 (b)     use of containment devices such as fume hoods or glove boxes;

 (c)     procedures for safe removal of contaminated waste; and

 (d)     decontamination procedures. 

The employer shall review and evaluate the effectiveness of the chemical hygiene plan at least annually and update it as necessary.

Setting up a Safe and Healthy Laboratory

A laboratory can only be safe and hygienic if the work practices and procedures that are followed there are safe and hygienic. Such practices are fostered by first giving responsibility and authority for laboratory safety and chemical hygiene to a laboratory safety officer who, together with a safety committee of laboratory personnel, decides what tasks must be accomplished and assigns responsibility for carrying out each of them.

The safety committee’s specific tasks include conducting periodic laboratory inspections and summarizing the results in a report submitted to the laboratory safety officer. These inspections are properly done with a checklist. Another important aspect of safety management is periodic inspections of safety equipment to ensure that all equipment is in good working order and in designated locations. Before this can be done, an annual inventory of all the safety equipment must be made; this includes a brief description, including size or capacity and manufacturer. Of no less importance is a semiannual inventory of all laboratory chemicals, including proprietary products. These should be classified into groups of chemically similar substances and also classified according to their fire hazard. Another essential safety classification depends on the degree of hazard associated with a substance, since the treatment a substance receives is directly related to the harm it can cause and the ease with which the harm is unleashed. Each chemical is put into one of three hazard classes chosen on the basis of grouping according to the order of magnitude of risk involved; they are:

1. ordinary hazard substances
2. high-hazard substances
3. extremely hazardous materials.

Ordinary hazard substances are those that are relatively easily controlled, are familiar to laboratory personnel and present no unusual risk. This class ranges from innocuous substances such as sodium bicarbonate and sucrose to concentrated sulphuric acid, ethylene glycol and pentane.

High-hazard substances present much greater hazards than ordinary hazards. They require special handling or, sometimes, monitoring, and present high fire or explosion hazards or severe health risks. In this group are chemicals that form unstable explosive compounds on standing (e.g., hydroperoxides formed by ethers) or substances that have high acute toxicities (e.g., sodium fluoride, which has an oral toxicity of 57 mg/kg in mice), or that have chronic toxicities such as carcinogens, mutagens or teratogens. Substances in this group often have the same kind of hazard as those in the group that follows. The difference is one of degree—those in group 3, the extremely hazardous materials, have either a greater intensity of hazard, or their order of magnitude is much greater, or the dire effects can be released far more easily.

Extremely hazardous materials, when not handled correctly, can very readily cause a serious accident resulting in severe injury, loss of life or extensive property damage. Extreme caution must be exercised in dealing with these substances. Examples of this class are nickel tetracarbonyl (a volatile, extremely poisonous liquid, the vapours of which have been lethal in concentrations as low as 1 ppm) and triethylaluminium (a liquid that spontaneously ignites on exposure to air and reacts explosively with water).

One of the most important of the safety committee’s tasks is to write a comprehensive document for the laboratory, a laboratory safety and chemical hygiene plan, that fully describes its safety policy and standard procedures for carrying out laboratory operations and fulfilling regulatory obligations; these include guidelines for working with substances that may fall into any of the three hazard categories, inspecting safety equipment, responding to a chemical spill, chemical waste policy, standards for laboratory air quality and any recordkeeping required by regulatory standards. The laboratory safety and chemical hygiene plan must be kept in the laboratory or must be otherwise easily accessible to its workers. Other sources of printed information include: chemical information sheets (also called material safety data sheets, MSDSs), a laboratory safety manual, toxicological information and fire hazard information. The inventory of laboratory chemicals and three associated derivative lists (classification of chemicals according to chemical class, fire safety class and the three degrees of hazard) must also be kept with these data.

A file system for records of safety-related activities is also required. It is not necessary that this file either be in the laboratory or be immediately accessible to laboratory workers. The records are mainly for the use of laboratory personnel who oversee laboratory safety and chemical hygiene and for the perusal of regulatory agency inspectors. It should thus be easily available and kept up to date. It is advisable that the file be kept outside the laboratory in order to reduce the possibility of its destruction in the event of a fire. The documents on file should include: records of laboratory inspections by the safety committee, records of inspections by any local regulatory agencies including fire departments and state and federal agencies, records dealing with hazardous waste disposal, records of taxes levied on various classes of hazardous waste, where applicable, a second copy of the inventory of laboratory chemicals, and copies of other pertinent documents dealing with the facility and its personnel (e.g., records of attendance of personnel at annual laboratory safety sessions).

Causes of Illness and Injury in the Laboratory

Measures for the prevention of personal injury, illness and anxiety are an integral part of the plans for the day-to-day operation of a well-run laboratory. The people who are affected by unsafe and unhygienic conditions in a laboratory include not only those who work in that laboratory but also neighbouring personnel and those who provide mechanical and custodial services. Since personal injuries in laboratories stem largely from inappropriate contact between chemicals and people, inappropriate mixing of chemicals or inappropriate supply of energy to chemicals, protecting health entails preventing such undesirable interactions. This, in turn, means suitably confining chemicals, combining them properly and closely regulating the energy supplied to them. The main kinds of personal injury in the laboratory are poisoning, chemical burns and injury resulting from fires or explosions. Fires and explosions are a source of thermal burns, lacerations, concussions and other severe bodily harm.

Chemical attack on the body. Chemical attack takes place when poisons are absorbed into the body and interfere with its normal function through disturbance of metabolism or other mechanisms. Chemical burns, or the gross destruction of tissue, usually occur by contact with either strong acids or strong alkalis. Toxic materials that have entered the body by absorption through the skin, eyes or mucous membranes, by ingestion or by inhalation, can cause systemic poisoning, usually by being spread via the circulatory system.

Poisoning is of two general types—acute and chronic. Acute poisoning is characterized by ill effects appearing during or directly after a single exposure to a toxic substance. Chronic poisoning becomes evident only after the passage of time, which may take weeks, months, years or even decades. Chronic poisoning is said to occur when each of these conditions is met: the victim must have been subjected to multiple exposures over long periods of time and to metabolically significant amounts of a chronic poison.

Chemical burns, usually encountered when liquid corrosives are spilled or splashed on the skin or in the eyes, also occur when those tissues come in contact with corrosive solids, ranging in size from powdery dusts to fairly large crystals, or with corrosive liquids dispersed in the air as mists, or with such corrosive gases as hydrogen chloride. The bronchial tubes, lungs, tongue, throat and epiglottis can also be attacked by corrosive chemicals in either the gaseous, liquid or solid states. Toxic chemicals also, of course, may be introduced into the body in any of these three physical states, or in the form of dusts or mists.

Injury through fires or explosions. Both fires or explosions may produce thermal burns. Some of the injuries caused by explosions, however, are particularly characteristic of them; they are injuries engendered either by the concussive force of the detonation itself or by such of its effects as glass fragments hurled through the air, causing loss of fingers or limbs in the first case, or skin lacerations or loss of vision, in the second.

Laboratory injuries from other sources. A third class of injuries may be caused neither by chemical attack nor by combustion. Rather they are produced by a miscellany of all other sources—mechanical, electrical, high-energy light sources (ultraviolet and lasers), thermal burns from hot surfaces, sudden explosive shattering of screw-capped glass chemical containers from the unexpected build-up of high internal gas pressures and lacerations from the sharp, jagged edges of newly broken glass tubing. Among the most serious sources of injury of a mechanical origin are tall, high-pressure gas cylinders tipping over and falling to the floor. Such episodes can injure legs and feet; in addition, should the cylinder stem break during the fall, the gas cylinder, propelled by the rapid, massive, uncontrolled escape of gas, becomes a deadly, undirected missile, a potential source of greater, more widespread harm.

Injury Prevention

Safety sessions and information dissemination. Injury prevention, dependent on performance of laboratory operations in a safe and prudent manner, is, in turn, dependent on laboratory workers being trained in correct laboratory methodology. Although they have received some of this training in their undergraduate and graduate education, it must be supplemented and reinforced by periodic laboratory safety sessions. Such sessions, which should emphasize understanding the physical and biological bases of safe laboratory practice, will enable laboratory workers to reject questionable procedures easily and to select technically sound methods as a matter of course. The sessions should also acquaint laboratory personnel with the kinds of data needed to design safe procedures and with sources of such information.

Workers must also be provided with ready access, from their work stations, to pertinent safety and technical information. Such materials should include laboratory safety manuals, chemical information sheets and toxicological and fire hazard information.

Prevention of poisoning and chemical burns. Poisoning and chemical burns have a common feature—the same four sites of entry or attack: (1) skin, (2) eyes, (3) mouth to stomach to intestines and (4) nose to bronchial tubes to lungs. Prevention consists in making these sites inaccessible to poisonous or corrosive substances. This is done by placing one or more physical barriers between the person to be protected and the hazardous substance or by ensuring that the ambient laboratory air is not contaminated. Procedures that use these methods include working behind a safety shield or using a fume hood, or utilizing both methods. The use of a glove box, of course, of itself affords a twofold protection. Minimization of injury, should contamination of tissue occur, is accomplished by removing the toxic or corrosive contaminant as quickly and completely as possible.

Prevention of acute poisoning and chemical burns in contrast with the prevention of chronic poisoning. Although the basic approach of isolation of the hazardous substance from the person to be protected is the same in preventing acute poisoning, chemical burns and chronic poisoning, its application must be somewhat different in preventing chronic poisoning. Whereas acute poisoning and chemical burns may be likened to massive assault in warfare, chronic poisoning has the aspect of a siege. Usually produced by much lower concentrations, exerting their influence through multiple exposures over long periods of time, its effects surface gradually and insidiously through sustained and subtle action. Corrective action involves either first detecting a chemical capable of causing chronic poisoning before any physical symptoms appear, or recognizing one or more aspects of a laboratory worker’s discomfort as possibly being physical symptoms connected with chronic poisoning. Should chronic poisoning be suspected, medical attention must be sought promptly. When a chronic poison is found at a concentration exceeding the allowable level, or even approaching it, steps must be taken either to eliminate that substance or, at the very least, to reduce its concentration to a safe level. Protection against chronic poisoning often requires that protective equipment be used for all or much of the workday; however, for reasons of comfort, the use of a glove box or a self-contained breathing apparatus (SCBA) is not always feasible.

Protection against poisoning or chemical burns. Protection against contamination of the skin by a particular splashed corrosive liquid or scattered poisonous airborne solid is best done by the use of safety gloves and a laboratory apron made of a suitable natural or synthetic rubber or polymer. The term suitable here is taken to mean a material which is neither dissolved, swelled nor in any other way attacked by the substance against which it must afford protection, nor should it be permeable to the substance. The use of a safety shield on the laboratory bench interposed between apparatus in which chemicals are being heated, reacted or distilled and the experimenter is a further safeguard against chemical burns and poisoning via skin contamination. Since the speed with which a corrosive or a poison is washed from the skin is a critical factor in preventing or minimizing the damage these substances can inflict, a safety shower, conveniently located in the laboratory, is an indispensable piece of safety equipment.

The eyes are best protected from splashed liquids by safety goggles or face shields. Airborne contaminants, in addition to gases and vapours, include solids and liquids when they are present in a finely subdivided state as dusts or mists. These are most effectively kept out of the eyes by conducting operations in a fume hood or glove box, although goggles afford some protection against them. To afford additional protection while the hood is being used, goggles may be worn. The presence of easily accessible eyewash fountains in the laboratory will often eliminate, and certainly will, at least, reduce eye damage through contamination by splashed corrosives or poisons.

The mouth to stomach to intestines route is usually connected with poisoning rather than with attack by corrosives. When toxic materials are ingested, it usually happens unwittingly through the chemical contamination of foods or cosmetics. Sources of such contamination are food stored in refrigerators with chemicals, food and beverages consumed in the laboratory, or lipstick kept or applied in the laboratory. Prevention of this kind of poisoning is done by avoiding practices known to cause it; this is feasible only when refrigerators to be used exclusively for food, and dining space outside of the laboratory, are made available.

The nose to bronchial tubes to lungs route, or respiratory route, of poisoning and chemical burns deals exclusively with airborne substances, whether gases, vapours, dusts or mists. These airborne materials may be kept from the respiratory systems of people within and outside of the laboratory by the concurrent practices of: (1) confining operations that either use or produce them to the fume hood (2) adjusting the laboratory air supply so that the air is changed 10 to 12 times per hour and (3) keeping the laboratory air pressure negative with respect to that of the corridors and rooms around it. Fume- or dust-producing operations that involve very bulky pieces of apparatus or containers the size of a 218-l drum, which are too large to be enclosed by an ordinary fume hood, should be done in a walk-in hood. In general, respirators or SCBA should not be used for any laboratory operations other than those of an emergency nature.

Chronic mercury poisoning, produced by the inhalation of mercury vapours, is occasionally found in laboratories. It is encountered when a pool of mercury that has accumulated in a hidden location—under floorboards, in drawers or a closet—has been emitting vapours over a long enough period of time to affect the health of laboratory personnel. Good laboratory housekeeping will avert this problem. Should a hidden source of mercury be suspected, the laboratory air must be checked for mercury either by the use of a special detector designed for the purpose or by sending an air sample for analysis.

Preventing fires and explosions and extinguishing fires. The principal cause of laboratory fires is the accidental ignition of flammable liquids. Flammable liquid is defined, in the fire safety sense, as being a liquid having a flashpoint of less than 36.7 °C. Ignition sources known to have caused this kind of laboratory fire include open flames, hot surfaces, electric sparks from switches and motors found in such equipment as stirrers, household-type refrigerators and electric fans, and sparks produced by static electricity. When ignition of a flammable liquid occurs, it takes place, not in the liquid itself, but above it, in the mixture of its vapours with air (when the concentration of vapour falls between certain upper and lower limits).

Preventing laboratory fires is accomplished by confining the vapours of flammables completely within the containers in which the liquids are kept or the apparatus in which they are used. If it is not possible to contain these vapours completely, their rate of escape should be made as low as possible and a continuous vigorous flow of air should be supplied to sweep them away, so as to keep their concentration at any given time well below the lower critical concentration limit. This is done both when reactions involving a flammable liquid are run in a fume hood and when drums of flammables are stored in safety solvent cabinets vented to an exhaust.

A particularly unsafe practice is the storage of such flammables as ethanol in a household-type refrigerator. These refrigerators will not keep vapours of stored flammable liquids from the sparks of its switches, motors and relays. No containers of flammables must ever be put in this type of refrigerator. This is especially true of open vessels and trays containing flammable liquids. However, even flammables in screw-capped bottles, kept in this type of refrigerator, have caused explosions, presumably by vapours leaking through a faulty seal or by the bottles breaking. Flammable liquids that require refrigeration must be kept only in explosion-proof refrigerators.

A significant source of fires that occur when large quantities of flammables are poured or siphoned from one drum to another is sparks produced through the accumulation of electric charge produced by a moving fluid. Spark generation of this sort can be prevented by electrically grounding both drums.

Most chemical and solvent fires that occur in the laboratory and are of manageable size, may be extinguished with either a carbon dioxide or dry-chemical type fire extinguisher. One or more 4.5 kg extinguishers of either kind should be supplied to a laboratory, according to its size. Certain special types of fires require other kinds of extinguishing agents. Many metal fires are put out with sand or graphite. Burning metal hydrides require graphite or powdered limestone.

When clothing is set afire in the laboratory, the flames must be put out quickly to minimize the injury caused by thermal burns. A wall-mounted wrap-around fire blanket extinguishes such fires effectively. It may be used for unassisted smothering of flames by the person whose clothing is on fire. Safety showers may also be used to extinguish these fires.

There are limits to the total volumes of flammable liquids that may be safely kept in a particular laboratory. Such limits, generally written into local fire codes, vary and depend on the materials of construction of the laboratory and on whether it is equipped with an automatic fire-extinguishing system. They usually range from about 55 to 135 litres.

Natural gas is often available from numbers of valves located throughout a typical laboratory. These are the most common sources of gas leaks, along with the rubber tubes and burners leading from them. Such leaks, when not detected soon after their onset, have led to severe explosions. Gas detectors, designed to indicate the level of gas concentration in the air, may be used to locate the source of such leakage quickly.

Prevention of injury from miscellaneous sources. Harm from tall, high-pressure gas cylinders falling, among the most familiar in this group of accidents, is avoided easily by strapping or chaining these cylinders securely to a wall or laboratory bench and putting cylinder caps on all unused and empty cylinders.

Most of the injuries from jagged edges of broken glass tubing are sustained through breakage while the tubing is being put into corks or rubber stoppers. They are avoided by lubricating the tube with glycerol and protecting the hands with leather work gloves.

(a)     General laboratory ventilation. This system should: Provide a source of air for breathing and for input to local ventilation devices; it should not be relied on for protection from toxic substances released into the laboratory; ensure that laboratory air is continually replaced, preventing increase of air concentrations of toxic substances during the working day; direct air flow into the laboratory from non-laboratory areas and out to the exterior of the building.

(b)    Hoods. A laboratory hood with 2.5 linear feet (76 cm) of hood space per person should be provided for every 2 workers if they spend most of their time working with chemicals; each hood should have a continuous monitoring device to allow convenient confirmation of adequate hood performance before use. If this is not possible, work with substances of unknown toxicity should be avoided or other types of local ventilation devices should be provided.

(c)     Other local ventilation devices. Ventilated storage cabinets, canopy hoods, snorkels, etc. should be provided as needed. Each canopy hood and snorkel should have a separate exhaust duct.

(d)    Special ventilation areas. Exhaust air from glove boxes and isolation rooms should be passed through scrubbers or other treatment before release into the regular exhaust system. Cold rooms and warm rooms should have provisions for rapid escape and for escape in the event of electrical failure.

(e)     Modifications. Any alteration of the ventilation system should be made only if thorough testing indicates that worker protection from airborne toxic substances will continue to be adequate.

(f)     Performance. Rate: 4-12 room air changes/hour is normally adequate general ventilation if local exhaust systems such as hoods are used as the primary method of control.

(g)    Quality. General air flow should not be turbulent and should be relatively uniform throughout the laboratory, with no high velocity or static areas; airflow into and within the hood should not be excessively turbulent; hood face velocity should be adequate (typically 60-100 lf/min) (152-254 cm/min).

(h)     Evaluation. Quality and quantity of ventilation should be evaluated on installation, regularly monitored (at least every 3 months), and reevaluated whenever a change in local ventilation is made.

Incompatible Materials

Incompatible materials are a pair of substances that, on contact or mixing, produce either a harmful or potentially harmful effect. The two members of an incompatible pair may be either a pair of chemicals or a chemical and a material of construction such as wood or steel. The mixing or contact of two incompatible materials leads either to a chemical reaction or to a physical interaction that generates a large amount of energy. Specific harmful or potentially harmful effects of these combinations, which can ultimately lead to serious injury or damage to the health, include liberation of large amounts of heat, fires, explosions, production of a flammable gas or generation of a toxic gas. Since a fairly extensive variety of substances is usually found in laboratories, the occurrence of incompatibles in them is quite common and presents a threat to life and health if they are not handled correctly.

Incompatible materials are seldom mixed intentionally. Most often, their mixing is the result of a simultaneous accidental breaking of two adjacent containers. Sometimes it is the effect of leakage or dripping, or results from the mixing of gases or vapours from nearby bottles. Although in many cases in which a pair of incompatibles is mixed, the harmful effect is easily observed, in at least one instance, a not readily detectable chronic poison is formed. This occurs as the result of the reaction of formaldehyde gas from 37% formalin with hydrogen chloride that has escaped from concentrated hydrochloric acid to form the potent carcinogen bis(chloromethyl) ether. Other instances of not immediately detectable effects are the generation of odourless, flammable gases.

Keeping incompatibles from mixing through the simultaneous breaking of adjacent containers or through escape of vapours from nearby bottles is simple—the containers are moved far apart. The incompatible pair, however, must first be identified; not all such identifications are simple or obvious. To minimize the possibility of overlooking an incompatible pair, a compendium of incompatibles should be consulted and scanned occasionally to acquire an acquaintance with less familiar examples. Preventing a chemical from coming in contact with incompatible shelving material, through dripping or through a bottle breaking, is done by keeping the bottle in a glass tray of sufficient capacity to hold all of its contents.

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Occupational health professionals have generally relied on the following hierarchy of control techniques to eliminate or minimize worker exposures: substitution, isolation, ventilation, work practices, personal protective clothing and equipment. Usually a combination of two or more of these techniques is applied. Although this article focuses primarily on the application of ventilation techniques, the other approaches are briefly discussed. They should not be ignored when attempting to control exposure to chemicals by ventilation.

The occupational health professional should always think of the concept of source-path-receiver. The primary focus should be on control at the source with control of the path the second focus. Control at the receiver should be considered the last choice. Whether it is during the start-up or design phases of a process or during the evaluation of an existing process, the procedure for control of exposure to air contaminants should start at the source and progress to the receiver. It is likely that all or most of these control strategies will need to be used.

Substitution

The principle of substitution is to eliminate or reduce the hazard by substituting non-toxic or less toxic materials or redesigning the process to eliminate escape of contaminants into the workplace. Ideally, substitute chemicals would be non-toxic or the process redesign would completely eliminate exposure. However, since this is not always possible the subsequent controls in the above hierarchy of controls are attempted.

Note that extreme care should be taken to assure that substitution does not result in a more hazardous condition. While this focus is on the toxicity hazard, the flammable and chemical reactivity of substitutes must also be considered when assessing this risk.

Isolation

The principle of isolation is to eliminate or reduce the hazard by separating the process emitting the contaminant from the worker. This is accomplished by completely enclosing the process or locating it a safe distance away from people. However, to accomplish this, the process may need to be operated and/or controlled remotely. Isolation is particularly useful for jobs requiring few workers and when control by other methods is difficult. Another approach is to perform hazardous operations on off shifts where fewer workers may be exposed. Sometimes the use of this technique does not eliminate exposure but reduces the number of people who are exposed.

Ventilation

Two types of exhaust ventilation are commonly employed to minimize airborne exposure levels of contaminants. The first is called general or dilution ventilation. The second is referred to as source control or local exhaust ventilation (LEV) and is discussed in more detail later in this article.

These two types of exhaust ventilation should not be confused with comfort ventilation, whose main purpose is to provide measured amounts of outdoor air for breathing and to maintain design temperature and humidity. Various types of ventilation are discussed elsewhere in this Encyclopaedia.

Work Practices

Work practices control encompasses the methods workers employ to perform operations and the extent to which they follow the correct procedures. Examples of this control procedure are given throughout this Encyclopaedia wherever general or specific processes are discussed. General concepts such as education and training, principles of management and social support systems include discussions of the importance of work practices in controlling exposures.

Personal Protective Equipment

Personal protective equipment (PPE) is considered the last line of defence for control of worker exposure. It encompasses the use of respiratory protection and protective clothing. It is frequently used in conjunction with other control practices, particularly to minimize the effects of unexpected releases or accidents. These issues are discussed in more detail in the chapter Personal protection.

Local Exhaust Ventilation

The most efficient and cost-effective form of contaminant control is LEV. This involves capture of the chemical contaminant at its source of generation. There are three types of LEV systems:

1. enclosures
2. exterior hoods
3. receiving hoods.

Enclosures are the preferable type of hood. Enclosures primarily are designed to contain the materials generated within the enclosure. The more complete the enclosure the more completely the contaminant will be contained. Complete enclosures are those that have no openings. Examples of complete enclosures include glove boxes, abrasive blasting cabinets and toxic gas storage cabinets (see figure 1, figure 2 and figure 3). Partial enclosures have one or more sides open but the source is still inside the enclosure. Examples of partial enclosures are a spray paint booth (see figure 4) and a laboratory hood. Often it might appear that the design of enclosures is more art than science. The basic principle is to design a hood with the smallest opening possible. The volume of air required is usually based on the area of all openings and maintaining an airflow velocity into the opening of 0.25 to 1.0 m/s. The control velocity chosen will depend on the operation’s characteristics, including the temperature and the degree to which the contaminant is propelled or generated. For complex enclosures, extreme care must be taken to assure that the exhaust flow is evenly distributed throughout the enclosure, particularly if the openings are distributed. Many enclosure designs are evaluated experimentally and if demonstrated to be effective are included as design plates in the American Conference of Governmental Industrial Hygienists’ industrial ventilation manual (ACGIH 1992).

Figure 1. Complete enclosure: Glovebox

Louis DiBernardinis

Figure 2. Complete enclosure: Toxic gas storage cabinet

Louis DiBernardinis

Figure 3. Complete enclosure: Abrasive blasting cabinet

Michael McCann

Figure 4. Partial enclosure: Spray paint booth

Louis DiBernardinis

Often, total enclosure of the source is not possible, or is not necessary. In these cases, another form of local exhaust, an exterior or capture hood, can be used. An exterior hood prevents the release of toxic materials into the workplace by capturing or entraining them at or close to the source of generation, usually a work station or process operation. Considerably less air volume is usually required than for the partial enclosure. However, since the contaminant is generated outside the hood, it must be designed and used properly in order to be as effective as a partial enclosure. The most effective control is a complete enclosure.

To work effectively, the air inlet of an exterior hood must be of appropriate geometrical design and placed near the point of chemical release. The distance away will depend on the size and shape of the hood and the velocity of air needed at the generation source to capture the contaminant and bring it into the hood. Generally, the closer to the generation source, the better. Design face or slot velocities are typically in the range of 0.25 to 1.0 and 5.0 to 10.0 m/s, respectively. Many design guidelines exist for this class of exhaust hoods in Chapter 3 of the ACGIH manual (ACGIH 1992) or in Burgess, Ellenbecker and Treitman (1989). Two types of exterior hoods that find frequent application are “canopy” hoods and “slot” hoods.

Canopy hoods are used primarily for capture of gases, vapours and aerosols released in one direction with a velocity that can be used to aid capture. These are sometimes called “receiving” hoods. This type of hood is generally used when the process to be controlled is at elevated temperatures, to make use of the thermal updraft, or the emissions are directed upward by the process. Examples of operations that may be controlled in this manner include drying ovens, melting furnaces and autoclaves. Many equipment manufacturers recommend specific capture hood configurations that are suitable for their units. They should be consulted for advice. Design guidelines are also provided in the ACGIH manual, Chapter 3 (ACGIH 1992). For example, for an autoclave or oven where the distance between the hood and the hot source does not exceed approximately the diameter of the source or 1 m, whichever is smaller, the hood may be considered a low canopy hood. Under such conditions, the diameter or cross-section of the hot air column will be approximately the same as the source. The diameter or side dimensions of the hood therefore need only be 0.3 m larger than the source.

The total flow rate for a circular low canopy hood is

Q_t=4.7 (D_f)^2.33 (D_t)^0.42

where:

Q_t =       total hood air flow in cubic feet per minute, ft³/min

D_f =       diameter of hood, ft

D_t =       difference between temperature of the hood source, and the ambient, °F.

Similar relationships exist for rectangular hoods and high canopy hoods. An example of a canopy hood can be seen in figure 5.

Figure 5. Canopy hood: Oven exhaust

Louis DiBernardinis

Slot hoods are used for control of operations that cannot be performed inside a containment hood or under a canopy hood. Typical operations include barrel filling, electroplating, welding and degreasing. Examples are shown in figure 6 and figure 7.

Figure 6. Exterior hood: Welding

Michael McCann

Figure 7. Exterior hood: Barrel filling

Louis DiBernardinis

The required flow can be calculated from a series of equations determined empirically by the size and shape of the hood and the distance of the hood from the source. For example, for a flanged slot hood, the flow is determined by

Q = 0.0743LVX

where:

Q =      total hood air flow, m³/min

L =       the length of the slot, m

V =       the velocity needed at the source to capture it, m/min

X =       distance from the source to the slot, m.

The velocity needed at the source is sometimes called “capture velocity” and is usually between 0.25 and 2.5 m/s. Guidelines for selecting an appropriate capture velocity are provided in the ACGIH manual. For areas with excessive cross-drafts or for high-toxicity materials, the upper end of the range should be selected. For particulates, higher capture velocities will be necessary.

Some hoods may be some combination of enclosure, exterior and receiving hoods. For example, the spray paint booth shown in figure 4 is a partial enclosure that is also a receiving hood. It is designed to provide efficient capture of particles generated by making use of the particle momentum created by the rotating grinding wheel in the direction of the hood.

Care must be used in selecting and designing local exhaust systems. Considerations should include (1) ability to enclose the operation, (2) source characteristics (i.e., point source vs. widespread source) and how the contaminant is generated, (3) capacity of existing ventilation systems, (4) space requirements and (5) toxicity and flammability of contaminants.

Once the hood is installed, a routine monitoring and maintenance programme for the systems shall be implemented to assure its effectiveness in preventing exposure to workers (OSHA 1993). Monitoring of the standard laboratory chemical hood has become standardized since the 1970s. However, there is no such standardized procedure for other forms of local exhaust; therefore, the user must devise his or her own procedure. The most effective would be a continuous flow monitor. This could be as simple as a magnetic or water pressure gauge measuring static pressure at the hood (ANSI/AIHA 1993). The required hood static pressure (cm of water) will be known from the design calculations, and flow measurements can be made at the time of installation to verify them. Whether or not a continuous flow monitor is present, there should be some periodic evaluation of the hood performance. This can be done with smoke at the hood to visualize capture and by measuring total flow in the system and comparing that to the design flow. For enclosures it is usually advantageous to measure face velocity through the openings.

Personnel must also be instructed in the correct use of these types of hoods, particularly where the distance from the source and the hood can be easily changed by the user.

If local exhaust systems are designed, installed and used correctly they can be an effective and economical means of controlling toxic exposures.

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