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Wednesday, 16 February 2011 00:49

Indoor Air: Methods for Control and Cleaning

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The quality of air inside a building is due to a series of factors that include the quality of outside air, the design of the ventilation/airconditioning system, the way that the system works and is maintained and the sources of indoor pollution. In general terms, the level of concentration of any contaminant in an indoor space will be determined by the balance between the generation of the pollutant and the rate of its elimination.

As for the generation of contaminants, the sources of pollution may also be external or internal. The external sources include atmospheric pollution due to industrial combustion processes, vehicular traffic, power plants and so on; pollution emitted near the intake shafts where air is drawn into the building, such as that from refrigeration towers or the exhaust vents of other buildings; and emanations from contaminated soil such as radon gas, leaks from gasoline tanks or pesticides.

Among the sources of internal pollution, it is worth mentioning those associated with the ventilation and air-conditioning systems themselves (chiefly the microbiological contamination of any segment of such systems), the materials used to build and decorate the building, and the occupants of the building. Specific sources of indoor pollution are tobacco smoke, laboratories, photocopiers, photographic labs and printing presses, gyms, beauty parlours, kitchens and cafeterias, bathrooms, parking garages and boiler rooms. All these sources should have a general ventilation system and air extracted from these areas should not be recycled through the building. When the situation warrants it, these areas should also have a localized ventilation system that operates by extraction.

Evaluating the quality of indoor air comprises, among other tasks, the measurement and evaluation of contaminants that may be present in the building. Several indicators are used to ascertain the quality of air inside a building. They include the concentrations of carbon monoxide and carbon dioxide, total volatile organic compounds (TVOC), total suspended particles (TSP) and the rate of ventilation. Various criteria or recommended target values exist for the evaluation of some of the substances found in interior spaces. These are listed in different standards or guidelines, such as the guidelines for the quality of interior air promulgated by the World Health Organization (WHO), or the standards of the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE).

For many of these substances, however, there are no defined standards. For now the recommended course of action is to apply the values and standards for industrial environments provided by the American Conference of Governmental Industrial Hygienists (ACGIH 1992). Safety or correction factors are then applied on the order of one-half, one-tenth or one-hundredth of the values specified.

The methods of control of indoor air can be divided in two main groups: control of the source of pollution, or control of the environment with ventilation and air cleaning strategies.

Control of the Source of Pollution

The source of pollution can be controlled by various means, including the following:

  1. Elimination. Eliminating the source of pollution is the ideal method for the control of indoor air quality. This measure is permanent and requires no future maintenance operations. It is applied when the source of pollution is known, as in the case of tobacco smoke, and it requires no substitution for polluting agents.
  2. Substitution. In some cases, substitution of the product that is the source of contamination is the measure that should be used. Changing the kind of products used (for cleaning, decoration, etc.) with others that provide the same service but are less toxic or present less risk to the people who use them is sometimes possible.
  3. Isolation or spatial confinement. These measures are designed to reduce exposure by limiting access to the source. The method consists in interposing barriers (partial or total) or containments around the source of pollution to minimize emissions to the surrounding air and to limit the access of people to the area near the source of pollution. These spaces should be equipped with supplementary ventilation systems that can extract air and provide a directed flow of air where needed. Examples of this approach are closed ovens, boiler rooms and photocopying rooms.
  4. Sealing the source. This method consists of using materials that emit minimal levels of pollution or that emit none at all. This system has been suggested as a way to inhibit the dispersal of loose asbestos fibres from old insulation, as well as to inhibit the emission of formaldehyde from walls treated with resins. In buildings contaminated with radon gas, this technique is used to seal cinder blocks and crevices in basement walls: polymers are used that prevent the immission of radon from the soil. Basement walls may also be treated with epoxy paint and a polymeric sealant of polyethylene or polyamide to prevent contamination that may seep in through walls or from the soil.
  5. Ventilation by localized extraction. Localized ventilation systems are based on the capture of the pollutant at, or as close as possible to, the source. The capture is accomplished by a bell designed to trap the pollutant in a current of air. The air then flows by conduits with the help of a fan to be purified. If the extracted air cannot be purified or filtered, then it should be vented outside and should not be recycled back into the building.

 

Control of the Environment

The indoor environments of nonindustrial buildings usually have many sources of pollution and, in addition, they tend to be scattered. The system most commonly employed to correct or prevent pollution problems indoors, therefore, is ventilation, either general or by dilution. This method consists of moving and directing the flow of air to capture, contain and transport pollutants from their source to the ventilation system. In addition, general ventilation also permits the control of the thermal characteristics of the indoor environment by air conditioning and recirculating air (see “Aims and principles of general and dilution ventilation”, elsewhere in this chapter).

In order to dilute internal pollution, increasing the volume of outside air is advisable only when the system is of the proper size and does not cause a lack of ventilation in other parts of the system or when the added volume does not prevent proper air-conditioning. For a ventilation system to be as effective as possible, localized extractors should be installed at the sources of pollution; air mixed with pollution should not be recycled; occupants should be placed near air diffusion vents and sources of pollution near extraction vents; pollutants should be expelled by the shortest possible route; and spaces that have localized sources of pollution should be kept at negative pressure relative to outside atmospheric pressure.

Most ventilation deficiencies seem to be linked to an inadequate amount of outside air. An improper distribution of ventilated air, however, can also result in poor air quality problems. In rooms with very high ceilings, for instance, where warm (less dense) air is supplied from above, air temperature may become stratified and ventilation will then fail to dilute the pollution present in the room. The placement and location of air diffusion vents and air return vents relative to the occupants and the sources of contamination is a consideration that requires special attention when the ventilation system is being designed.

Air Cleaning Techniques

Air cleaning methods should be precisely designed and selected for specific, very concrete types of pollutants. Once installed, regular maintenance will prevent the system from becoming a new source of contamination. The following are descriptions of six methods used to eliminate pollutants from air.

Filtration of particles

Filtration is a useful method to eliminate liquids or solids in suspension, but it should be borne in mind that it does not eliminate gases or vapours. Filters may capture particles by obstruction, impact, interception, diffusion and electrostatic attraction. Filtration of an indoor air conditioning system is necessary for many reasons. One is to prevent the accumulation of dirt that may cause a diminution of its heating or cooling efficiency. The system may also be corroded by certain particles (sulphuric acid and chlorides). Filtration is also necessary to prevent a loss of equilibrium in the ventilation system due to deposits on the fan blades and false information being fed to the controls because of clogged sensors.

Indoor air filtration systems benefit from placing at least two filters in series. The first, a pre-filter or primary filter, retains only the larger particles. This filter should be changed often and will lengthen the life of the next filter. The secondary filter is more efficient than the first, and can filter out fungal spores, synthetic fibres and in general finer dust than that collected by the primary filter. These filters should be fine enough to eliminate irritants and toxic particles.

A filter is selected based on its effectiveness, its capacity to accumulate dust, its loss of charge and the required level of air purity. A filter’s effectiveness is measured according to ASHRAE 52-76 and Eurovent 4/5 standards (ASHRAE 1992; CEN 1979). Their capacity for retention measures the mass of dust retained multiplied by the volume of air filtered and is used to characterize filters that retain only large particles (low and medium efficiency filters). To measure its retention capacity, a synthetic aerosol dust of known concentration and granulometry is forced through a filter. the portion retained in the filter is calculated by gravimetry.

The efficiency of a filter is expressed by multiplying the number of particles retained by the volume of air filtered. This value is the one used to characterize filters that also retain finer particles. To calculate the efficiency of a filter, a current of atmospheric aerosol is forced through it containing an aerosol of particles with a diameter between 0.5 and 1 μm. The amount of captured particles is measured with an opacitimeter, which measures the opacity caused by the sediment.

The DOP is a value used to characterize very high-efficiency particulate air (HEPA) filters. The DOP of a filter is calculated with an aerosol made by vapourizing and condensing dioctylphthalate, which produces particles 0.3 μm in diameter. This method is based on the light-scattering property of drops of dioctylphthalate: if we put the filter through this test the intensity of scattered light is proportional to the surface concentration of this material and the penetration of the filter can be measured by the relative intensity of scattered light before and after filtering the aerosol. For a filter to earn the HEPA designation it must be better than 99.97 per cent efficient on the basis of this test.

Although there is a direct relationship between them, the results of the three methods are not directly comparable. The efficiency of all filters diminishes as they clog up, and they can then become a source of odours and contamination. The useful life of a high efficiency filter can be greatly extended by using one or several filters of a lower rating in front of the high efficiency filter. Table 1 shows the initial, final and mean yields of different filters according to criteria established by ASHRAE 52-76 for particles 0.3 μm in diameter.

Table 1. The effectiveness of filters (according to ASHRAE standard 52-76) for particles of 3 mm diameter

Filter description

ASHRAE 52-76

Efficiency (%)

 

Dust spot (%)

Arrestance (%)

Initial

Final

Median

Medium

25–30

92

1

25

15

Medium

40–45

96

5

55

34

High

60–65

97

19

70

50

High

80–85

98

50

86

68

High

90–95

99

75

99

87

95% HEPA

95

99.5

99.1

99.97% HEPA

99.97

99.7

99.97

 

Electrostatic precipitation

This method proves useful for controlling particulate matter. Equipment of this sort works by ionizing particles and then eliminating them from the air current as they are attracted to and captured by a collecting electrode. Ionization occurs when the contaminated effluent passes through the electrical field generated by a strong voltage applied between the collecting and the discharge electrodes. The voltage is obtained by a direct current generator. The collecting electrode has a large surface and is usually positively charged, while the discharge electrode consists of a negatively charged cable.

The most important factors that affect the ionization of particles are the condition of the effluent, its discharge and the characteristics of the particles (size, concentration, resistance, etc.). The effectiveness of capture increases with humidity, and the size and density of the particles, and decreases with the increased viscosity of the effluent.

The main advantage of these devices is that they are highly effective at collecting solids and liquids, even when particle size is very fine. In addition, these systems may be used for heavy volumes and high temperatures. The loss of pressure is minimal. The drawbacks of these systems are their high initial cost, their large space requirements and the safety risks they pose given the very high voltages involved, especially when they are used for industrial applications.

Electrostatic precipitators are used in a full range, from industrial settings to reduce the emission of particles to domestic settings to improve the quality of indoor air. The latter are smaller devices that operate at voltages in the range of 10,000 to 15,000 volts. They ordinarily have systems with automatic voltage regulators which ensure that enough tension is always applied to produce ionization without causing a discharge between both electrodes.

Generation of negative ions

This method is used to eliminate particles suspended in air and, in the opinion of some authors, to create healthier environments. The efficacy of this method as a way to reduce discomfort or illness is still being studied.

Gas adsorption

This method is used to eliminate polluting gases and vapours like formaldehyde, sulphur dioxide, ozone, nitrogen oxides and organic vapours. Adsorption is a physical phenomena by which gas molecules are trapped by an adsorbent solid. The adsorbent consists of a porous solid with a very large surface area. To clean this kind of pollutant from the air, it is made to flow through a cartridge full of the adsorbent. Activated carbon is the most widely used; it traps a wide range of inorganic gases and organic compounds. Aliphatic, chlorinated and aromatic hydrocarbons, ketones, alcohols and esters are some examples.

Silica gel is also an inorganic adsorbent, and is used to trap more polar compounds such as amines and water. There are also other, organic adsorbents made up of porous polymers. It is important to keep in mind that all adsorbent solids trap only a certain amount of pollutant and then, once saturated, need to be regenerated or replaced. Another method of capture through adsorbent solids is to use a mixture of active alumina and carbon impregnated with specific reactants. Some metallic oxides, for instance, capture mercury vapour, hydrogen sulphide and ethylene. It must be borne in mind that carbon dioxide is not retained by adsorption.

Gas absorption

Eliminating gases and fumes by absorption involves a system that fixes molecules by passing them through an absorbent solution with which they react chemically. This is a very selective method and it uses reagents specific to the pollutant that needs to be captured.

The reagent is generally dissolved in water. It also must be replaced or regenerated before it is used up. Because this system is based on transferring the pollutant from the gaseous phase to the liquid phase, the reagent’s physical and chemical properties are very important. Its solubility and reactivity are especially important; other aspects that play an important part in this transfer from gaseous to liquid phase are pH, temperature and the area of contact between gas and liquid. Where the pollutant is highly soluble, it is sufficient to bubble it through the solution to fix it to the reagent. Where the pollutant is not as readily soluble the system that must be employed must ensure a greater area of contact between gas and liquid. Some examples of absorbents and the contaminants for which they are especially suited are given in table 2.

Table 2. Reagents used as absorbents for various contaminants


Absorbent

Contaminant

Diethylhydroxamine

Hydrogen sulphide

Potassium permangenate

Odiferous gases

Hydrochloric and sulphuric acids

Amines

Sodium sulphide

Aldehydes

Sodium hydroxide

Formaldehyde


Ozonization

This method of improving the quality of indoor air is based on the use of ozone gas. Ozone is generated from oxygen gas by ultraviolet radiation or electric discharge, and employed to eliminate contaminants dispersed in air. The great oxidizing power of this gas makes it suitable for use as an antimicrobial agent, a deodorant and a disinfectant and it can help to eliminate noxious gases and fumes. It is also employed to purify spaces with high concentrations of carbon monoxide. In industrial settings it is used to treat the air in kitchens, cafeterias, food and fish processing plants, chemical plants, residual sewage treatment plants, rubber plants, refrigeration plants and so on. In office spaces it is used with air conditioning installations to improve the quality of indoor air.

Ozone is a bluish gas with a characteristic penetrating smell. At high concentrations it is toxic and even fatal to man. Ozone is formed by the action of ultraviolet radiation or an electric discharge on oxygen. The intentional, accidental and natural production of ozone should be differentiated. Ozone is an extremely toxic and irritating gas both at short-term and long-term exposure. Because of the way it reacts in the body, no levels are known for which there are no biological effects. These data are discussed more fully in the chemicals section of this Encyclopaedia.

Processes that employ ozone should be carried out in enclosed spaces or have a localized extraction system to capture any release of gas at the source. Ozone cylinders should be stored in refrigerated areas, away from any reducing agents, inflammable materials or products that may catalyze its breakdown. It should be kept in mind that if ozonizers function at negative pressures, and have automatic shut-off devices in case of failure, the possibility of leaks is minimized.

Electrical equipment for processes that employ ozone should be perfectly insulated and maintenance on them should be done by experienced personnel. When using ozonizers, conduits and accessory equipment should have devices that shut ozonizers down immediately when a leak is detected; in case of a loss of efficiency in the ventilation, dehumidifying or refrigeration functions; when there occurs an excess of pressure or a vacuum (depending on the system); or when the output of the system is either excessive or insufficient.

When ozonizers are installed, they should be provided with ozone specific detectors. The sense of smell cannot be trusted because it can become saturated. Ozone leaks can be detected with reactive strips of potassium iodide that turn blue, but this is not a specific method because the test is positive for most oxidants. It is better to monitor for leaks on a continuing basis using electrochemical cells, ultraviolet photometry or chemiluminesence, with the chosen detection device connected directly to an alarm system that acts when certain concentrations are reached.

 

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