Extremely low frequency (ELF) and very low frequency (VLF) electric and magnetic fields encompass the frequency range above static (> 0 Hz) fields up to 30 kHz. For this paper ELF is defined as being in the frequency range > 0 to 300 Hz and VLF in the range > 300 Hz to 30 kHz. In the frequency range > 0 to 30 kHz, the wavelengths vary from ∞(infinity) to 10 km and so the electric and magnetic fields act essentially independently of each other and must be treated separately. The electric field strength (E) is measured in volts per metre (V/m), the magnetic field strength (H) is measured in amperes per metre (A/m) and the magnetic flux density (B) in tesla (T).

Considerable debate about possible adverse health effects has been expressed by workers using equipment that operates in this frequency range. By far the most common frequency is 50/60 Hz, used for the generation, distribution and use of electric power. Concerns that exposure to 50/60 Hz magnetic fields may be associated with an increased cancer incidence have been fuelled by media reports, distribution of misinformation and ongoing scientific debate (Repacholi 1990; NRC 1996).

The purpose of this article is to provide an overview of the following topic areas:

• sources, occupations and applications
• dosimetry and measurement
• interaction mechanisms and biological effects
• human studies and effects on health
• protective measures
• occupational exposure standards.

Summary descriptions are provided to inform workers of the types and strengths of fields from major sources of ELF and VLF, biological effects, possible health consequences and current exposure limits. An outline of safety precautions and protective measures is also given. While many workers use visual display units (VDUs), only brief details are given in this article since they are covered in greater detail elsewhere in the Encyclopaedia.

Much of the material contained here can be found in greater detail in a number of recent reviews (WHO 1984, 1987, 1989, 1993; IRPA 1990; ILO 1993; NRPB 1992, 1993; IEEE 1991; Greene 1992; NRC 1996).

Sources of Occupational Exposure

Levels of occupational exposure vary considerably and are strongly dependent upon the particular application. Table 1 gives a summary of typical applications of frequencies in the range > 0 to 30 kHz.

Table 1. Applications of equipment operating in the range > 0 to 30 kHz

Power generation and distribution

The principal artificial sources of 50/60 Hz electric and magnetic fields are those involved in power generation and distribution, and any equipment using electric current. Most such equipment operates at the power frequencies of 50 Hz in most countries and 60 Hz in North America. Some electric train systems operate at 16.67 Hz.

High voltage (HV) transmission lines and substations have associated with them the strongest electric fields to which workers may be routinely exposed. Conductor height, geometrical configuration, lateral distance from the line, and the voltage of the transmission line are by far the most significant factors in considering the maximum electric field strength at ground level. At lateral distances of about twice the line height, the electric field strength decreases with distance in an approximately linear fashion (Zaffanella and Deno 1978). Inside buildings near HV transmission lines, the electric field strengths are typically lower than the unperturbed field by a factor of about 100,000, depending on the configuration of the building and the structural materials.

Magnetic field strengths from overhead transmission lines are usually relatively low compared to industrial applications involving high currents. Electrical utility employees working in substations or on the maintenance of live transmission lines form a special group exposed to larger fields (of 5 mT and higher in some cases). In the absence of ferromagnetic materials, the magnetic field lines form concentric circles around the conductor. Apart from the geometry of the power conductor, the maximum magnetic flux density is determined only by the magnitude of the current. The magnetic field beneath HV transmission lines is directed mainly transverse to the line axis. The maximum flux density at ground level may be under the centre line or under the outer conductors, depending on the phase relationship between the conductors. The maximum magnetic flux density at ground level for a typical double circuit 500 kV overhead transmission lines system is approximately 35 μT per kiloampere of current transmitted (Bernhardt and Matthes 1992). Typical values for the magnetic flux density up to 0.05 mT occur in workplaces near overhead lines, in substations and in power stations operating at frequencies of 16 2/3, 50, or 60 Hz (Krause 1986).

Industrial processes

Occupational exposure to magnetic fields comes predominantly from working near industrial equipment using high currents. Such devices include those used in welding, electroslag refining, heating (furnaces, induction heaters) and stirring.

Surveys on induction heaters used in industry, performed in Canada (Stuchly and Lecuyer 1985), in Poland (Aniolczyk 1981), in Australia (Repacholi, unpublished data) and in Sweden (Lövsund, Oberg and Nilsson 1982), show magnetic flux densities at operator locations ranging from 0.7 μT to 6 mT, depending on the frequency used and the distance from the machine. In their study of magnetic fields from industrial electro-steel and welding equipment, Lövsund, Oberg and Nilsson (1982) found that spot-welding machines (50 Hz, 15 to 106 kA) and ladle furnaces (50 Hz, 13 to 15 kA) produced fields up to 10 mT at distances up to 1 m. In Australia, an induction heating plant operating in the range 50 Hz to 10 kHz was found to give maximum fields of up to 2.5 mT (50 Hz induction furnaces) at positions where operators could stand. In addition maximum fields around induction heaters operating at other frequencies were 130 μT at 1.8 kHz, 25 μT at 2.8 kHz and in excess of 130 μT at 9.8 kHz.

Since the dimensions of coils producing the magnetic fields are often small there is seldom high exposure to the whole body, but rather local exposure mainly to the hands. Magnetic flux density to the hands of the operator may reach 25 mT (Lövsund and Mild 1978; Stuchly and Lecuyer 1985). In most cases the flux density is less than 1 mT. The electric field strength near the induction heater is usually low.

Workers in the electrochemical industry may be exposed to high electric and magnetic field strengths because of electrical furnaces or other devices using high currents. For instance, near induction furnaces and industrial electrolytic cells magnetic flux densities can be measured as high as 50 mT.

Visual display units

The use of visual display units (VDUs) or video display terminals (VDTs) as they are also called, grows at an ever increasing rate. VDT operators have expressed concerns about possible effects from emissions of low-level radiations. Magnetic fields (frequency 15 to 125 kHz) as high as 0.69 A/m (0.9 μT) have been measured under worst-case conditions close to the surface of the screen (Bureau of Radiological Health 1981). This result has been confirmed by many surveys (Roy et al. 1984; Repacholi 1985 IRPA 1988). Comprehensive reviews of measurements and surveys of VDTs by national agencies and individual experts concluded that there are no radiation emissions from VDTs that would have any consequences for health (Repacholi 1985; IRPA 1988; ILO 1993a). There is no need to perform routine radiation measurements since, even under worst-case or failure mode conditions, the emission levels are well below the limits of any international or national standards (IRPA 1988).

A comprehensive review of emissions, summary of the applicable scientific literature, standards and guidelines has been provided in the document (ILO 1993a).

Medical applications

Patients suffering from bone fractures that do not heal well or unite have been treated with pulsed magnetic fields (Bassett, Mitchell and Gaston 1982; Mitbreit and Manyachin 1984). Studies are also being conducted on the use of pulsed magnetic fields to enhance wound healing and tissue regeneration.

Various devices generating magnetic field pulses are used for bone growth stimulation. A typical example is the device that generates an average magnetic flux density of about 0.3 mT, a peak strength of about 2.5 mT, and induces peak electric field strengths in the bone in the range of 0.075 to 0.175 V/m (Bassett, Pawluk and Pilla 1974). Near the surface of the exposed limb, the device produces a peak magnetic flux density of the order of 1.0 mT causing peak ionic current densities of about 10 to 100 mA/m² (1 to 10 μA/cm²) in tissue.

Measurement

Prior to the commencement of measurements of ELF or VLF fields, it is important to obtain as much information as possible about the characteristics of the source and the exposure situation. This information is required for the estimation of the expected field strengths and the selection of the most appropriate survey instrumentation (Tell 1983).

Information about the source should include:

• frequencies present, including harmonics
• power transmitted
• polarization (orientation of E field)
• modulation characteristics (peak and average values)
• duty cycle, pulse width, and pulse repetition frequency
• antenna characteristics, such as type, gain, beam width and scan rate.

Information about the exposure situation must include:

• distance from the source
• existence of any scattering objects. Scattering by plane surfaces can enhance the E field by a factor of 2. Even greater enhancement may result from curved surfaces, e.g., corner reflectors.

Results of surveys conducted in occupational settings are summarized in table 2.

Table 2. Occupational sources of exposure to magnetic fields

Source: Allen 1991; Bernhardt 1988; Krause 1986; Lövsund, Oberg and Nilsson 1982; Repacholi, unpublished data; Stuchly 1986; Stuchly and Lecuyer 1985, 1989.

Instrumentation

An electric or magnetic field-measuring instrument consists of three basic parts: the probe, the leads and the monitor. To ensure appropriate measurements, the following instrumentation characteristics are required or are desirable:

• The probe must respond only to the E field or the H field and not to both simultaneously.
• The probe must not produce significant perturbation of the field.
• The leads from the probe to the monitor must not disturb the field at the probe significantly, or couple energy from the field.
• The frequency response of the probe must cover the range of frequencies required to be measured.
• If used in the reactive near-field, the dimensions of the probe sensor should preferably be less than a quarter of a wavelength at the highest frequency present.
• The instrument should indicate the root mean square (rms) value of the measured field parameter.
• The response time of the instrument should be known. It is desirable to have a response time of about 1 second or less, so that intermittent fields are easily detected.
• The probe should be responsive to all polarization components of the field. This may be accomplished either by inherent isotropic response, or by physical rotation of the probe through three orthogonal directions.
• Good overload protection, battery operation, portability and rugged construction are other desirable characteristics.
• Instruments provide an indication of one or more of the following parameters: average E field (V/m) or mean square E field (V²/m²); average H field (A/m) or mean square H field (A²/m²).

Surveys

Surveys are usually conducted to determine whether fields existing in the workplace are below limits set by national standards. Thus the person taking the measurements must be fully familiar with these standards.

All occupied and accessible locations should be surveyed. The operator of the equipment under test and the surveyor should be as far away as practicable from the test area. All objects normally present, which may reflect or absorb energy, must be in position. The surveyor should take precautions against radiofrequency (RF) burns and shock, particularly near high-power, low-frequency systems.

Interaction Mechanisms and Biological Effects

Interaction mechanisms

The only established mechanisms by which ELF and VLF fields interact with biological systems are:

• Electric fields which induce a surface charge on an exposed body which results in currents (measured in mA/m²) inside the body, the magnitude of which is related to the surface charge density. Depending on the exposure conditions, size, shape and position of the exposed body in the field, the surface charge density can vary greatly, resulting in a variable and non-uniform distribution of currents inside the body.
• Magnetic fields also act on humans by inducing electric fields and currents inside the body.
• Electric charges induced in a conducting object (e.g., an automobile) exposed to ELF or VLF electric fields may cause current to pass through a person in contact with it.
• Magnetic field coupling to a conductor (for example, a wire fence) causes electric currents (of the same frequency as the exposing field) to pass through the body of a person in contact with it.
• Transient discharges (sparks) can occur when people and metal objects exposed to a strong electric field come into sufficiently close proximity.
• Electric or magnetic fields may interfere with implanted medical devices (e.g., unipolar cardiac pacemakers) and cause malfunction of the device.

The first two interactions listed above are examples of direct coupling between persons and ELF or VLF fields. The last four interactions are examples of indirect coupling mechanisms because they can occur only when the exposed organism is in the vicinity of other bodies. These bodies can include other humans or animals and objects such as automobiles, fences or implanted devices.

While other mechanisms of interaction between biological tissues and ELF or VLF fields have been postulated or there is some evidence to support their existence (WHO 1993; NRPB 1993; NRC 1996), none has been shown to be responsible for any adverse consequence to health.

Health effects

The evidence suggests that most of the established effects of exposure to electric and magnetic fields in the frequency range > 0 to 30 kHz result from acute responses to surface charge and induced current density. People can perceive the effects of the oscillating surface charge induced on their bodies by ELF electric fields (but not by magnetic fields); these effects become annoying if sufficiently intense. A summary of the effects of currents passing through the human body (thresholds for perception, let-go or tetanus) are given in table 3.

Table 3. Effects of currents passing through the human body

Source: Bernhardt 1988a.

Human nerve and muscle cells have been stimulated by the currents induced by exposure to magnetic fields of several mT and 1 to 1.5 kHz; threshold current densities are thought to be above 1 A/m². Flickering visual sensations can be induced in the human eye by exposure to magnetic fields as low as about 5 to 10 mT (at 20 Hz) or electric currents directly applied to the head. Consideration of these responses and of the results of neurophysiological studies suggests that subtle central nervous system functions, such as reasoning or memory, may be affected by current densities above 10 mA/m² (NRPB 1993). Threshold values are likely to remain constant up to about 1 kHz but rise with increasing frequency thereafter.

Several in vitro studies (WHO 1993; NRPB 1993) have reported metabolic changes, such as alterations in enzyme activity and protein metabolism and decreased lymphocyte cytotoxicity, in various cell lines exposed to ELF and VLF electric fields and currents applied directly to the cell culture. Most effects have been reported at current densities between about 10 and 1,000 mA/m², although these responses are less clearly defined (Sienkiewicz, Saunder and Kowalczuk 1991). However, it is worth noting that the endogenous current densities generated by the electrical activity of nerves and muscles are typically as high as 1 mA/m² and may reach up to 10 mA/m² in the heart. These current densities will not adversely affect nerve, muscle and other tissues. Such biological effects will be avoided by restricting the induced current density to less than 10 mA/m² at frequencies up to about 1 kHz.

Several possible areas of biological interaction which have many health implications and about which our knowledge is limited include: possible changes in night-time melatonin levels in the pineal gland and alterations in circadian rhythms induced in animals by exposure to ELF electric or magnetic fields, and possible effects of ELF magnetic fields on the processes of development and carcinogenesis. In addition, there is some evidence of biological responses to very weak electric and magnetic fields: these include the altered mobility of calcium ions in brain tissue, changes in neuronal firing patterns, and altered operand behaviour. Both amplitude and frequency “windows” have been reported which challenge the conventional assumption that the magnitude of a response increases with increasing dose. These effects are not well established and do not provide a basis for establishing restrictions on human exposure, although further investigations are warranted (Sienkievicz, Saunder and Kowalczuk 1991; WHO 1993; NRC 1996).

Table 4 gives the approximate ranges of induced current densities for various biological effects in humans.

Table 4. Approximate current density ranges for various biological effects

Source: Sienkiewicz et al. 1991.

Occupational Exposure Standards

Nearly all standards having limits in the range > 0-30 kHz have, as their rationale, the need to keep induced electric fields and currents to safe levels. Usually the induced current densities are restricted to less than 10 mA/m². Table 5 gives a summary of some current occupational exposure limits.

Table 5. Occupational limits of exposure to electric and magnetic fields in the frequency range > 0 to 30 kHz (note that f is in Hz)

Protective Measures

Occupational exposures that occur near high voltage transmission lines depend on the worker’s location either on the ground or at the conductor during live-line work at high potential. When working under live-line conditions, protective clothing may be used to reduce the electric field strength and current density in the body to values similar to those that would occur for work on the ground. Protective clothing does not weaken the influence of the magnetic field.

The responsibilities for the protection of workers and the general public against the potentially adverse effects of exposure to ELF or VLF electric and magnetic fields should be clearly assigned. It is recommended that the competent authorities consider the following steps:

• development and adoption of exposure limits and the implementation of a compliance programme
• development of technical standards to reduce the susceptibility to electromagnetic interference, for example, for pacemakers
• development of standards defining zones with limited access around sources of strong electric and magnetic fields because of electromagnetic interference (e.g., for pacemakers and other implanted devices). The use of appropriate warning signs should be considered.
• requirement of specific assignment of a person responsible for the safety of workers and the public at each site with high exposure potentials
• development of standardized measurement procedures and survey techniques
• requirements for the education of workers on the effects of exposure to ELF or VLF electric and magnetic fields and the measures and rules which are designed to protect them
• drafting of guidelines or codes of practice for worker safety in ELF or VLF electric and magnetic fields. ILO (1993a) provides excellent guidance for such a code.

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Radiofrequency (RF) electromagnetic energy and microwave radiation is used in a variety of applications in industry, commerce, medicine and research, as well as in the home. In the frequency range from 3 to 3 x 10⁸ kHz (that is, 300 GHz) we readily recognize applications such as radio and television broadcasting, communications (long-distance telephone, cellular telephone, radio communication), radar, dielectric heaters, induction heaters, switched power supplies and computer monitors.

High-power RF radiation is a source of thermal energy that carries all of the known implications of heating for biological systems, including burns, temporary and permanent changes in reproduction, cataracts and death. For the broad range of radiofrequencies, cutaneous perception of heat and thermal pain is unreliable for detection, because the thermal receptors are located in the skin and do not readily sense the deep heating of the body caused by these fields. Exposure limits are needed to protect against these adverse health effects of radiofrequency field exposure.

Occupational Exposure

Induction heating

By applying an intense alternating magnetic field a conducting material can be heated by induced eddy currents. Such heating is used for forging, annealing, brazing and soldering. Operating frequencies range from 50/60 to several million Hz. Since the dimensions of the coils producing the magnetic fields are often small, the risk of high-level whole-body exposure is small; however, exposure to the hands can be high.

Dielectric heating

Radiofrequency energy from 3 to 50 MHz (primarily at frequencies of 13.56, 27.12 and 40.68 MHz) is used in industry for a variety of heating processes. Applications include plastic sealing and embossing, glue drying, fabric and textile processing, woodworking and the manufacture of such diverse products as tarpaulins, swimming pools, waterbed liners, shoes, travel check folders and so on.

Measurements reported in the literature (Hansson Mild 1980; IEEE COMAR 1990a, 1990b, 1991) show that in many cases, electric and magnetic leakage fields are very high near these RF devices. Often the operators are women of child-bearing age (that is, 18 to 40 years). The leakage fields are often extensive in some occupational situations, resulting in whole-body exposure of operators. For many devices, the electric and magnetic field exposure levels exceed all existing RF safety guidelines.

Since these devices may give rise to very high absorption of RF energy, it is of interest to control the leakage fields which emanate from them. Thus, periodic RF monitoring becomes essential to determine whether an exposure problem exists.

Communication systems

Workers in the fields of communication and radar are exposed only to low-level field strengths in most situations. However, the exposure of workers who must climb FM/TV towers can be intense and safety precautions are necessary. Exposure can also be substantial near transmitter cabinets that have their interlocks defeated and doors open.

Medical exposure

One of the earliest applications of RF energy was short-wave diathermy. Unshielded electrodes are usually used for this, leading possibly to high stray fields.

Recently RF fields have been used in conjunction with static magnetic fields in magnetic resonance imaging (MRI). Since the RF energy used is low and the field is almost fully contained within the patient enclosure, the exposure to operators is negligible.

Biological Effects

The specific absorption rate (SAR, measured in watts per kilogram) is widely used as a dosimetric quantity, and exposure limits can be derived from SARs. The SAR of a biological body depends upon such exposure parameters as frequency of the radiation, intensity, polarization, configuration of the radiation source and the body, reflection surfaces and body size, shape and electrical properties. Furthermore, the SAR spatial distribution inside the body is highly non-uniform. Non-uniform energy deposition results in non-uniform deep-body heating and may produce internal temperature gradients. At frequencies above 10 GHz, the energy is deposited close to the body surface. The maximum SAR occurs at about 70 MHz for the standard subject, and at about 30 MHz when the person is standing in contact with RF ground. At extreme conditions of temperature and humidity, whole-body SARs of 1 to 4 W/kg at 70 MHz are expected to cause a core temperature rise of about 2 ºC in healthy human beings in one hour.

RF heating is an interaction mechanism that has been studied extensively. Thermal effects have been observed at less than 1 W/kg, but temperature thresholds have generally not been determined for these effects. The time-temperature profile must be considered in assessing biological effects.

Biological effects also occur where RF heating is neither an adequate nor a possible mechanism. These effects often involve modulated RF fields and millimetre wavelengths. Various hypotheses have been proposed but have not yet yielded information useful for deriving human exposure limits. There is a need to understand the fundamental mechanisms of interaction, since it is not practical to explore each RF field for its characteristic biophysical and biological interactions.

Human and animal studies indicate that RF fields can cause harmful biological effects because of excessive heating of internal tissues. The body’s heat sensors are located in the skin and do not readily sense heating deep within the body. Workers may therefore absorb significant amounts of RF energy without being immediately aware of the presence of leakage fields. There have been reports that personnel exposed to RF fields from radar equipment, RF heaters and sealers, and radio-TV towers have experienced a warming sensation some time after being exposed.

There is little evidence that RF radiation can initiate cancer in humans. Nevertheless, a study has suggested that it may act as a cancer promoter in animals (Szmigielski et al. 1988). Epidemiological studies of personnel exposed to RF fields are few in number and are generally limited in scope (Silverman 1990; NCRP 1986; WHO 1981). Several surveys of occupationally exposed workers have been conducted in the former Soviet Union and Eastern European countries (Roberts and Michaelson 1985). However, these studies are not conclusive with respect to health effects.

Human assessment and epidemiological studies on RF sealer operators in Europe (Kolmodin-Hedman et al. 1988; Bini et al. 1986) report that the following specific problems may arise:

• RF burns or burns from contact with thermally hot surfaces

• numbness (i.e., paresthesia) in hands and fingers; disturbed or altered tactile sensitivity

• eye irritation (possibly due to fumes from vinyl-containing material)

• significant warming and discomfort of the legs of operators (perhaps due to current flow through legs to ground).

Mobile Phones

The use of personal radiotelephones is rapidly increasing and this has led to an increase in the number of base stations. These are often sited in public areas. However, the exposure to the public from these stations is low. The systems usually operate on frequencies near 900 MHz or 1.8 GHz using either analogue or digital technology. The handsets are small, low power radio transmitters that are held in close proximity to the head when in use. Some of the power radiated from the antenna is absorbed by the head. Numerical calculations and measurements in phantom heads show that the SAR values can be of the order of a few W/kg (see further ICNIRP statement, 1996). Public concern about the health hazard of the electromagnetic fields has increased and several research programmes are being devoted to this question (McKinley et al., unpublished report). Several epidemiological studies are ongoing with respect to mobile phone use and brain cancer. So far only one animal study (Repacholi et al. 1997) with transgenic mice exposed 1 h per day for 18 months to a signal similar to that used in digital mobile communication has been published. By the end of the experiments 43 of 101 exposed animals had lymphomas, compared to 22 of 100 in the sham-exposed group. The increase was statistically significant (p > 0.001). These results cannot easily be interpreted with relevance to human health and further research on this is needed.

Standards and Guidelines

Several organizations and governments have issued standards and guidelines for protection from excessive exposure to RF fields. A review of worldwide safety standards was given by Grandolfo and Hansson Mild (1989); the discussion here pertains only to the guidelines issued by IRPA (1988) and IEEE standard C 95.1 1991.

The full rationale for RF exposure limits is presented in IRPA (1988). In summary, the IRPA guidelines have adopted a basic limiting SAR value of 4 W/kg, above which there is considered to be an increasing likelihood that adverse health consequences can occur as a result of RF energy absorption. No adverse health effects have been observed due to acute exposures below this level. Incorporating a safety factor of ten to allow for possible consequences of long-term exposure, 0.4 W/kg is used as the basic limit for deriving exposure limits for occupational exposure. A further safety factor of five is incorporated to derive limits for the general public.

Derived exposure limits for the electric field strength (E), the magnetic field strength (H) and the power density specified in V/m, A/m and W/m² respectively, are shown in figure 1. The squares of the E and H fields are averaged over six minutes, and it is recommended that the instantaneous exposure not exceed the time-averaged values by more than a factor of 100. Furthermore, the body-to-ground current should not exceed 200 mA.

Figure 1. IRPA (1988) exposure limits for electric field strength E, magnetic field strength H and power density

Standard C 95.1, set in 1991, by the IEEE gives limiting values for occupational exposure (controlled environment) of 0.4 W/kg for the average SAR over a person’s entire body, and 8 W/kg for the peak SAR delivered to any one gram of tissue for 6 minutes or more. The corresponding values for exposure to the general public (uncontrolled environment) are 0.08 W/kg for whole-body SAR and 1.6 W/kg for peak SAR. The body-to-ground current should not exceed 100 mA in a controlled environment and 45 mA in an uncontrolled environment. (See IEEE 1991 for further details.) The derived limits are shown in figure 2.

Figure 2. IEEE (1991) exposure limits for electric field strength E, magnetic field strength H and power density

Further information on radiofrequency fields and microwaves can be found in, for instance, Elder et al. 1989, Greene 1992, and Polk and Postow 1986.

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A laser is a device which produces coherent electromagnetic radiant energy within the optical spectrum from the extreme ultraviolet to the far infrared (submillimetre). The term laser is actually an acronym for light amplification by stimulated emission of radiation. Although the laser process was theoretically predicted by Albert Einstein in 1916, the first successful laser was not demonstrated until 1960. In recent years lasers have found their way from the research laboratory to the industrial, medical and office setting as well as construction sites and even households. In many applications, such as videodisk players and optical fibre communication systems, the laser’s radiant energy output is enclosed, the user faces no health risk, and the presence of a laser embedded in the product may not be obvious to the user. However, in some medical, industrial or research applications, the laser’s emitted radiant energy is accessible and may pose a potential hazard to the eye and skin.

Because the laser process (sometimes referred to as “lasing”) can produce a highly collimated beam of optical radiation (i.e., ultraviolet, visible or infrared radiant energy), a laser can pose a hazard at a considerable distance—quite unlike most hazards encountered in the workplace. Perhaps it is this characteristic more than anything else that has led to special concerns expressed by workers and by occupational health and safety experts. Nevertheless, lasers can be used safely when appropriate hazard controls are applied. Standards for the safe use of lasers exist worldwide, and most are “harmonized” with each other (ANSI 1993; IEC 1993). All of the standards make use of a hazard classification system, which groups laser products into one of four broad hazard classes according to the laser’s output power or energy and its ability to cause harm. Safety measures are then applied commensurate to the hazard classification (Cleuet and Mayer 1980; Duchene, Lakey and Repacholi 1991).

Lasers operate at discrete wavelengths, and although most lasers are monochromatic (emitting one wavelength, or single colour), it is not uncommon for a laser to emit several discrete wavelengths. For example, the argon laser emits several different lines within the near ultraviolet and visible spectrum, but is generally designed to emit only one green line (wavelength) at 514.5 nm and/or a blue line at 488 nm. When considering potential health hazards, it is always crucial to establish the output wavelength(s).

All lasers have three fundamental building blocks:

1. an active medium (a solid, liquid or gas) that defines the possible emission wavelengths
2. an energy source (e.g., electric current, pump lamp or chemical reaction)
3. a resonant cavity with output coupler (generally two mirrors).

Most practical laser systems outside of the research laboratory also have a beam delivery system, such as an optical fibre or articulated arm with mirrors to direct the beam to a work station, and focusing lenses to concentrate the beam on a material to be welded, etc. In a laser, identical atoms or molecules are brought to an excited state by energy delivered from the pump lamp. When the atoms or molecules are in an excited state, a photon (“particle” of light energy) can stimulate an excited atom or molecule to emit a second photon of the same energy (wavelength) travelling in phase (coherent) and in the same direction as the stimulating photon. Thus light amplification by a factor of two has taken place. This same process repeated in a cascade causes a light beam to develop that reflects back and forth between the mirrors of the resonant cavity. Since one of the mirrors is partially transparent, some light energy leaves the resonant cavity forming the emitted laser beam. Although in practice, the two parallel mirrors are often curved to produce a more stable resonant condition, the basic principle holds for all lasers.

Although several thousand different laser lines (i.e., discrete laser wavelengths characteristic of different active media) have been demonstrated in the physics laboratory, only 20 or so have been developed commercially to the point where they are routinely applied in everyday technology. Laser safety guidelines and standards have been developed and published which basically cover all wavelengths of the optical spectrum in order to allow for currently known laser lines and future lasers.

Laser Hazard Classification

Current laser safety standards throughout the world follow the practice of categorizing all laser products into hazard classes. Generally, the scheme follows a grouping of four broad hazard classes, 1 through 4. Class 1 lasers cannot emit potentially hazardous laser radiation and pose no health hazard. Classes 2 through 4 pose an increasing hazard to the eye and skin. The classification system is useful since safety measures are prescribed for each class of laser. More stringent safety measures are required for the highest classes.

Class 1 is considered an “eye-safe”, no-risk grouping. Most lasers that are totally enclosed (for example, laser compact disc recorders) are Class 1. No safety measures are required for a Class 1 laser.

Class 2 refers to visible lasers that emit a very low power that would not be hazardous even if the entire beam power entered the human eye and was focused on the retina. The eye’s natural aversion response to viewing very bright light sources protects the eye against retinal injury if the energy entering the eye is insufficient to damage the retina within the aversion response. The aversion response is composed of the blink reflex (approximately 0.16–0.18 second) and a rotation of the eye and movement of the head when exposed to such bright light. Current safety standards conservatively define the aversion response as lasting 0.25 second. Thus, Class 2 lasers have an output power of 1 milliwatt (mW) or less that corresponds to the permissible exposure limit for 0.25 second. Examples of Class 2 lasers are laser pointers and some alignment lasers.

Some safety standards also incorporate a subcategory of Class 2, referred to as “Class 2A”. Class 2A lasers are not hazardous to stare into for up to 1,000 s (16.7 min). Most laser scanners used in point-of-sales (super-market checkout) and inventory scanners are Class 2A.

Class 3 lasers pose a hazard to the eye, since the aversion response is insufficiently fast to limit retinal exposure to a momentarily safe level, and damage to other structures of the eye (e.g., cornea and lens) could also take place. Skin hazards normally do not exist for incidental exposure. Examples of Class 3 lasers are many research lasers and military laser rangefinders.

A special subcategory of Class 3 is termed “Class 3A” (with the remaining Class 3 lasers termed “Class 3B”). Class 3A lasers are those with an output power between one and five times the accessible emission limits (AEL) for the Class 1 or Class 2, but with an output irradiance not exceeding the relevant occupational exposure limit for the lower class. Examples are many laser alignment and surveying instruments.

Class 4 lasers may pose a potential fire hazard, a significant skin hazard or a diffuse-reflection hazard. Virtually all surgical lasers and material processing lasers used for welding and cutting are Class 4 if not enclosed. All lasers with an average power output exceeding 0.5 W are Class 4. If a higher power Class 3 or Class 4 is totally enclosed so that hazardous radiant energy is not accessible, the total laser system could be Class 1. The more hazardous laser inside the enclosure is termed an embedded laser.

Occupational Exposure Limits

The International Commission on Non-Ionizing Radiation Protection (ICNIRP 1995) has published guidelines for human exposure limits for laser radiation that are periodically updated. Representative exposure limits (ELs) are provided in table 1 for several typical lasers. Virtually all laser beams exceed permissible exposure limits. Thus, in actual practice, the exposure limits are not routinely used to determine safety measures. Instead, the laser classification scheme—which is based upon the ELs applied under realistic conditions—is really applied to this end.

Table 1. Exposure limits for typical lasers

All standards/guidelines have MPE’s at other wavelengths and exposure durations.

Note: To convert MPE’s in mW/cm² to mJ/cm², multiply by exposure time t in seconds. For example, the He-Ne or Argon MPE at 0.1 s is 0.32 mJ/cm².

Source: ANSI Standard Z-136.1(1993); ACGIH TLVs (1995) and Duchene, Lakey and Repacholi (1991).

Laser Safety Standards

Many nations have published laser safety standards, and most are harmonized with the international standard of the International Electrotechnical Commission (IEC). IEC Standard 825-1 (1993) applies to manufacturers; however, it also provides some limited safety guidance for users. The laser hazard classification described above must be labelled on all commercial laser products. A warning label appropriate to the class should appear on all products of Classes 2 through 4.

Safety Measures

The laser safety classification system greatly facilitates the determination of appropriate safety measures. Laser safety standards and codes of practice routinely require the use of increasingly more restrictive control measures for each higher classification.

In practice, it is always more desirable to totally enclose the laser and beam path so that no potentially hazardous laser radiation is accessible. In other words, if only Class 1 laser products are employed in the workplace, safe use is assured. However, in many situations, this is simply not practical, and worker training in safe use and hazard control measures is required.

Other than the obvious rule—not to point a laser at a person’s eyes—there are no control measures required for a Class 2 laser product. For lasers of higher classes, safety measures are clearly required.

If total enclosure of a Class 3 or 4 laser is not feasible, the use of beam enclosures (e.g., tubes), baffles and optical covers can virtually eliminate the risk of hazardous ocular exposure in most cases.

When enclosures are not feasible for Class 3 and 4 lasers, a laser controlled area with controlled entry should be established, and the use of laser eye protectors is generally mandated within the nominal hazard zone (NHZ) of the laser beam. Although in most research laboratories where collimated laser beams are used, the NHZ encompasses the entire controlled laboratory area, for focused beam applications, the NHZ may be surprisingly limited and not encompass the entire room.

To assure against misuse and possible dangerous actions on the part of unauthorized laser users, the key control found on all commercially manufactured laser products should be utilized.

The key should be secured when the laser is not in use, if people can gain access to the laser.

Special precautions are required during laser alignment and initial set-up, since the potential for serious eye injury is very great then. Laser workers must be trained in safe practices prior to laser set-up and alignment.

Laser-protective eyewear was developed after occupational exposure limits had been established, and specifications were drawn up to provide the optical densities (or ODs, a logarithmic measure of the attenuation factor) that would be needed as a function of wavelength and exposure duration for specific lasers. Although specific standards for laser eye protection exist in Europe, further guidelines are provided in the United States by the American National Standards Institute under the designations ANSI Z136.1 and ANSI Z136.3.

Training

When investigating laser accidents in both laboratory and industrial situations, a common element emerges: lack of adequate training. Laser safety training should be both appropriate and sufficient for the laser operations around which each employee will work. Training should be specific to the type of laser and the task to which the worker is assigned.

Medical Surveillance

Requirements for medical surveillance of laser workers vary from country to country in accordance with local occupational medicine regulations. At one time, when lasers were confined to the research laboratory and little was known about their biological effects, it was quite typical that each laser worker was periodically given a thorough general ophthalmological examination with fundus (retinal) photography to monitor the status of the eye. However, by the early 1970s, this practice was questioned, since the clinical findings were almost always negative, and it became clear that such exams could identify only acute injury which was subjectively detectable. This led the WHO task group on lasers, meeting in Don Leaghreigh, Ireland, in 1975, to recommend against such involved surveillance programmes and to emphasize testing of visual function. Since that time, most national occupational health groups have continuously reduced medical examination requirements. Today, complete ophthalmological examinations are universally required only in the event of a laser eye injury or suspected overexposure, and pre-placement visual screening is generally required. Additional examinations may be required in some countries.

Laser Measurements

Unlike some workplace hazards, there is generally no need to perform measurements for workplace monitoring of hazardous levels of laser radiation. Because of the highly confined beam dimensions of most laser beams, the likelihood of changing beam paths and the difficulty and expense of laser radiometers, current safety standards emphasize control measures based upon hazard class and not workplace measurement (monitoring). Measurements must be performed by the manufacturer to assure compliance with laser safety standards and proper hazard classification. Indeed, one of the original justifications for laser hazard classification related to the great difficulty of performing proper measurements for hazard evaluation.

Conclusions

Although the laser is relatively new to the workplace, it is rapidly becoming ubiquitous, as are programmes concerned with laser safety. The keys to the safe use of lasers are first to enclose the laser radiant energy if at all possible, but if not possible, to set up adequate control measures and to train all personnel working with lasers.

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Light and infrared (IR) radiant energy are two forms of optical radiation, and together with ultraviolet radiation, they form the optical spectrum. Within the optical spectrum, different wavelengths have considerably different potentials for causing biological effects, and for this reason the optical spectrum may be further subdivided.

The term light should be reserved for wavelengths of radiant energy between 400 and 760 nm, which evoke a visual response at the retina (CIE 1987). Light is the essential component of the output of illuminating lamps, visual displays and a wide variety of illuminators. Aside from the importance of illumination for seeing, some light sources may, however, pose unwanted physiological reactions such as disability and discomfort glare, flicker and other forms of eye stress due to poor ergonomic design of workplace tasks. The emission of intense light is also a potentially hazardous side-effect of some industrial processes, such as arc welding.

Infrared radiation (IRR, wavelengths 760 nm to 1 mm) may also be referred to quite commonly as thermal radiation (or radiant heat), and is emitted from any warm object (hot engines, molten metals and other foundry sources, heat-treated surfaces, incandescent electric lamps, radiant heating systems, etc.). Infrared radiation is also emitted from a large variety of electrical equipment such as electric motors, generators, transformers and various electronic equipment.

Infrared radiation is a contributory factor in heat stress. High ambient air temperature and humidity and a low degree of air circulation can combine with radiant heat to produce heat stress with the potential for heat injuries. In cooler environments, unwelcome or poorly designed sources of radiant heat can also produce discomfort—an ergonomic consideration.

Biological Effects

Occupational hazards presented to the eye and skin by visible and infrared forms of radiation are limited by the eye’s aversion to bright light and the pain sensation in the skin resulting from intense radiant heating. The eye is well-adapted to protect itself against acute optical radiation injury (due to ultraviolet, visible or infrared radiant energy) from ambient sunlight. It is protected by a natural aversion response to viewing bright light sources that normally protects it against injury arising from exposure to sources such as the sun, arc lamps and welding arcs, since this aversion limits the duration of exposure to a fraction (about two-tenths) of a second. However, sources rich in IRR without a strong visual stimulus can be hazardous to the lens of the eye in the case of chronic exposure. One can also force oneself to stare at the sun, a welding arc or a snow field and thereby suffer a temporary (and sometimes a permanent) loss of vision. In an industrial setting in which bright lights appear low in the field of view, the eye’s protective mechanisms are less effective, and hazard precautions are particularly important.

There are at least five separate types of hazards to the eye and skin from intense light and IRR sources, and protective measures must be chosen with an understanding of each. In addition to the potential hazards presented by ultraviolet radiation (UVR) from some intense light sources, one should consider the following hazards (Sliney and Wolbarsht 1980; WHO 1982):

1. Thermal injury to the retina, which can occur at wavelengths from 400 nm to 1,400 nm. Normally the danger of this type of injury is posed only by lasers, a very intense xenon-arc source or a nuclear fireball. The local burning of the retina results in a blind spot (scotoma).
2. Blue-light photochemical injury to the retina (a hazard principally associated with blue light of wavelengths from 400 nm to 550 nm) (Ham 1989). The injury is commonly termed “blue light” photoretinitis; a particular form of this injury is named, according to its source, solar retinitis. Solar retinitis was once referred to as “eclipse blindness” and associated “retinal burn”. Only in recent years has it become clear that photoretinitis results from a photochemical injury mechanism following exposure of the retina to shorter wavelengths in the visible spectrum, namely, violet and blue light. Until the 1970s, it was thought to be the result of a thermal injury mechanism. In contrast to blue light, IRA radiation is very ineffective in producing retinal injuries. (Ham 1989; Sliney and Wolbarsht 1980).
3. Near-infrared thermal hazards to the lens (associated with wavelengths of approximately 800 nm to 3,000 nm) with potential for industrial heat cataract. The average corneal exposure to infrared radiation in sunlight is of the order of 10 W/m². By comparison, glass and steel workers exposed to infrared irradiances of the order of 0.8 to 4 kW/m² daily for 10 to 15 years have reportedly developed lenticular opacities (Sliney and Wolbarsht 1980). These spectral bands include IRA and IRB (see figure 1). The American Conference of Governmental Industrial Hygienists (ACGIH) guideline for IRA exposure of the anterior of the eye is a time-weighted total irradiance of 100 W/m² for exposure durations exceeding 1,000 s (16.7 min) (ACGIH 1992 and 1995).
4. Thermal injury of the cornea and conjunctiva (at wavelengths of approximately 1,400 nm to 1 mm). This type of injury is almost exclusively limited to exposure to laser radiation.
5. Thermal injury of the skin. This is rare from conventional sources but can occur across the entire optical spectrum.

The importance of wavelength and time of exposure

Thermal injuries (1) and (4) above are generally limited to very brief exposure durations, and eye protection is designed to prevent these acute injuries. However, photochemical injuries, such as are mentioned in (2) above, can result from low dose rates spread over the entire workday. The product of the dose rate and the exposure duration always results in the dose (it is the dose that governs the degree of photochemical hazard). As with any photochemical injury mechanism, one must consider the action spectrum which describes the relative effectiveness of different wavelengths in causing a photobiological effect. For example, the action spectrum for photochemical retinal injury peaks at approximately 440 nm (Ham 1989). Most photochemical effects are limited to a very narrow range of wavelengths; whereas a thermal effect can occur at any wavelength in the spectrum. Hence, eye protection for these specific effects need block only a relatively narrow spectral band in order to be effective. Normally, more than one spectral band must be filtered in eye protection for a broad-band source.

Sources of Optical Radiation

Sunlight

The greatest occupational exposure to optical radiation results from exposure of outdoor workers to the sun’s rays. The solar spectrum extends from the stratospheric ozone-layer cut-off of about of 290-295 nm in the ultraviolet band to at least 5,000 nm (5 μm) in the infrared band. Solar radiation can attain a level as high as 1 kW/m² during the summer months. It can result in heat stress, depending upon ambient air temperature and humidity.

Artificial sources

The most significant artificial sources of human exposure to optical radiation include the following:

1. Welding and cutting. Welders and their co-workers are typically exposed not only to intense UV radiation, but also to intense visible and IR radiation emitted from the arc. Under rare instances, these sources have produced acute injury to the retina of the eye. Eye protection is mandatory for these environments.
2. Metals industries and foundries. The most significant source of visible and infrared exposure are from molten and hot metal surfaces in the steel and aluminium industries and in foundries. Worker exposure typically ranges from 0.5 to 1.2 kW/m².
3. Arc lamps. Many industrial and commercial processes, such as those involving photochemical curing lamps, emit intense, short-wave visible (blue) light as well as UV and IR radiation. While the likelihood of harmful exposure is low due to shielding, in some cases accidental exposure can occur.
4. Infrared lamps. These lamps emit predominantly in the IRA range and are generally used for heat treatment, paint drying and related applications. These lamps do not pose any significant exposure hazard to humans since the discomfort produced upon exposure will limit exposure to a safe level.
5. Medical treatment. Infrared lamps are used in physical medicine for a variety of diagnostic and therapeutic purposes. Exposures to the patient vary considerably according to the type of treatment, and IR lamps require careful use by staff members.
6. General lighting. Fluorescent lamps emit very little infrared and are generally not bright enough to pose a potential hazard to the eye. Tungsten and tungsten-halogen incandescent lamps emit a large fraction of their radiant energy in the infrared. Additionally, the blue light emitted by tungsten-halogen lamps can pose a retinal hazard if a person stares at the filament. Fortunately, the eye’s aversion response to bright light prevents acute injury even at short distances. Placing glass “heat” filters over these lamps should minimize/eliminate this hazard.
7. Optical projectors and other devices. Intense light sources are used in searchlights, film projectors and other light-beam collimating devices. These may pose a retinal hazard with the direct beam at very close distances.

Measurement of Source Properties

The most important characteristic of any optical source is its spectral power distribution. This is measured using a spectroradiometer, which consists of suitable input optics, a monochromator and a photodetector.

In many practical situations, a broad-band optical radiometer is used to select a given spectral region. For both visible illumination and safety purposes, the spectral response of the instrument will be tailored to follow a biological spectral response; for example, lux-meters are geared to the photopic (visual) response of the eye. Normally, aside from UVR hazard meters, the measurement and hazard analysis of intense light sources and infrared sources is too complex for routine occupational health and safety specialists. Progress is being made in standardizations of safety categories of lamps, so that measurements by the user will not be required in order to determine potential hazards.

Human Exposure Limits

From knowledge of the optical parameters of the human eye and the radiance of a light source, it is possible to calculate irradiances (dose rates) at the retina. Exposure of the anterior structures of the human eye to infrared radiation may also be of interest, and it should be further borne in mind that the relative position of the light source and the degree of lid closure can greatly affect the proper calculation of an ocular exposure dose. For ultraviolet and short-wavelength light exposures, the spectral distribution of the light source is also important.

A number of national and international groups have recommended occupational exposure limits (ELs) for optical radiation (ACGIH 1992 and 1994; Sliney 1992). Although most such groups have recommended ELs for UV and laser radiation, only one group has recommended ELs for visible radiation (i.e., light), namely, the ACGIH, an agency well-known in the field of occupational health. The ACGIH refers to its ELs as threshold limit values, or TLVs, and as these are issued yearly, there is an opportunity for a yearly revision (ACGIH 1992 and 1995). They are based in large part on ocular injury data from animal studies and from data from human retinal injuries resulting from viewing the sun and welding arcs. TLVs are furthermore based on the underlying assumption that outdoor environmental exposures to visible radiant energy are normally not hazardous to the eye except in very unusual environments, such as snow fields and deserts, or when one actually fixes the eyes on the sun.

Optical Radiation Safety Evaluation

Since a comprehensive hazard evaluation requires complex measurements of spectral irradiance and radiance of the source, and sometimes very specialized instruments and calculations as well, it is rarely carried out onsite by industrial hygienists and safety engineers. Instead, the eye protective equipment to be deployed is mandated by safety regulations in hazardous environments. Research studies evaluated a wide range of arcs, lasers and thermal sources in order to develop broad recommendations for practical, easier-to-apply safety standards.

Protective Measures

Occupational exposure to visible and IR radiation is seldom hazardous and is usually beneficial. However, some sources emit a considerable amount of visible radiation, and in this case, the natural aversion response is evoked, so there is little chance of accidental overexposure of the eyes. On the other hand, accidental exposure is quite likely in the case of artificial sources emitting only near-IR radiation. Measures which can be taken to minimize the unnecessary exposure of staff to IR radiation include proper engineering design of the optical system in use, wearing appropriate goggles or face visors, limiting access to persons directly concerned with the work, and ensuring that workers are aware of the potential hazards associated with exposure to intense visible and IR radiation sources. Maintainance staff who replace arc lamps must have adequate training so as to preclude hazardous exposure. It is unacceptable for workers to experience either skin erythema or photokeratitis. If these conditions do occur, working practices should be examined and steps taken to ensure that overexposure is made unlikely in the future. Pregnant operators are at no specific risk to optical radiation as regards the integrity of their pregnancy.

Eye protector design and standards

The design of eye protectors for welding and other operations presenting sources of industrial optical radiation (e.g., foundry work, steel and glass manufacture) started at the beginning of this century with the development of Crooke’s glass. Eye protector standards which evolved later followed the general principle that since infrared and ultraviolet radiation are not needed for vision, those spectral bands should be blocked as best as possible by currently available glass materials.

The empirical standards for eye protective equipment were tested in the 1970s and were shown to have included large safety factors for infrared and ultraviolet radiation when the transmission factors were tested against current occupational exposure limits, whereas the protection factors for blue light were just sufficient. Some standards’ requirements were therefore adjusted.

Ultraviolet and infrared radiation protection

A number of specialized UV lamps are used in industry for fluorescence detection and for photocuring of inks, plastic resins, dental polymers and so on. Although UVA sources normally pose little risk, these sources may either contain trace amounts of hazardous UVB or pose a disability glare problem (from fluorescence of the eye’s crystalline lens). UV filter lenses, glass or plastic, with very high attenuation factors are widely available to protect against the entire UV spectrum. A slight yellowish tint may be detectable if protection is afforded to 400 nm. It is of paramount importance for this type of eyewear (and for industrial sunglasses) to provide protection for the peripheral field of vision. Side shields or wraparound designs are important to protect against the focusing of temporal, oblique rays into the nasal equatorial area of the lens, where cortical cataract frequently originates.

Almost all glass and plastic lens materials block ultraviolet radiation below 300 nm and infrared radiation at wavelengths greater than 3,000 nm (3 μm), and for a few lasers and optical sources, ordinary impact-resistant clear safety eyewear will provide good protection (e.g., clear polycarbonate lenses effectively block wavelengths greater than 3 μm). However, absorbers such as metal oxides in glass or organic dyes in plastics must be added to eliminate UV up to about 380–400 nm, and infrared beyond 780 nm to 3 μm. Depending upon the material, this may be either easy or very difficult or expensive, and the stability of the absorber may vary somewhat. Filters that meet the American National Standards Institute’s ANSI Z87.1 standard must have the appropriate attenuation factors in each critical spectral band.

Protection in various industries

Fire-fighting

Fire-fighters may be exposed to intense near-infrared radiation, and aside from the crucially important head and face protection, IRR attenuating filters are frequently prescribed. Here, impact protection is also important.

Foundry and glass industry eyewear

Spectacles and goggles designed for ocular protection against infrared radiation generally have a light greenish tint, although the tint may be darker if some comfort against visible radiation is desired. Such eye protectors should not be confused with the blue lenses used with steel and foundry operations, where the objective is to check the temperature of the melt visually; these blue spectacles do not provide protection, and should be worn only briefly.

Welding

Infrared and ultraviolet filtration properties can be readily imparted to glass filters by means of additives such as iron oxide, but the degree of strictly visible attenuation determines the shade number, which is a logarithmic expression of attenuation. Normally a shade number of 3 to 4 is used for gas welding (which calls for goggles), and a shade number of 10 to 14 for arc welding and plasma arc operations (here, helmet protection is required). The rule of thumb is that if the welder finds the arc comfortable to view, adequate attenuation is provided against ocular hazards. Supervisors, welder’s helpers and other persons in the work area may require filters with a relatively low shade number (e.g., 3 to 4) to protect against photokeratitis (“arc eye” or “welder’s flash”). In recent years a new type of welding filter, the autodarkening filter has appeared on the scene. Regardless of the type of filter, it should meet ANSI Z87.1 and Z49.1 standards for fixed welding filters specified for dark shade (Buhr and Sutter 1989; CIE 1987).

Autodarkening welding filters

The autodarkening welding filter, whose shade number increases with the intensity of the optical radiation impinging upon it, represents an important advance in the ability of welders to produce consistently high-quality welds more efficiently and ergonomically. Formerly, the welder had to lower and raise the helmet or filter each time an arc was started and quenched. The welder had to work “blind” just prior to striking the arc. Furthermore, the helmet is commonly lowered and raised with a sharp snap of the neck and head, which can lead to neck strain or more serious injuries. Faced with this uncomfortable and cumbersome procedure, some welders frequently initiate the arc with a conventional helmet in the raised position—leading to photokeratitis. Under normal ambient lighting conditions, a welder wearing a helmet fitted with an autodarkening filter can see well enough with the eye protection in place to perform tasks such as aligning the parts to be welded, precisely positioning the welding equipment and striking the arc. In the most typical helmet designs, light sensors then detect the arc flash virtually as soon as it appears and direct an electronic drive unit to switch a liquid crystal filter from a light shade to a preselected dark shade, eliminating the need for the clumsy and hazardous manoeuvres practised with fixed-shade filters.

The question has frequently been raised whether hidden safety problems may develop with autodarkening filters. For example, can afterimages (“flash blindness”) experienced in the workplace result in permanently impaired vision? Do the new types of filter really offer a degree of protection that is equivalent or better than that which conventional fixed filters can provide? Although one can answer the second question in the affirmative, it must be understood that not all autodarkening filters are equivalent. Filter reaction speeds, the values of the light and dark shades achieved under a given intensity of illumination, and the weight of each unit may vary from one pattern of equipment to another. The temperature dependence of the unit’s performance, the variation in the degree of shade with electrical battery degradation, the “resting state shade” and other technical factors vary depending upon each manufacturer’s design. These considerations are being addressed in new standards.

Since adequate filter attenuation is afforded by all systems, the single most important attribute specified by the manufacturers of autodarkening filters is the speed of filter switching. Current autodarkening filters vary in switching speed from one tenth of a second to faster than 1/10,000th of a second. Buhr and Sutter (1989) have indicated a means of specifying the maximum switching time, but their formulation varies relative to the time-course of switching. Switching speed is crucial, since it gives the best clue to the all-important (but unspecified) measure of how much light will enter the eye when the arc is struck as compared with the light admitted by a fixed filter of the same working shade number. If too much light enters the eye for each switching during the day, the accumulated light-energy dose produces “transient adaptation” and complaints about “eye strain” and other problems. (Transient adaptation is the visual experience caused by sudden changes in one’s light environment, which may be characterized by discomfort, a sensation of having been exposed to glare and temporary loss of detailed vision.) Current products with switching speeds of the order of ten milliseconds will better provide adequate protection against photoretinitis. However, the shortest switching time—of the order of 0.1 ms—has the advantage of reducing transient adaptation effects (Eriksen 1985; Sliney 1992).

Simple check tests are available to the welder short of extensive laboratory testing. One might suggest to the welder that he or she simply look at a page of detailed print through a number of autodarkening filters. This will give an indication of each filter’s optical quality. Next, the welder may be asked to try striking an arc while observing it through each filter being considered for purchase. Fortunately, one can rely on the fact that light levels which are comfortable for viewing purposes will not be hazardous. The effectiveness of UV and IR filtration should be checked in the manufacturer’s specification sheet to make sure that unnecessary bands are filtered out. A few repeated arc strikings should give the welder a sense of whether discomfort will be experienced from transient adaptation, although a one-day trial would be best.

The resting or failure state shade number of an autodarkening filter (a failure state occurs when the battery fails) should provide 100% protection for the welder’s eyes for at least one to several seconds. Some manufacturers use a dark state as the “off” position and others use an intermediate shade between the dark and the light shade states. In either case, the resting state transmittance for the filter should be appreciably lower than the light shade transmittance in order to preclude a retinal hazard. In any case, the device should provide a clear and obvious indicator to the user as to when the filter is switched off or when a system failure occurs. This will ensure that the welder is warned in advance in case the filter is not switched on or is not operating properly before welding is begun. Other features, such as battery life or performance under extreme temperature conditions may be of importance to certain users.

Conclusions

Although technical specifications can appear to be somewhat complex for devices that protect the eye from optical radiation sources, safety standards exist which specify shade numbers, and these standards provide a conservative safety factor for the wearer.

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