The level of risk for adverse health effects to the skin and eyes from exposure to UV light is determined by the UV light wavelengths present, UV light irradiance values and personal exposure time.
This necessitates the strict implementation of exposure limit values, which take into account the effectiveness of different UV light wavelengths in eliciting biological effects, in order to protect against overexposure to UV light in the workplace.
The UV light exposure limit values (ELVs) are based on studies of thresholds for acute effects and derived from statistical consideration. It is generally the case that acute effects will only occur if UV light exposure exceeds a threshold level, which will usually vary from person to person. Therefore, exceeding an exposure limit value will not necessarily result in an adverse health effect.
The risk of an adverse health effect will increase as personal exposure levels increase above the exposure limit value. The majority of effects covered in the previous chapter will occur in the healthy adult working population at levels substantially above the exposure limit values defined later in this chapter.
However, we also know that persons who are abnormally photosensitive or those taking or in contact with certain drugs or chemicals may suffer adverse health effects at levels below the exposure limit values.
Chronic effects are not attributed to a threshold below which they will not occur and therefore the risk of occurrence cannot be reduced to zero. The risk will be reduced by reducing personal exposure. Observing the exposure limit values for exposure to artificial sources of UV light should reduce the risk to levels at which society has accepted with respect to exposure to naturally occurring UV light.
The Control of Artificial Optical Radiation at Work Regulations 2010 and the Optical Radiation Directive is based on exposure limit values defined by the International Commission on Non-Ionising Radiation Protection (ICNIRP).
The exposure limit values apply to workers and wherever possible to members of the public, exposed to artificial UV light sources, where UV light irradiance values and exposure durations can be controlled. This includes not only processes involving UV light, but also exposure to other artificial sources, including general lighting, which may contribute to an individuals total UV light exposure.
UV light exposure limits are applicable for personal exposure to incoherent UV light sources such as arcs, gas and vapour discharges, fluorescent tubes, incandescent lamps and LEDs. Most of these UV light sources will be broadband, however single emission lines can be produced from low-pressure gas discharges. Broadband UV light sources consist of either a number of monochromatic lines or a continuous spectral distribution, often with superimposed lines.
The exposure limits should be applied to UV light radiant exposure, measured with an instrument having cosine angular response and oriented perpendicular to the most directly exposed surfaces of the body when assessing skin exposure and along (or parallel to) the line(s) of sight of each exposed individual when assessing ocular exposure.
The exposure limit values do not apply to UV light sources, but to people.
The question is not whether a UV light source exceeds the exposure limits, but whether exposure to a UV light source could result in a person exceeding the exposure limits.
In cases of persons subjected to UV light exposure from artificial sources – it is necessary to assess the level of risk for adverse health effects by determining personal UV light exposure levels and comparing with the exposure limit values. This will determine whether the existing control measures are adequate or whether more needs to be done to control the risk.
UV light exposure limit values within an 8 hour period
The exposure limit values are derived from the ICNIRP guidelines which are based on maximum permissible UV light exposure values for an 8-hour working day and take into account the normal 24 hour light/dark cycle where cellular repair can take place after exposure is discontinued.
Therefore, in cases where continuous exposure for longer than 8 hours is possible, for example, 10-12 hour extended shifts or even double shifts, special care needs to be taken.
There are 2 UV light exposure limit values depending on the wavelength or wavelength range of the UV lamp. It is, therefore, necessary to know the UV light wavelengths present in order to select the applicable exposure limit value. It is important to understand that in some cases both exposure limit values will apply and in these cases, it is necessary to comply with both, which is achieved by adopting the most restrictive exposure limit for the eyes.
Exposure Limit Values for the Skin and Eye
Where UV light in the spectral region 180 nm – 400 nm (UV-A, UV-B and UV-C) is incident upon the unprotected skin and eye…
… for broadband UV light sources the UV light effective radiant exposure (Heffmax) must not exceed 30 J/m2 eff within an 8 hour period per day.
Exposure Limit Value for the eye
Where UV light in the spectral region 315 nm – 400 nm (UV-A) is incident upon the unprotected eyes …
… the UV-A light radiant exposure (HUV-Amax) must not exceed 10,000 J/m2 (unweighted) within an 8 hour period per day.
The maximum permissible UV light radiant exposure values are derived from an envelope action spectrum combining the photokeratitis and skin erythema action spectra, which provide the UV light radiant exposure required to produce these biological effects as a function of wavelength, i.e. the relative effectiveness of different wavelengths in producing these biological effects. They are set below the energies required for the development of these observable effects. In addition, they take account of the risk of producing lenticular opacities (cataractogenesis).
The result is a smooth curve illustrating a maximum sensitivity at the wavelength 270 nm, i.e. UV light at this wavelength is the most effective in eliciting erythema and photokeratitis and is allocated the most restrictive maximum permissible exposure value of 30 J/m2 eff.
Therefore, by definition, the effectiveness of UV light at this wavelength of 270 nm is given the value 1. The effectiveness at other wavelengths is then related to this value and derive the spectral weighting function (Sλ) for each wavelength.
For broadband UV light sources …
… the maximum permissible UV light effective radiant exposure value (Heffmax) of 30 J/m2 eff takes into account the variations in the effectiveness of different UV light wavelengths in causing biological hazardous effects such as erythema, photo conjunctivitis and photokeratitis. This is necessary because some UV light wavelengths have a very significant effect, others a proportionally less effect and some almost none at all, depending on the effect in question. The result is a measurement which is directly proportional to the biological hazardous effects.
… the maximum permissible UV-A light radiant exposure value (HUV-Amax) of 10,000 J/m2 (unweighted) is in addition to the maximum permissible UV light effective radiant exposure value (Heffmax) of 30 J/m2 eff.
It is necessary that compliance is achieved with both exposure limit values. This is achieved by adopting the most restrictive exposure limit for the eyes.
The UV light exposure limit values are intended to preclude any significant acute photobiological effects and reduce the risks for chronic effects (delayed effects) as much as possible by limiting life-long UV exposure. They are based upon the best available evidence, set below threshold levels of UV light exposure where observable adverse health effects would occur and incorporate significant safety margins.
The safety margins for UV-A are greater than for UV-B and UV-C. The margin of safety for acute adverse effects on the skin varies from 3 to 20 for lightly pigmented skin types, depending on wavelength. On the basis of acute adverse effects on the eyes, the safety factor for UV-A exposure varies from around 7 at 320 nm to more than 100 at 390 nm.
The UV light exposure limit values do not attempt to set a dividing line between ‘safe’ and ‘dangerous’ levels of UV light. What they do is define a level of UV light exposure, below which it is believed that nearly all individuals may be repeatedly exposed without adverse health effects. Some people may be unusually photosensitive or may be exposed to photosensitising agents, in which case these exposure limit values may not provide adequate protection.