Patty's Industrial Hygiene, Physical and Biological Agents. Группа авторов
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Название: Patty's Industrial Hygiene, Physical and Biological Agents
Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Жанр: Химия
isbn: 9781119816225
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Because the biological effects of exposure to optical radiation depend on the wavelength, assessment of broadband optical radiation hazards must take into account both the spectral distribution of the radiation received and the biological action spectra for the effects of interest. Exposure criteria have been developed for the assessment of the potential for some adverse health effects associated with various portions of the optical radiation spectrum.
4.1.1 UV Hazards to Skin and Eye
Guidelines for exposure to UV radiation have been developed by the American Conference of Governmental Industrial Hygienists (ACGIH) (15), the International Commission on Non‐Ionizing Radiation Protection (ICNIRP) (16), and the CIE (17).
The ACGIH and ICNIRP guidelines for broadband UV radiation between 180 and 400 nm recommend that the radiant exposure to the unprotected skin or eye, spectrally weighted by a defined relative spectral effectiveness function S(λ), be limited to 30 J m−2 (3 mJ cm−2) in an eight hour period. A plot of S(λ) is provided in Figure 7. This function is sometimes referred to as the “actinic hazard” function. The guidelines define an effective irradiance Eeff as
where Eλ is the spectral irradiance measured in W m−2 nm−1 and Δλ is the bandwidth in nanometers of the spectral interval. The maximum permissible exposure time at a given effective irradiance may be calculated by dividing 30 J m−2 by the effective irradiance. (See Section 5.1, Eq. (24).) This guideline is based on the principle that tissue damage due to the photochemical effects of UV radiation depends on the cumulative dose (that is, the radiant exposure), which is equal to the effective irradiance integrated over the exposure time.
FIGURE 7 Relative spectral effectiveness function S(λ) used for spectral weighting of incident UV radiation in the ACGIH and ICNIRP exposure guidelines (13, 14). S(λ) is dimensionless.
Source: From Ref. (15). Reprinted with permission of ACGIH.
The relative spectral effectiveness function S(λ) was derived from an action spectrum, measured in terms of radiant exposure, that was drawn to form an envelope around experimentally determined threshold doses for minimal erythema and barely detectable photokeratitis (14, 16). This action spectrum defines the harmonized ACGIH Threshold Limit Values (TLV®) and ICNIRP exposure limits for monochromatic radiation between 180 and 400 nm. S(λ) represents the ratio between the TLV for radiant exposure to monochromatic radiation at 270 nm (30 J m−2) and the TLV for radiant exposure to monochromatic radiation at wavelength λ. The TLV and S(λ) are thus inversely proportional to each other. The ACGIH/ICNIRP exposure limits include a safety factor that may be as low as 1.5–2.0 for acute photokeratitis at some wavelengths (16). The safety factor for erythema is higher because this effect has higher dose thresholds than photokeratitis (18). Dark pigmentation may confer additional protection against erythema; on the other hand, photosensitive individuals and individuals simultaneously exposed to photosensitizing chemicals may suffer erythema at effective doses that are below the ACGIH/ICNIRP exposure limits.
CIE has established an erythema reference action spectrum defined by a spectral weighting function Ser(λ) (17). The erythemal effective irradiance Eer can be expressed by Eq. (11).
The erythemal effective radiant exposure is the integral of Eer over time. CIE has also defined the standard erythema dose (SED) as an erythemal effective radiant exposure equal to 100 J m−2 (17). Unlike the TLV, the SED is intended to be used as a reference dose, not as an exposure limit. Nevertheless, it may be of interest to compare the TLV at wavelength λ with the erythemal effective radiant exposure that is equivalent to 1 SED for monochromatic radiation at wavelength λ:
(12)
The ACGIH/ICNIRP exposure limit and CIE SED curves are presented in Figure 8. At wavelengths longer than 300 nm, the two curves nearly coincide.
The exposure limits defined by the ACGIH/ICNIRP action spectrum are considered protective against photochemical effects leading to erythema and photokeratitis, but concerns remained that these limits might not protect the lens of the eye against thermal effects resulting from absorption of UV‐A at high dose rates over short time periods (16, 18). Therefore, the ACGIH established a TLV of 10 000 J m−2 for radiant exposures to UV‐A, without spectral weighting, over periods less than 1000 seconds. The TLV for unweighted UV‐A irradiance is 10 W m−2 for periods longer than 1000 seconds. This guideline is based on the premise that ambient outdoor irradiances of about 10 W m−2 due to solar UV‐A are not harmful to the eyes (16, 18).
4.1.2 Retinal Hazards
The ocular media are effectively transparent to visible radiation and transmit some IR‐A and UV‐A radiation as well. Transmitted optical radiation is focused by the lens onto the retina to form a retinal image. The irradiance received at the retinal image of an optical source is proportional to the radiance of the source. Photochemical or thermal damage may result from excessive irradiance at the retinal image.
ICNIRP (19) and ACGIH (20) have established guidelines for protection against retinal hazards. The relevant spectral weighting functions used by ACGIH are generally harmonized with those recommended by ICNIRP, but the exposure limits recommended by ICNIRP and ACGIH differ in some cases.
FIGURE 8 Comparison of the ACGIH TLV for narrowband radiation (15) with the erythemal effective radiant exposure (J m−2) equivalent to 1 SED (17) at wavelength λ.
4.1.2.1 Blue‐Light Hazard
High retinal irradiances from short‐wavelength optical radiation may cause photochemical damage to the retina, leading to loss of visual acuity. This hazard is referred to as the “blue‐light” hazard because the effect is greatest in the blue region of the optical spectrum between 435 and 440 nm.
The ACGIH TLV for protection against the blue‐light hazard (20) is similar, but not identical, to the ICNIRP exposure limits (19). The source spectral radiance Lλ is weighted by a blue‐light hazard function B(λ) over the range 305–700 nm:
(13)