UV to some extent include glass and polycarbonate. Crown glass used in eyeglass lenses transmits close to 90% of UV radiation at wavelengths longer than 340 nm, about 50% of radiation at 310 nm, and 10% of radiation at 295 nm (48). Polycarbonate plastic has better UV‐shielding characteristics but still transmits about 10% of radiation at 320 nm and 50% of radiation at 350 nm (48). CR‐39 (allyl diglycol carbonate), another plastic used for eyeglass lenses, transmits about 10% of radiation at 360 nm and 50% of radiation at 375 nm (48). UV absorption by glass or plastics may be enhanced by UV‐absorbing additives or films. The UV protection afforded by sunglasses is not indicated by the darkness of the tint (49).
The UV‐shielding properties of fabrics depend on a number of characteristics, including fiber type, color, thickness, weight, and density of the weave or knit (50). Ultraviolet protection factor (UPF) for a fabric is defined as the inverse of the fraction of erythemal‐weighted solar spectral irradiance between 290 and 400 nm that is transmitted through the fabric. For example, if the weighted irradiance transmitted through a fabric is one‐tenth of the weighted irradiance incident on the fabric, the UPF for that fabric is 10. Wool, silk, nylon, and polyester generally have higher UPFs than cotton, linen, or rayon. UPFs tend to increase with darker color, tighter weave, heavier weight, and greater thickness. Bleaching a fabric reduces its UPF by removing UV‐absorbing impurities. Washing increases the UPF of cotton fabrics, probably by tightening the weave or knit through shrinkage (50). Wetting may decrease UPF, and though UPF is a good predictor of the protection afforded by loose‐fitting clothing, it may underestimate exposure to the skin under tight‐fitting clothing (51). Because UPFs are determined using simulated solar UV radiation, they might not accurately represent the degree of protection afforded against UV radiation of wavelengths less than 290 nm, such as germicidal UV radiation. Also, the UPF for a fabric does not provide a good measure of protection against the effects of UV‐A radiation, including photoaging and reaction to photosensitizers (52). The general principle of selecting tightly woven fabrics or leather for good protection should, however, hold true throughout the UV spectrum.
Studies of UV attenuation by materials used in disposable elastomeric gloves (53) found that chloroprene, nitrile, and latex provided significantly better UV protection than vinyl gloves in the UV‐A and UV‐B regions. Vinyl gloves transmitted 70% or more of incident radiation at wavelengths greater than 300 nm. All four materials were highly protective at wavelengths less than 290 nm, transmitting less than 1% of incident radiation. Stretching significantly increased transmittance in many gloves samples, an effect that was spectrally dependent. Glove transmittance increased with water or saline wetting. Color and thickness of gloves were not found to be a consistent predictor of protection (53).
UV barrier creams, such as sunscreens, use the UV‐absorptive properties of various chemicals to attenuate UV exposure to the skin. Organic molecules used as active agents in sunscreens generally contain aromatic rings. The inorganic chemicals zinc oxide and titanium dioxide are also commonly used. Sunscreens are assigned a sun protection factor (SPF), which is defined as the ratio of the minimum erythemal dose (MED) for skin treated with a specified amount of sunscreen to the MED for unprotected skin. SPFs are determined experimentally by testing the sunscreen product on fair‐skinned human subjects exposed to simulated solar UV radiation between 290 and 400 nm (54). SPF is not necessarily a good measure of protection from UV‐C radiation since that band is nearly absent from solar UV, nor from photoeffects other than erythema, such as photoaging. The use of sunscreens for prevention of occupational skin cancer is controversial, in part because the effectiveness of sunscreens has not been demonstrated for some types of skin cancer, and in part because use of sunscreen may give a false sense of security leading to more prolonged exposure and neglect of other UV protective measures (55).
5.3.1.2 Coverage for Personal Protection from UV Radiation
All skin exposed to excessive levels of UV radiation should be covered. Protective equipment and apparel worn in addition to regular work clothes could include face shield, long sleeves, gloves, and drapes to protect the neck. Sunscreens are best used as a secondary shield in combination with protective equipment and clothing, to attenuate the radiation transmitted through the outer protective shielding and to cover gaps. Sunscreen should be applied in sufficient quantity (at least an ounce of lotion to cover all exposed skin) and reapplied about every 2 h (56). UV‐protective eyewear should fit close to the eyes (57).
5.3.2 Visible and IR Shielding
Various absorptive filter media are available for eyewear to attenuate visible and IR radiation. Sunglasses marketed to airline pilots as protective against blue light exposure were found to provide sufficient protection against the solar blue‐light hazard at flying altitude as well as at ground level (49). Protective eyewear with known attenuation in the blue region should be used for protection against the blue‐light hazard from artificial sources.
Tinted eyewear that does not attenuate light uniformly across the visible spectrum may alter color perception, potentially inhibiting the ability to distinguish color‐coded safety signals and markings. Under the ANSI Z87.1‐2015 standard for eye and face protection (58), visible light filters that transmit 13.9% or more of visible light (and are thus suitable for wearing while driving a vehicle) must comply with the light transmittance requirements of the ANSI Z80.3‐2010 standard for nonprescription sunglasses, which include traffic signal recognition. However, brown‐tinted sunglasses that met the ANSI Z80.3 standard were found to distort color perception among railroad workers in Canada such that yellow railroad signals appeared red to some individuals (59). When color distortion could compromise safety or job performance, a neutral gray tint, which attenuates visible wavelengths uniformly, should be considered when selecting filtering eyewear, provided that adequate attenuation of any blue‐light hazards present can be achieved without excessive darkening of vision overall.
In some cases, extremely high radiant energy absorbed by a filter could potentially raise its temperature high enough for the filter to become a significant source of IR itself, or even to melt. A reflective coating of certain metals such as gold or copper can block some of the radiation from entering a filter, enhancing the filter's performance, though with some reduction in transmission of visible light. Alternatively, interference filters have recently been developed for use in near‐IR protective eyewear that may provide IR reflectance and visible light transmittance similar to that of reflective filters while showing more resistance to mechanical damage such as cracking, scratching and delamination (60, 61).
Reflective aluminized clothing, illustrated in Figure 10, is widely used for protection against radiant heat stress. Barrier creams with reflective flecks are available for application to skin that could be exposed to high levels of IR.
5.3.3 Filter Shade Numbers
The ANSI Z87.1‐2015 standard for eye and face protection established transmittance requirements for general‐purpose filter lenses, which are identified by shade numbers (58). Shade number is determined by the manufacturer of the lens using the following formula:
(29)
where TL is the luminous transmittance (the nominal fraction of photopic‐weighted visible light that is transmitted through the filter). A maximum effective transmittance in the “far‐UV” region (200–315 nm) and a maximum IR average transmittance (780–2000 nm) are specified by the standard for each shade number. The transmittance of blue‐light‐weighted radiation for a shade must be less than the luminous transmittance for that shade.
FIGURE 10 Firefighters on a field training exercise wear aluminized clothing for protection against radiant heat.
Source: Photo by Airman 1st Class Kathrine McDowell, U.S. Air Force.