6.3 Photocuring of Coatings in Manufacturing
Several regions of the optical radiation spectrum have been used in manufacturing for drying and curing coatings and other materials.
In the powder coating process, an alternative to solvent‐based painting or finishing, pigment in a resin powder matrix is deposited on the substrate by electrostatic attraction and heat is then used to melt the powder so that it forms a smooth, solid layer. IR lamps or ovens are often used as the heat source for the melting process.
Photocuring processes that produce cross‐linking in polymers require higher quantum energies and thus a much shorter wavelength, typically in the UV region. UV‐A is the most commonly used wavelength band for photocuring. However, xenon lamps, mercury‐vapor lamps, and metal halide lamps used for high‐powered photocuring may also emit UV‐B and UV‐C radiation. High‐power lamps for photocuring are typically enclosed in opaque housings, but stray radiation may still exceed exposure limits for UV radiation (80). There is also a potential blue‐light hazard if the source is viewed. In the case of pulsed xenon photocuring systems, the risk of retinal thermal damage should also be evaluated. Enclosures should be designed with minimal openings. Direct light from the source should not be visible to the operator or other personnel. Reflected light should be minimized by painting surfaces with black matte paint. Because ozone may be generated by UV‐C radiation, source enclosures should be ventilated to the outside. Workers should wear eye and skin protection for UV. Electrical safety and excess heat are additional concerns due to the power requirements of these photocuring systems.
Photocuring systems that use ultraviolet light‐emitting diode (ULED) technology are also available. ULEDs emit radiation in a relatively narrow band, for instance 380–420 nm, eliminating the risk of ozone generation and exposure to UV‐B and UV‐C radiation. ULED photocuring sources might still produce high UV‐A irradiances of about 1 W cm−2, sufficient to cause overexposure in a few seconds or minutes. Blue‐light exposure could also be a hazard if the source is viewed. Potentially exposed skin and eyes should be protected.
6.4 Germicidal Lamps
Low‐pressure mercury‐vapor lamps for germicidal use emit radiation primarily in a narrow peak at 254 nm. The TLV for radiant exposure to the eye or skin at this wavelength is 60 J m−2 (15), which is equivalent to a continuous irradiance of 0.002 W m−2 in an eight hours period or 60 W m−2 in one second. Some germicidal lamps also emit a small amount of radiation at 185 nm, which generates ozone. The high irradiances needed to inactivate pathogens could present a hazard to exposed personnel within a few seconds (69). Therefore, germicidal UV sources should be enclosed, baffled, or carefully positioned to prevent overexposure. Upper room germicidal irradiation is used to disinfect room air in institutions such as hospitals, prisons, and homeless shelters. The practice in designing upper room germicidal systems is to prevent irradiances in excess of 0.002 W m−2 anywhere in the lowest 2 m (6.5 ft) of a room. A personal dosimetry study on workers in a hospital, a school, an office, and a shelter found that the eight hours UV‐C dose was a small fraction of the TLV even when some irradiances measured at eye level exceeded 0.002 W m−2 by factors of 3–6 (45). The design criterion for the maximum allowable irradiance in the lower room thus appeared to be very conservative.
Excimer lamps emitting radiation at 222 nm are increasingly being used as germicidal radiation sources. Because 222 nm radiation has lower penetration through the stratum corneum than 254 nm radiation, it is less damaging to skin (81). The TLV for 222 nm narrowband radiation, logarithmically interpolated from tabulated values (15), is 228 J m−2. In a small pilot study, human volunteers radiated with a commercial 222 nm sterilizing lamp developed erythema at a threshold dose of 400–500 J m−2 and DNA lesions at doses of 630–1010 J m−2, well below the 3000 J m−2 dose that can effectively kill pathogens (82). Secondary excimer emission peaks at 234 and 257 nm could have contributed to the adverse effects on the skin. These longer wavelengths can be removed with a bandpass filter (81). As with 254 nm radiation, UV‐absorbing eye protection is recommended to prevent photokeratitis from 222 nm germicidal radiation (81).
6.5 Stage and Studio Lighting
High‐powered lamps for stage or studio illumination can be a significant source of exposure to performers and production crew members. A study of 11 models of metal halide or halogen photoflood lights used in television studios and theaters found that, for several photofloods, the TLV for actinic UV, UV‐A, or blue‐light exposure could be exceeded in a few minutes. Exposures were higher in the television studios than in the theater stages (83).
Blue LEDs used in stage lighting can cause blue‐light hazard overexposure in minutes (84). Stage luminaires that used a combination of blue, green, and red LEDs to produce white light were found to be similar to other white stage lighting tested in terms of the ratio of the blue‐light hazard‐weighted radiance to the illuminance (perceived brightness) of the source. Due to the low photopic response to blue light (around 450 nm), blue LED arrays used as blue stage lighting did not appear very bright; therefore viewing time might not be limited by aversion (84). Exposure to UV‐A from “blacklights” used for fluorescent effects during performances could exceed dose limits for protection of the lens in about an hour of viewing, and exposure duration would not be limited by behavioral aversion to bright light (85).
Stage and studio lights should be equipped with UV filters, which are very effective at reducing actinic UV and UV‐A exposure (86). Protecting performers from blue light may pose a difficult challenge because these workers must often look toward the lights for extended periods and might not be able to wear protective eyeglasses for cosmetic reasons. The possibility of using contact lenses that absorb UV and blue light has been suggested (83).
6.6 Dental Curing and Bleaching
Both UV light and short‐wavelength visible light are used in dentistry for curing resins in composite fillings and for tooth whitening treatments. Light is used to activate photoinitiators, which initiate polymerization of the resin. Photoinitiators can be activated by radiation only in specific bands; for example, the commonly used photoinitiator camphoroquinine has an activity peak between 470 and 480 nm. Photocuring units use halogen lamps, LEDs, plasma‐arc lamps, or lasers, and produce irradiance levels that range from 3000 to over 10 000 W m−2 (87). The duration of irradiation is typically just a few minutes for curing but may last up to an hour for bleaching. Dental personnel is at risk from exposure to radiation reflected from the patient's teeth. Calculations of the maximum permissible exposure time to blue‐light radiance from 26 models of dental lamps indicated that the blue‐light dose could be exceeded in several seconds or several minutes, depending on the type of lamp (88).
In an evaluation of the spectral transmittance of 18 different filters used in protective eyeglasses, handheld filters, or stationary filters for dental curing or bleaching lights, it was found that nine of the filters had transmittance less than 0.1% in the blue‐light region and would provide adequate attenuation when used with any dental lamp on the market. Six of the filters were found to provide inadequate protection if used with the lamps for which they were recommended (88).
6.7 Glassblowing and Foundries
Glassblowers and foundry workers can be exposed to excessive near‐IR irradiances to the eyes when working at open furnaces . Significant UV radiation may