Читать книгу Patty's Industrial Hygiene, Physical and Biological Agents - Группа авторов - Страница 142

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).