Ultraviolet light, carcinogenicity

Few split-dose experiments have been performed with chemicals. Popescu et aL (1984) observed that with split doses of carcinogen separated by 2 to 24 hours only N-acetoxy-2-fluorenylacetamide enhanced transformation of Syrian hamster embryo ceUs while doses of N-methyl-N -nitro-N-nitrosoguanidine, mitomycin C or ultraviolet light were less effective than single doses, and no effect of dose fractionation was observed with methyl methanesulfonate. 

Photocarcinogenesis Carcinogenic effects associated with exposure to ultraviolet light. 

Phenanthrene absorbs ultraviolet light and causes production of singlet oxygen, which in turn leads to free radical production. Although a large body of literature exists on the toxicity and carcinogenicity of other PAHs, primarily benzo[d ]pyrene, toxicity data for phenanthrene are limited. 

Substances with xenobiotic bonds are not necessarily more toxic than those without them. The aflatoxins (mycotoxins that are produced by Aspergillusflavus) are the most carcinogenic substances known but lack xenobiotic bonds, whereas the synthetic Freons are almost biologically inert but are also very stable. (They escape the lower atmosphere, reaching the stratosphere, where they are split into free radicals that react and destroy ozone, which helps to shield the Earth from too much ultraviolet light. These ozone-depleting reactions have dramatic environmental effects, but the direct toxicity of the Freons is modest.)... 

Caution. Carbon monoxide is a toxic gas and should be used in an efficient fume hood. Avoid looking directly at the ultraviolet light source. Protective goggles should be worn. Carbon tetrachloride is a carcinogen and must be handled with care in a hood. 

Itoh, T., Cado, D., Kamide, R., and linn, S. (2004). DDB2 gene disruption leads to skin tumors and resistance to apoptosis after exposure to ultraviolet light but not a chemical carcinogen. Proc. Natl Acad. Sd. USA 101, 2052-2057. 

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