Ultraviolet Radiation and Toxic Chemical Mixtures

UV radiation from the Sun that impinges upon the Earth s surface is in wavelengths between 290 and 400 nm. The UV rays that reach the Earth are classified as UVA (long length waves of 320-400 nm) and UVB (short length waves of 290-320 nm). 

Although UVA plays an essential role in the formation of vitamin D by the human skin, it is harmful as it causes sunburn and cataract formation in the eyes. As discussed below, UVA also causes toxic effects when mixed with xenobiotic chemicals. UVB causes damage at the molecular level. It is absorbed by DNA and alters its structure. UV excites organic molecules and generates free radicals that are responsible for substitution, elimination, and polymerization reactions free radical formation and generation of ROS. Exposures to both UVA and UVB concurrent with exposures to xenobiotic chemicals produce unanticipated toxic effects. Following are examples of the mixture effects of UV. 

UV is known to induce nonmelanoma skin cancer. Ethanol and aloe emodin alone do not induce skin tumors in the absence of UV. When an ethanol solution of aloe emodin was painted onto the skin of mice in conjunction with UVB exposure, the mice developed melanin-containing skin tumors. The mechanism for the observed carcinogenesis induction by the mixture is unknown, 91 

UV exposure is known to induce photosensitive responses in humans. Photosensitivity is an adverse reaction that occurs when a chemical or drug is applied to the skin or taken internally at the same time that one is exposed to UV. Not all people are photosensitive to the same agents. Two distinct responses are observed phototoxic and photoallergic reactions. 

Phototoxicity is photosensitivity that is independent of immunological responses. Phototoxic responses are dose dependent and will affect almost anyone when sufficient dosage is applied or when taken concurrent with UV exposure. In phototoxic reactions, photoactivated chemicals cause direct cellular damage. UV absorption produces either excited state chemicals or metabolites of these chemicals. These, in turn, can be converted into either free radicals or singlet oxygen, either of which results in biomo-lecular oxidation)10  

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