Skin reactions to light
Ultraviolet radiation (UVR) can be helpful when used to treat diseases such as psoriasis, but it can also be harmful (Fig. 16.1). It is the leading cause of skin cancers, and causes or worsens several skin disorders. UVR is non-ionizing, but changes the skin chemically by reacting with endogenous light-absorbing chemi-cals (chromophores), which include DNA, RNA, uro-canic acid and melanin. Different types of skin (now conventionally divided into six types; Table 16.1) react differently to UVR, and require different degrees of protection against the sun.
The UVR spectrum is divided into three parts (Fig. 16.2), each having different effects on the skin, although UVC does not penetrate the ozone layer of the atmosphere and is therefore currently irrelevant to skin disease. Virtually all of the UVB is absorbed in the epidermis, whereas some 30% of UVA reaches the dermis. The B wavelengths (UVB: 290 –320 nm) cause sunburn and are effectively screened out by window glass. The A spectrum (UVA) is long-wave ultraviolet light, from 320 nm to the most violet colour perceptible to the eye (about 400 nm). It ages and tans the skin. The differences between the wavelengths can be recorded conveniently in the form of action spectra, which show how effective each is at producing different biological effects, such as clearing psoriasis or causing erythema.