Ultraviolet (UV) light is broken down into bands of light according to its physical characteristics and biologic effects:
- UVC: 100 to 290 nm wavelength. High-energy radiation that injures cells through direct DNA damage and the generation of free-radical species. Fortunately, UVC radiation is filtered by atmospheric ozone and does not reach the earth’s surface.
- UVB: 290 to 320 nm wavelength. Midrange radiation that is not completely filtered by atmospheric ozone, called the “burning” rays because it causes sunburn. UVB injures skin cells primarily through formation of DNA thymine dimers and 6 to 4 photoproducts that, if not repaired properly, cause gene mutations and lead to altered cell function and carcinogenesis.
- UVA: 320 to 400 nm wavelength. Long-wave radiation that is the lowest energy spectrum of ultraviolet. UVA is not filtered by atmospheric ozone, and a 150-fold greater amount of UVA strikes the surface of the earth compared to UVB. UVA damages skin cells predominantly through the formation of free radicals. UVB penetrates to the basal layer of the epidermis, whereas UVA penetrates to the middermis. Skin wrinkling following chronic sun exposure is due to UVA injury of dermal fibroblasts, resulting in altered collagen and elastin synthesis.
Combined, UVB and UVA are carcinogenic. Thus, it is imperative to warn patients about the damaging effects of ultraviolet radiation and how to properly protect against its adverse effects.
