The sun is essential for life.

The sun IS essential for life. Yet, sunlight can also be a source of such deleterious effects as sunburn, and suntanning, as well as premalignant and malignant lesions. These may all occur in individuals with normal responses to sunlight. In addition, there exist a variety of "abnormal" photosensitivity responses to sunlight that may result from either endogenous imbalances (e.g. the porphyrias) or from added exogenous factors (e.g. drug photosensitivity). The "normal" responses to sunlight, by and large, are produced preferentially by UVB (290-320 nm), with minor contribution by UVA (320-400 nm) wavelengths. In contrast, the "abnormal" photosensitivity responses are, for the most part, elicited predominantly by long UVA and, in some cases, visible light.