Cumulative effects of repeated subthreshold doses of ultraviolet radiation.

For fair Caucasian skin, the minimal delayed erythema dose (MED) 24 hr after exposure to broadband UVA is about 1200 times greater than the MED of broadband UVB, for both single and multiple daily exposures. Repeated daily exposure to doses less than MED results in cumulative effects manifest by gradual lowering of the daily dose threshold for delayed erythema and pigmentation induced by UVA or UVB. At threshold doses, UVB is more erythemogenic than melanogenic; the opposite is true for UVA. Repeated daily UVA exposure greatly enhances melanogenesis such that markedly suberythemogenic exposure doses of UVA result in true melanogenesis.