• immediate pigment darkening;
  • fluence rate;
  • skin type;
  • sunscreen;
  • UVA radiation

When human skin is irradiated with ultraviolet radiation (340–100 nm) there is an immediate pigment response, termed Immediate Pigment Darkening (IPD). This reaction is thought to be due to photooxidation of preexisting melanin, precursors and/or melanin metabolites because it appears during exposure. It has been demonstrated that UVA-induced skin reactions, including erythema and pigmentation, are oxygen dependent. Therefore, these reactions should also be irradiance (fluence rate) dependent. The purpose of this investigation was to determine the dependence of the IPD threshold on fluence rate. We exposed the forearm of 12 volunteers (skin type II-V) to monochromatic UVA radiation (362 nm) at a range of fluence rates of 6–115 mW/cm2 and determined the fluence at which pigment was perceptible. The threshold fluence for the IPD reaction increased by a factor of 2.7 as the fluence rate increased by a factor of 18. Therefore, we conclude that the IPD reaction following exposure to 362 nm radiation is dependent on fluence rate, and independent of skin type.