It is well established that ultraviolet radiation has immunomodulatory effects that may be involved in
skin cancer. Recent studies have shown that ultraviolet A radiation (320-400 nm) as well as ultraviolet B (290-320 nm) is immunosuppressive. This means
sunscreens that mainly absorb ultraviolet B (protection against
erythema) may be less effective in preventing ultraviolet radiation-induced immunosuppression than broad-spectrum products. We have studied the effects of ultraviolet A exposure on the human delayed-type
hypersensitivity response and compared the efficacy of
sunscreens having different levels of ultraviolet A protection under both solar-simulated radiation and outdoor real-life solar exposure conditions. Delayed-type
hypersensitivity was assessed using recall
antigens. In a first study, two groups of volunteers were exposed to ultraviolet A (either full spectrum ultraviolet A or ultraviolet A1) without prior application of
sunscreen and they were shown to exhibit significantly reduced delayed-type
hypersensitivity responses. In order to compare the efficacy of
sunscreens in preventing photoimmunosuppression, three groups of subjects received 10 cumulative exposures to solar-simulated radiation; one group was exposed unprotected and the other two were exposed after being applied either a ultraviolet B or a broad-spectrum
sunscreen, each with the same sun protection
factor 9, but with different ultraviolet A protection factors 9 and 2. Then, an outdoor study was conducted in which delayed-type
hypersensitivity was assessed before and after six daily exposures. Two different groups of subjects were treated with one of two
sunscreens having the same sun protection factor 25 but different ultraviolet A-protection factors. In unprotected volunteers, responses to delayed-type
hypersensitivity tests were significantly reduced irrespective of ultraviolet exposure conditions (full spectrum ultraviolet A, ultraviolet A1, solar-simulated radiation). The ultraviolet B
sunscreen failed to protect from solar- simulated radiation-induced immunosuppression. In contrast, the broad-spectrum
sunscreen having the same sun protection factor but providing high protection in the ultraviolet A range significantly reduced local ultraviolet-induced immunosuppression and prevented the distal effects. In the outdoor study, as compared with delayed-type
hypersensitivity responses obtained before sun exposure, no alteration of immune response was detected when the skin was protected by broad-spectrum
sunscreen sun protection factor 25 and ultraviolet A-protection factor 14. Conversely, a broad-spectrum
sunscreen sun protection factor 25 ultraviolet A-protection factor 6 failed to protect against the sun-impaired response. The above studies clearly demonstrate the role of ultraviolet A in the induction of photoimmunosuppression together with the need for
sunscreen products providing efficient photoprotection throughout the entire ultraviolet spectrum.