Background/Aims: Solar ultraviolet (UV) radiation is the most important environmental risk factor for melanoma and non‐melanoma skin cancer. It is known that tap water and saltwater baths have a photosensitizing capacity in subsequent UV irradiation of the skin. The aim of the present study was to determine the influence of sweating from heat or from physical activity on the photosensitivity of the skin.

Methods: Minimal erythema dose to ultraviolet B (MED/UVB) was determined on the inner forearms of 22 healthy volunteers with a Saalmann Multitester®. Sweating had been provoked by heat (sauna for 10 min) and, on another occasion, physical exercise (jogging for 15 min) followed by MED/UVB testing in a similar way.

Results: Compared to baseline MED/UVB there was a highly significant decrease in MED/UVB of up to 31.6% (median 15.8%, mean 15.9%±10.0) after 10 min in the sauna and up to 40.9% (median 16.3%, mean 17.2%±12.5) after 15 min jogging (P<0.0001).

Conclusion: Sweat influences the hydration of the horny layer of the skin, resulting in a decrease in reflection and dispersion of UV light. Moderate physical activities such as jogging and/or heat may facilitate erythema reaction. By sweating, outdoor workers and people practicing outdoor sports who are exposed to considerable amounts of solar UV radiation further increase their risk of sunburn.