Protection against the sun in Denmark is only necessary from mid April to September. The number of erythema units may reach about 16 per day in the summer sun and about 50% of the biological active radiation is found between 12 a.m. and 3 p.m. (summertime). The most biologically active wavelengths are found to be 300-325 nm and protection with sunscreens and clothes should include these wavelengths. Necessary sun protection factors for average and especially sensitive Danes were estimated. The average Dane needs a SPF of 4-8 and especially sensitive persons a SPF of 8-15 when exposed all day in summer. Most clothes meet these demands.