A sunscreen ointment has a Sun-Protection-Factor (SPF) of 15.How much of UV radiation it filters out.
"SPF 15" means that 1/15th of the burning radiation reaches the skin through the recommended thickness of sunscreen SPF 15 sunscreens filter out about 93% of UVB rays, while SPF 30 sunscreens filter out about 97%, SPF 50 sunscreens about 98% Ref: https://en.m.wikipedia.org/wiki/Sunscreen
15%
85%
93%
98%
Concept of health and disease