The sun gives off ultraviolet radiation that we divide into categories based on the wavelength.
- UVC - 100 to 290 nm
- UVB - 290 to 320 nm
- UVA - 320 to 400 nm
UVA is not affected by most sunscreen and penetrates deep into the skin. SOme research suggest it is UVA that causes skin cancer, not UVB.
Melanin is the pigment that makes skin brown. People with more melanin have darker skin. Melanin absorbs UVA and UVB and is thus a natural sunscreen, and is probably the reason that dark skinned people have a tenth of the skin cancer rate of fairer folks. When fair skinned people are exposed to the sun, this stimulates melanin production in their skin, thus giving them a 'tan' which protects them somewhat from the UVA and UVB.
If you use sunscreen, it may be that you inadvertantly expose yourself to much higher dosages to UVA than you would naturally since you are able to stay in the sun much longer without burning, and also without developing a ptoective "tan". There appears to be a link between the increase in skin cancer amongst fail skinned people and the increase in teh use of sun screen creams.