|MadSci Network: Cell Biology|
Sunblock literally blocks UV rays from being absorbed into the skin. Sunblock acts as a physical barrier and does not induce a chemical change in melanocytes or other skin cells. By preventing the absorption of the ultraviolet, it prevents the damage to the DNA of the cells that comprise the skin, which is what can lead to the development of melanoma. A common type of complete sunblock is zinc oxide. Sunscreens (as opposed to sunblocks) usually try to filter out UVA rays while allowing UVB rays to penetrate and 'tan' your skin. What might be confusing you is that the way in which your skin tans is that the UV rays themselves induce chemical changes in the melanocytes that produce the change in skin pigmentation. These chemical changes are not the same changes that result in the damage to the DNA. In general, UV light can be broken into two types based on wavelength: shorter UVB rays and longer UVA rays. Some people distinguish between UVB and UVA rays (UVB are 'good' UV rays that tan while UVA rays are 'bad' and responsible for the damage to the DNA). However, it should be remembered that while there is a difference in wavelength it isn't that great and scientists consider both to be capable of causing the DNA damage that results in melanoma.
Try the links in the MadSci Library for more information on Cell Biology.