Re: Does common sunblock induce Melanocyte change or does it prevent the...

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.