Re: why does UV B rays cause production of carcinoma cells

UVa is the ultra-violet from ~400 to 320nm.
UV-B is below that, i.e. less than 320 nm ( I am not being very precise- 
obviously at some stage it becomes X-rays).

UV-B is therefore more energetic, but that isn't really the issue. The 
main problem is that UVB is absorbed by DNA, the genetic material of the 
cell. When it is absorbed, it has the energy to cause thymidines to 
react, forming a cross-link between two thymidines called a cyclobutane 
photodimer, but the chemistry details are not so important. The main 
thing is that the UVB leads to mutation of the DNA with relatively high 
efficiency, and DNA damage leads to cell death, or cancer.
hope this helps
david