How does low level radiation cause biological damage?
Low level radiation like ultraviolet light can damage biological tissues by altering the DNA in the nuclei of the cells. Specifically, radiation can cause two thymine bases in DNA nucleotides to cross-link and form thymine dimers. These dimers can then alter the DNA replication process during cell division and cause either mutated or damaged cells to be produced. These mutations can often be repaired by mechanisms within the cell but bad enough damage over an extended period of time can cause damage that could be dangerous to the organism. Skin cancer is a perfect example.