The action spectrum shows that UVA does not cause immediate reaction, but rather UV begins to cause photokeratitis and skin redness (with Caucasians more sensitive) at wavelengths starting near the beginning of the UVB band at 315 nm, and rapidly increasing to 300 nm.....
In the past, UVA was considered not harmful or less harmful than UVB, but today it is known to contribute to skin cancer via
indirect DNA damage (free radicals such as reactive oxygen species). UVA can generate highly reactive chemical intermediates, such as hydroxyl and oxygen radicals, which in turn can damage DNA. The DNA damage caused indirectly to skin by UVA consists mostly of single-strand breaks in DNA, while the damage caused by UVB includes direct formation of
thymine dimers or other
pyrimidine dimers and double-strand DNA breakage.
[57] UVA is immunosuppressive for the entire body (accounting for a large part of the immunosuppressive effects of sunlight exposure), and is mutagenic for basal cell keratinocytes in skin.
[58]
UVB photons can cause direct DNA damage. UVB radiation
excites DNA molecules in skin cells, causing aberrant
covalent bonds to form between adjacent
pyrimidine bases, producing a
dimer. Most UV-induced pyrimidine dimers in DNA are removed by the process known as
nucleotide excision repair that employs about 30 different proteins.
[53] Those pyrimidine dimers that escape this repair process can induce a form of programmed cell death (
apoptosis) or can cause DNA replication errors leading to
mutation.