UVA/UVB

When sunlight penetrates the top layers of the skin, ultraviolet radiation (referred to as UVA or UVB) bombards the genetic material, the DNA, inside the skin cells and damages it.

UVB radiation is the primary cause of sunburn and primarily affects the skin’s outer layers. UVB is most intense at midday when sunlight is brightest. It is interesting to note that slightly more than 70% of the yearly UVB dose is received during summer and only 28% is received during the remainder of the year.

On the other hand, UVA radiation penetrates more deeply and efficiently; however, UVA's intensity also tends to be less variable both during the day and throughout the year than UVB's. For example, only about half of the yearly UVA dose is received during the summer months and the balance is spread over the rest of the year. UVA is also not filtered through window glass (as is UVB).

Damaging effects of UV radiation

Both UVA and UVB rays damage the skin, inducing genetic injury, wrinkles, lower immunity against infection, aging skin disorders and cancer, although the reasons for this are not yet fully understood. The following are some ways in which cancer may develop and some defensive actions that the skin uses to protect itself against DNA damage.

Oxidation and antioxidants

UV radiation affects the production of oxidants, also called free radicals. These free radicals are unstable molecules produced by normal chemical processes in the body that, in excess, can damage the body's cells and even alter their genetic material, contributing to the aging process and sometimes to cancer. The large surface area of the skin makes this organ a prime target for oxidants.

Defective DNA repair and protective enzymes

Some melanomas and other skin cancers are caused by a breakdown in the mechanisms that help repair DNA damage. This can occur due to various causes including an inherited condition called xeroderma pigmentosum (XP). A number of enzymes in the skin help protect against this damage. One repair enzyme called T4 endonuclease 5 (T4N5) is, in fact, being investigated in lotions to protect against skin cancers.

Breakdown of Immune Protection

Specific immune factors protect the skin, including white blood cells called T lymphocytes and specialized skin cells called Langerhans cells. These immune factors attack developing cancer cells at the earliest stages. Unfortunately, certain substances in the skin, in particular a chemical called urocanic acid, suppresses these immune factors when exposed to sunlight, setting the stage for skin cancers.

Defective Cell Death (Apoptosis)

Apoptosis is the last defense of the immune system. It is a natural process of cell-suicide, which occurs when cells are severely damaged. Apoptosis in the skin kills off cells harmed by UVA preventing them from becoming cancerous. (The peeling after sunburn is the result of these dead skin cells.) In some cases, however, genetic mutations or other factors derail apoptosis. If this occurs, the cells can become immortal and continue to proliferate, resulting in skin cancers.