Sunscreen products are rated according to their ability to prevent reddening of the skin for a period of 24 hours after sun exposure. Although it is well known that sunscreens do protect against skin reddening, their ability to protect against melanoma (the most dangerous form of skin cancer) is not clearly known.
When human skin is exposed to excess UV light from the sun, the body produces a substance called nitric oxide (NO). NO causes dilation of blood vessels at the skin surface, resulting in increased blood flow and reddening effect. NO is also an important immunoregulatory molecule that stimulates a natural anti-tumor immune response to UV light.
Researchers from the VA Medical Center in Memphis, TN measured the ability of five sunscreen preparations to inhibit NO production upon UV exposure in vitro. Four of the five suncreens tested directly inhibited NO production.
The authors conclude that, although sunscreens may in part prevent against reddening of the skin by blocking UV rays, another major mechanism of action includes blocking NO production. Skin reddening due to increased NO production is one of the human bodies' natural warning signs and it is a defense mechanism against UV induced sun damage. The authors conclude that, although sunscreen use is effective at reducing sun-induced reddening of the skin, its use may actually promote melanoma rather than prevent it.
Chiang, TM, et al., "Sunscreen ingredients inhibit inductible nitric oxide synthase (iNOS): a possible biochemical explanation for the sunscreen melanoma controversy." Melanoma Res., February 2005; 15(1):3-6.