We’ve learned a lot in recent years about the nitty-gritty of risk. For example, a history of sunburn is a key measure of risk: A person who has suffered three sunburns a decade has a threefold risk of developing melanoma (the most severe form of skin cancer) compared with someone with fewer burns. Levels of exposure in childhood and adolescence are good predictors of the risk of basal cell cancer. Squamous cell cancers – which tend to occur on exposed areas such as the face, neck and arms – follow a geographical pattern: They are most common in the sunniest part of the world.
But we know how to protect ourselves from the cancer-causing rays of the sun too.
It is recommended, by the Canadian Cancer Society, dermatologists and others, that we limit our exposure to the sun, particularly during peak UV hours, wear protective items such as hats, sunglasses and long sleeves and use sunscreen.
In recent weeks, there has been important new information about sunscreen published in both the United States and Britain.
The U.S. Food and Drug Administration, for example, has unveiled new labelling rules decreeing that for a sunscreen to say it can help prevent cancer it must have a sun protection factor of at least 15 and protect against both UVA and UVB rays. The FDA also says sunscreens can no longer assert they are waterproof, just water-resistant, and claims of a SPF over 50 are no longer permitted.
The SPF is a measure of how much longer skin covered with a product take to redden (or develop erythema, to use the scientific term) in response to UV radiation compared with unprotected skin.
A SPF 15 sunscreen limits exposure to about 7 per cent of UVB radiation, and a SPF 30 to about 3 per cent.
However, as the British Drug and Therapeutics Bulletin notes, those figures are based on the assumption that sunscreen is applied at a thickness of two milligrams for each square centimetre – a level at which it would run off the skin and be unsightly. In real life people apply much less, so you should assume that your SPF 30 sunscreen is offering no better than SPF 15 protection.
The point is this: We are exposed to various types of radiation all the time, from natural and man-made sources. We can protect ourselves from these exposures – and benefit from some even – but the key is to have a little basic scientific knowledge and apply it appropriately and practically in our daily lives.
