How does sunscreen work?

SKIN CANCER is America’s most common cancer, and is becoming more so. One estimate suggests that 20% of Americans will suffer from it over the course of their lives. The problem is a global one. The main risk factor for skin cancer is cumulative ultraviolet (UV) radiation exposure over decades. The heatwaves that scorched America, China and Europe in the past week are becoming more common and a growing share of the world’s population is exposed to dangerously high temperatures. Extreme heat makes sunscreen all the more essential. How does it work and how important is it?

Sunscreen is a relatively modern invention. Franz Greiter, a Swiss chemist, made one of the first formulas in 1946 after being sunburned while mountain climbing. In the 1940s lounging in the sun became popular and so too did tanning oils and sunscreens. As studies revealed the dangers of UV exposure, demand for protective products grew further. In 1962, Mr Greiter developed a sun-protection factor (SPF) rating based on roughly how long it would take for sun-exposed skin to go red after applying a given product compared with unprotected skin (though in practice, times vary for a host of reasons). Mr Greiter’s original formula had an SPF of just 2.1.

Sunscreens contain UV filters which either absorb or reflect UV rays so that less of them hit the skin. The dreaded ghostly cast of many formulas comes from titanium dioxide, a white powder used as a filter. There are now also chemical sunscreens which are often transparent (though oilier): most use oxybenzone and avobenzone. Modern formulas normally protect against two types of UV radiation. The most intense rays, known as UV-B, cause sunburn and blistering and are more carcinogenic at higher temperatures. It is these rays which SPF ratings refer to. Most formulas also protect against UV-A rays. These shorter wavelengths can pass through windows and cloud cover and penetrate the skin more deeply. This unleashes “free radicals”, unbalanced molecules that react in unpredictable ways, damaging cell DNA (and causing wrinkles). That may play a role in causing cancer too, though the science remains inconclusive.

Children, the fairer skinned and the freckled are most vulnerable to UV rays. (Freckles are often evidence of too much sun exposure: they are caused by a buildup of melanin in the skin, as the body tries to protect itself.) Skin pigmentation provides some extra protection from sunburn: in 2018 America’s Centres for Disease Control and Prevention, a government agency that advises on health care, found that white people were 25 times more likely to get skin cancer than black people (though the risks were substantial for both).

Applied too sparingly, sunscreen is mostly useless for everyone. Doctors recommend roughly two fingers worth of product on the face and neck, and a teaspoon for each limb. Over the day, the skin’s natural oils and sweat break down sunscreen’s film barrier. That makes reapplication essential. And despite innovations, in a warming world sunscreen alone does not provide sufficient protection. Cooler countries may need to follow the lead of the hottest by eschewing the midday heat. ■

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