New research suggests that outdoor activities are exposing us to higher levels of harmful ultraviolet rays than previously thought.
The present estimate of UV exposure, the UV index, is based on measuring how much radiation hits a horizontal surface. But this often underestimates how much UV hits a vertical surface when the sun is low in the sky, says Peter Hoeppe, a biometeorologist at Geo Risk Research in Munich, Germany.
His team has developed a new way of estimating UV exposure that it hopes is more accurate. The researchers measured the UV radiation hitting 27 surfaces inclined at different angles at three different locations in Germany.
Using data from three years' worth of readings taken every 2 minutes, Hoeppe's team constructed a three-dimensional model of the human body showing accurate UV exposure in different solar conditions and body postures.
Their model also takes into account UV reflected off nearby surfaces. It predicts that even at midday, parts of the body will receive higher doses of UV than the UV index estimates (see diagram). And in some circumstances a person standing would receive more UV than if lying down. "In many cases dermatologists are underestimating the amount of UV exposure," he says.
Snow can reflect as much as 60 per cent of UV, says Hoeppe, which means that skiers are at a greater risk of skin cancer than previously thought. Mike Clark at the National Radiological Protection Board, which is responsible for measuring and disseminating the UK's UV index to weather services, says the major risk is still from sunbathing in a horizontal position in the midday sun. "When the sun is low in the sky, people are less likely to be sunbathing, especially while standing up," he points out.
Sara Hiom of Cancer Research UK's SunSmart campaign says there is a danger of overcomplicating the message. The bottom line, she says, is cover up.