People outdoors are exposed to higher levels of the sun's harmful ultraviolet (UV) rays than previously thought, a study shows.
UV rays may be higher than thought on the slopes
Present measures of UV exposure - the Solar Index - are taken from how much sunlight hits flat surfaces.
But German researchers at Geo Risk Research in Munich say this underestimates levels hitting sloped surfaces.
They describe their new measuring device in New Scientist.
The Solar UV Index is a system developed by the World Health Organization to help people gauge their potential risk of sun damage when outdoors.
It uses horizontal surfaces to measure the sun's UV rays from which the solar index can be estimated.
Professor Peter Hoeppe and colleagues in Munich used new types of monitoring systems that they believed would estimate UV exposure more accurately.
Their systems measured the UV radiation hitting 27 surfaces inclined at different angles at three different locations in Germany.
The team analysed three years' worth of readings that had been taken every two minutes by the systems.
They created a three-dimensional image of the human body, with the surface divided into about 20,000 triangles.
3-D body map
They then used the data to colour the triangles according to UV radiation exposure the body would get at different angles.
This allowed them to visualise the UV exposure of any kind of body posture and spatial orientation.
Professor Hoeppe's team found UV exposure to parts of the skin were many times higher than those estimated with current measuring devices, particularly when the sun was lower in the sky and when the person was standing.
"In many cases, dermatologists are underestimating the amount of UV exposure," Professor Hoeppe said.
"The human being only has a few surfaces which are horizontally orientated if the person is standing - the top of the head and the shoulders for example.
"The rest of the surfaces are vertical.
"If the sun is high in the sky then the horizontal measurement may be OK, but when the sun is lower the vertical surfaces receive a lot of radiation," he said.
He said skiers might be at particularly high risk because snow slopes can reflect as much as 60% of UV.
Doctors could use the three-dimensional body pictures to educate people about which areas of their skin were at greatest risk of sun damage in different circumstances, he said.
But he did not think the horizontal scanning devices used worldwide to monitor UV radiation needed to be replaced by the new system.
"For the everyday forecasts it would be too much information," he said.
Dr Mike Clark, scientific spokesman for the NRPB said: "The researchers have a point that the results may not be so accurate later in the day, and also in the winter months when the sun is lower in the sky."
But he said the risks were much lower in the UK.
"Temperatures are cooler and people less likely to be sunbathing.
"Nevertheless, we will look at this work carefully and see if it has any implications for how we measure solar radiation intensities," he said.
Sara Hiom, Cancer Research UK's SunSmart Campaign coordinator, said most important message was still to avoid strong sun and to cover up and use sunscreen when in the sun.