By Victoria Gill
Science reporter, BBC News
Researchers have developed an indicator that turns an appropriate shade of pink to alert wearers of sunburn.
The thin film device could be worn as a wrist band to warn wearers they risk receiving a potentially harmful dose of ultraviolet rays.
UV rays drive a chemical reaction in the indicator, releasing an acid into a dye, and causing it to change colour.
The research team from the University of Strathclyde report their work in the journal Chemical Communications.
Professor of chemistry, Andrew Mills, who led the team, describes this combination of a UV-driven reaction with an acid-sensitive dye as "intelligent ink".
"People think of chemical reactions as happening in test tubes," he said. "But here you have a reaction in a very thin layer of ink film that produces a colour change."
Other indicators are already available that detect and measure UV. But what's special about this one, said Professor Mills, is that it can be adjusted to give an instant signal at the point when sun exposure is about to cause damage.
Don't feel the burn
Professor Mills has made a prototype of the film, combining a dye that gradually changes colour from yellow to blue, and a central strip of dye that turns pink.
"This delayed reaction is the novel feature," Professor Mills explained. As soon as the indicator turns pink, he says "you should get out of the sun because if you stay you'll burn".
When the line turns pink, it's time to get out of the sun
The device could also be adapted to different skin types; adding an alkali to the dye would increase the delay before the colour change.
"Our plan is to start a company that will make products out of this technology, such as wrist bands or clothing labels," Professor Mills said.
"We've already been approached by a number of skincare product manufacturers who are interested in the technology."
Jodie Moffat, health information officer from Cancer Research UK, said that anything highlighting the damage that UV exposure can cause would be of value.
According to the charity, more than 2,300 people die from skin cancer each year in the UK.
Ms Moffat said she could imagine "this sort of device being used to encourage people to protect their skin".
But, she added, it would need to be thoroughly tested to ensure it reflected exposure levels in real life situations.