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Colour-changing stickers and hair dye that react to ultraviolet light could help people gauge when they are at risk of sunburn.

Alex Mariakakis at the University of Washington in Seattle worked with a team at Microsoft to develop printable stickers that change colour from purple to light pink throughout the day to indicate cumulative exposure to UV light.听The patches contain a reference colour scale to indicate UV exposure, as measured by UV index (UVI) hours, a standard measure of UV radiation.

They also accommodate听for different skin tones, which have different minimum UV-exposure thresholds for sunburn, says Mariakakis. The three versions of the patch each display thresholds corresponding to 0, 3.33, 6.67 and 11.11 UVI hours, which roughly match the minimum amount of UVI hours that can cause sunburn in people of differing skin tones.

For example, people with very pale skin 鈥� Type I on the Fitzpatrick scale of skin tone 鈥� may burn after 2.22 to 3.33 UVI hours of exposure.

After being taught how to compare the colour-changing section with the reference colours, 35 participants were able to glance at the patch and determine whether the reading would indicate sunburn risk for three different skin tones with an accuracy of 73 per cent.

The patches are made from a UV-sensitive ink and can be printed in an ordinary inkjet printer. The ink consists of a photoacid generator 鈥� a compound that produces acid when exposed to UV light 鈥� as well as a pH-sensitive dye that responds to that acid.

鈥淎s you鈥檙e more exposed to UV, it generates more acid and the dye changes colour,鈥� says Bichlien Nguyen at Microsoft Research in Redmond, Washington.

The researchers also made from pigments that change colour in the presence of UV light 鈥� from clear to pink, for example.听In one version, they tested a monochromatic design that increases in colour saturation with rising UV intensity. In a second, they used a tiered design that included three dyes, each with different saturations moving from the top of a person鈥檚 head to the ends of their hair.

鈥淚f you鈥檙e going outside and it鈥檚 a cloudy day, only the top part of your hair would change,鈥� says Mariakakis.

Because the hair dye changes colour irreversibly, it is only able to indicate UV intensity at a given moment rather than cumulative UV exposure throughout a day, like the patch. The hair dye currently only works on lighter-coloured hair.