While exposure to some sunlight is necessary for the body to produce Vitamin D, we all know that getting too much sun is not a good idea – among other things, it can cause sunburn, skin cancer and premature aging of the skin. With that in mind, a cheap new paper wristband has been designed to let us know when to seek the shade.

Created by scientists at Australia's RMIT University and Spain's University of Granada, the one-time-use disposable device features four paper discs arranged in a row, each one with a progressively less-happy smiley face printed on it.

Those faces are printed using a special ink containing phosphomolybdic acid, which gradually turns from colorless to blue as it's exposed to ultraviolet (UV) light. Going from left to right along the row, each face is coated with an increasing number of transparent film layers, which block some of the incoming UV rays.

Upon initial exposure to the sun, none of those faces are visible. As the user stays outside, though, the faces sequentially appear – the first/smiliest one indicates that the user has only received 25 percent of their maximum safe amount of exposure, with the next two indicating 50 and 75 percent. Once the last/grumpiest face has shown up, they've reached 100 percent and should get inside.

By varying the number of film layers covering the faces, the wristbands can be made in six levels of sensitivity, each one aimed at users with different skin colors – after all, people with dark skin can withstand (and in fact need) greater amounts of sunlight than those with fair skin. And while we have seen other UV-sensitive wristbands before, the researchers claim that those typically only track overall UV radiation, whereas the new one distinguishes between UVA, UVB and UVC, which each affect human health differently.

Down the road, the technology could conceivably also be used to produce sensors for monitoring industrial and consumer products which degrade with prolonged UV exposure.

A paper on the research, which was led by RMIT's Prof. Vipul Bansal, was recently published in the journal Nature Communications.

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