As any dermatologist will tell you, it's important to know when to get out of the sunlight – or at least, when to apply more sunscreen. As a result, there are now various UV exposure-monitoring devices that tell us when to seek the shade. Not everyone wants to buy one, however, plus some of the single-use models contain environmentally-harmful materials. With that in mind, scientists have developed cheap, disposable eco-friendly sensors that are made of paper.

Using an inkjet printer, non-toxic titanium dioxide and a food dye are both applied to a paper substrate. When exposed to UV light for long enough, the titanium dioxide acts as a photocatalyst, degrading the dye and causing it to change color. That color-change, which can easily be detected by the unaided eye, lets users know that they're in danger of getting sunburned.

Additionally, by applying substances to the paper that act as UV neutral density filters, it's possible to determine how much sunlight exposure is required to cause the color-change to take place. In this way, developers of the sensors can account for users' different skin tones, and their use of different SPF levels of sunscreen.

The research was conducted at Australia's University of New South Wales, and is described in a paper that was recently published in the journal ACS Sensors.