If you are relying on your burning skin to tell you when it's time to get out of the sun, you're doing it wrong. Sunburns occur when you've had too much exposure to ultraviolet (UV) rays, and that can put you at risk for skin cancer — the most common form of cancer in the United States, according to Centers for Disease Control and Prevention (CDC).
To help you know exactly when you've gotten your sunshine fix, USC Viterbi Professor Andrea Armani and graduate research lead Michele Lee have created some pretty amazing wearable technology. The two developed a waterproof sticker that you can easily place anyway on your body like a Band-Aid. When you've gotten your necessary dose of vitamin D, the sticker changes to a burnt orange color. The color change also indicates that you've gotten enough UV exposure, and it's time to get out of the sun.
This adhesive is not a substitute for sunscreen and is fully functional with sunblock coverage. The CDC recommends wearing protection with a minimum of SPF 15.