The Smartsun UV monitor is a wristband which alerts wearers when they are being exposed to dangerous levels of UV rays from the sun. It does this by changing color from yellow to pink to indicate that it's time to take shelter from the sun.
The wristband is designed for single use. It begins its life with a yellow cover, turning beige when approaching the UV threshold before becoming pink when it's time to head indoors.
The Smartsun is able to do this because it incorporates an acid-releasing agent which reacts to changing UV light levels. This agent affects the material's pH levels, causing a dye to be released and change the wristband's color.
The developers of Smartsun say that if you are wearing sunscreen, you should also apply sunscreen to the wristband so that it is aligned with your level of sun protection and takes longer to turn pink.