Adult Sun Signals UV sensors come in an 18-piece package and monitor UV exposure by changing colors to help your family avoid sunburn and help reduce the risk of skin cancer. SunSignals are fun, easy to use, inexpensive and provide an easy way to tell when you've had enough sun. SunSignals are 7/8" round sensors that start out yellow and turn deep orange when exposed to harmful ultraviolet B (UVB) radiation from the sun. SunSignals are applied directly to clothing or skin and clearly indicate when a maximum recommended level of sun exposure has been reached.Knowing when you've had too much sun is no easy task. With no practical way to measure ultraviolet rays, it's difficult to know how much sunscreen to use, when to reapply it, or when to get out of the sun. A person may be out in the sun for two hours one day, and not get sunburned. Another day, that same person might be out for only 30 minutes, but still get sunburned, even though it was cloudy. Time of day, time of year, altitude, cloud cover, and the angle of the sun all affect the intensity of the sun's rays. The only way to know if you've had too much sun is to measure the sun's ultraviolet B (UVB) rays. By monitoring UVB rays, you can know at any time if you're at risk for over-exposure to the sun. Therefore, SunSignals UV Sensors take away the guesswork and provide the most reliable way for most people to monitor their exposure to the sun! You get 18 SunSignals UV Sensors per package!