How can ultraviolet waves be helpful to humans?

UV light, when the body is over-exposed, can cause skin cancer, premature skin aging, eye damage and problems with the immune system. That is why it is important to apply sunscreen when you are out for a sun tan. It can also be helpful to have a UV monitor to measure the current intensity of the UV radiation from the sun as it can then let you know how long you should stay unsheltered in the sun.