Ultraviolet waves, X-rays and gamma rays have high frequencies and carry a lot of energy. When they are absorbed by living tissue they can damage cells. Too much ultraviolet from the Sun or a sunbed can cause sunburn, damage the eyes and lead to skin cancer.
X-rays and gamma rays are ionising radiation. They can knock electrons out of atoms and damage molecules such as DNA inside cells. A low dose can cause mutations in DNA, which may make a cell become cancerous. A high dose can kill cells or cause radiation burns.
The risks are managed. Radiographers stand behind screens or leave the room, because repeated exposure to X-rays could harm their cells. Patients wear lead aprons over areas not being scanned, and the exposure time is kept as short as possible. People protect their skin from ultraviolet with sunscreen and clothing. Doctors weigh the risk against the benefit. For an X-ray of a suspected broken bone the risk is small, and finding the break is worth that risk. In radiotherapy, gamma rays are aimed at a tumour from several directions to kill cancer cells while limiting damage to healthy tissue.