Why are some electromagnetic waves harmful to humans?

Electromagnetic (EM) waves form the electromagnetic spectrum with a range of wavelengths. EM waves that have wavelengths shorter than those of visible light include ultraviolet light, X rays and gamma rays, all of which are harmful to humans. This is because they are forms of ionising radiation, meaning that they ionise atoms in human tissue which they are incident on. For example, DNA may be damaged by the radiation, leading to sickness or increasing the chance of cancer due to genetic mutations. Very high doses of radiation can cause immediate skin burning or worse. The severity of the effect of the dose is measured in sieverts (Sv) and depends on the size of the dose and the type of radiation.