Changes in atoms and in the nuclei of atoms can generate or absorb electromagnetic waves over a wide range of frequencies. Gamma rays originate from changes in the nucleus of an atom. (Higher tier) Radio waves can be produced by oscillations in electrical circuits. When a radio wave is absorbed, for example by an aerial, it creates an alternating current in the circuit with the same frequency as the radio wave itself.
Ultraviolet waves, X-rays and gamma rays can have hazardous effects on human body tissue. The harm depends on the type of radiation and the size of the dose. Radiation dose is a measure of the risk of harm from exposure of the body to the radiation. It is measured in sieverts (Sv), and 1000 millisieverts (mSv) = 1 sievert, so 2500 mSv is 2.5 Sv.
Ultraviolet waves can cause skin to age prematurely and increase the risk of skin cancer. X-rays and gamma rays are ionising radiation: they can cause the mutation of genes and cancer. Given data about doses, you should be able to draw conclusions about the risks, for example that a larger dose carries a larger risk.