Not every radiation is harmful. Radiation is basically energy traveling as waves or particles. It can be classified as ionizing and non-ionizing radiation.

Non-ionizing radiation includes visible light, infrared rays, microwaves, radio waves, etc. Non-ionizing radiation is generally not harmful.

Now, ionizing radiation includes gamma rays, x-rays, radiation given off by radioactive elements, nuclear power plants, nuclear weapons, etc. Although we are daily exposed to low levels of ionizing radiation, it doesn't cause any harm.

However, exposure to high levels of ionizing radiation in a short period of time can damage the DNA in our cells, causing cell dysfunction or even cell death. Also, sometimes when the damaged DNA tries to repair itself, there can be a change or mutation in the genes, leading to uncontrolled cell division and thus, causing cancer. High levels of ionizing radiation can even turn water in our body into hydrogen peroxide which is really toxic. Thus, the ionizing radiation can prove to be very harmful.