There are many types of electromagnetic radiation: gamma rays, x-rays, ultraviolet light, visible light, infrared light, radio waves, etc. Each type of radiation has a different effect on the body. 

Ionising radiation

Ionising radiation such as gamma rays, x-rays and a certain part of ultraviolet light (the short-wave UVC) can ionise atoms due to their large energy content. As a result, the DNA in the body can be damaged and body cells can change. Even a small dose of ionising radiation is hazardous to the body. In the long run, it can cause cancer.

Non-ionising radiation

Non-ionising radiation, which will be discussed hereinafter, initiates all kinds of biological processes but only poses a risk when the radiation is too intense. What are these biological processes and when are we dealing with a risk?

Ultraviolet light

UVA light and UVB light are the most energy-rich forms of non-ionising radiation. UVC is even richer in energy but belongs to ionising radiation, which we leave out of consideration here. Ultraviolet light easily causes photochemical reactions, even in our body. That is why it was once called "chemical radiation" at the time of its discovery. These photochemical reactions can be favourable (creation of vitamin D) but will cause damage if the dose is too high (cataract, eye inflammations, sunburn and even skin cancer).
You can read more about the damage caused by UV radiation on the skin at (NL) / (FR).

Visible light

Light-sensitive cells in the retina absorb the energy of the light and convert it to nerve impulses, which makes it possible for us to see. We obviously need light but light that is too intense can be hazardous. Laser light for example can cause irreparable damage to the eyes because the light-sensitive cells in the eyes are burned. Blue light, which releases the maximum energy of all visible light, can cause harmful photochemical reactions in the retina without being very intense. In the long run, this can cause bad eyesight. Sources of blue light include sunrays and to a lesser extent LED lamps. 

You can read about the risks of optical radiation in the section “Lamps and radiation”.

Infrared light and radio waves

Energy from infrared light and radio waves is converted to heat in the body. We can feel the heat of infrared light because its energy is absorbed mostly by the skin. On the other hand, we cannot feel radio waves because these emit their energy deeper in the body, under the heat-sensitive cells of the skin.

In principle, the conversion of infrared light and radio waves to heat does not pose any problems for our body. The human body is capable of producing or emitting heat by itself to maintain the body temperature. To a certain extent. Infrared radiation or radio waves that are too intense introduce so much heat in the body that it cannot remove that heat. It will put our body under pressure and that has to be avoided.

Our body itself also emits infrared radiation (and even some radio waves) because it is warm.

You can read more about this in the section “Mobile telephone”.

Electromagnetic fields with (extremely) low frequency

While infrared light and radio waves are converted to heat, electromagnetic fields with (extremely) low frequency produce an electric current in the body. That is why scientists use the term induced current.
Very small electric currents are present naturally in our body. Nerves are able to send signals by means of electrical impulses. But strong currents that are caused by external sources can also stimulate nerves and muscles or cause flashes of light in the field of vision.
You can read more about this in the section “Electricity”.

These biological processes do not necessarily lead to damage but they imply a risk. You can read in the section “Exposure limits” how the standards are drafted to protect people against these risks.