The EM Spectrum is an continuous spectrum of waves made of 7 types of transverse waves.
They all travel the same speed through a vacuum but travel different speeds in different materials.
All EM waves transfer energy from the source to the observer, for example, when you use an electric heater, infrared waves transfer energy from the thermal energy store of the heater to your thermal energy store.
The higher the frequency, the more energy the wave transfers, making it more dangerous more humans.
In order of increasing frequency and decreasing wavelength the categories of the EM spectrum go:
They also increase in danger to the body when absorbed.
Our eyes can only detect a limited range of frequencies of EM radiation.
When EM waves meet a boundary they can be absorbed, transmitted, refracted or reflected which depends on the wavelength.
The differences in how EM waves are trasmitted, reflected and absorbed have implications for human health:
Radio waves are trasmitted through the body without being absorbed
Some wavelengths of microwaves can be absorbed, causing the heating of cells which may be dangerous.
IR and visible light are mostly reflected or absorbed by the skin, causing heating too. IR can cause burns too if the skin gets too hot.
UV is also absorbed by the skin. But it's an ionising type of radiation and when absorbed by the skin can cause damage to the surface of your skin which can lead to skin cancer. It can also damage your eyes.
X-rays and gamma rays are also ionising and can cause mutation to cells which can cause cancer.