Why do people think mobile phones and microwaves are harmful to the human body?

Answer 1

Mobile phones emit waves radio waves which may increase your chances of getting brain cancer (if you have constant exposure to the phone or if you are often near a radio antenna).

Microwaves were actually first used for weapon testing, as they gave energy to water molecules which caused them to heat up and turn into a gas. The human body is made up of about 60% water. If you were exposed to a strong enough microwave without protection your blood (about 50% water) could boil in your veins, this of course would be extremely painful and could probably kill you (a extreme example of this is when you cook a raw egg in the microwave (causing it to explode which isn't recommended)). Microwave shields aren't perfect so pressing your face against the microwave every time you use it could potentially cause long term damage to your body (especially your brain).