Electro-magnetic fields (EMFs) are present wherever there is electric wiring.  Homeowners are often unaware of spots in their home where these fields are located.  Thus, it is not unusual to discover a baby's crib placed in a room opposite the spot where electrical wires enter the house on the outside.  Similarly, people often sleep (for 1/3rd of their lives) with their head inside the electro-magnetic field generated by the small transformer of a clock radio or on the other side of the wall from an indoor circuit breaker panel.   

Checking the home with an EMF meter detects such locations.  It can be argued that EMFs are not harmful - just as it could not be proven that cigarette smoking was causally linked to lung cancer.  However, it might be prudent to avoid spending a lot of time with your head in one of these fields.