Examples are power lines, building wiring, transformers , etc. The best way to determine the levels of the magnetic fields in an area is to have a survey done. Some utility companies will do a preliminary survey at little or no cost. An alternate is to purchase or rent a field meter and measure the area in question. When you have the survey performed, make sure the monitors in the area are switched off so you can get a true reading of the background fields.
Magnetic fields are a direct result of current, or the use of electricity.
If the amount of current you are using suddenly increases or decreases, then your magnetic fields will proportionately increase or decrease. The debate over the possible health effects of EMF continues, but to date research is inconclusive. Your local utility can often provide credible materials from reliable sources on this subject. Often, moving the equipment relatively small distances from the source may be enough to diminish the background magnetic fields and the interference. If the interference problem is minor, you may try setting your monitor to different refresh rates to find a level which is more comfortable.