Purpose: To provide heat to small spaces (space heaters) or whole houses (baseboard heaters). These are also present in some floors (floor heaters).
Issue: As well as putting forth electric fields from their design, because these units use a fair amount of current, they generate fairly large magnetic fields.
Better Option: When possible, gas, kerosine, propane, or wood are better heating options from an EMF perspective. If you need to use an electric heater, then using a gauss meter to determine the safe working or sleeping distance is best.