Electric fields are produced by voltage and magnetic fields are produced by current. A lamp that is plugged in but turned off produces an electric field. If the lamp is both plugged in and turned on it produces a magnetic field as well. Electric fields are easily shielded by conducting objects like trees and buildings. Magnetic fields are not easily shielded by most materials.
EMFs exist almost everywhere.
However, the intensity of magnetic and electric fields diminishes greatly as you move away from the source. In 1979 a study in Denver, Colorado first raised the possibility that residential proximity to power lines might be associated with childhood leukemia. Since that time there have been many studies which have attempted to clarify the relationship between electric and magnetic fields and the risk of cancer. However, the results of these studies have been inconsistent and sometimes conflicting. This current study by the University of Toronto and The Hospital for Sick Children was designed to improve the measure of EMF exposures and thereby better characterize the relationship with the risk of childhood leukemia.