What are Electric and Magnetic Fields (EMF)?

The Electromagnetic Spectrum

Electric and magnetic fields (EMF), are often described as invisible lines of force. They are present as a part of both the natural environment and environments produced by human activity. As shown in Figure 1 , these fields are part of the electromagnetic spectrum which is arranged in order of increasing frequency left-to-right. Frequency is the number of times every second that a field completes a full cycle (or oscillates), and is expressed in units of cycles per second, or Hertz (Hz).

Figure 1: The electromagnetic spectrum. The electric power system operates at 60 Hz in North America and 50 Hz in Europe (see transmission line tower symbol, second from left).

The high end of the spectrum comprises ionizing radiation, such as x-rays and gamma rays, with frequencies in the range of a billion-billion cycles per second. Ionizing radiation has enough energy to damage cells, and its use in medicine and nuclear energy is carefully managed. In the middle of the electromagnetic spectrum (millions to billions of cycles per second), are the radio-frequency (RF) fields we use every day for television, radio, microwave ovens, walkie-talkies, and cellular phones. RF fields are non-ionizing but at sufficiently high levels are able to heat tissues in the body. Various organizations, including most prominently, the International Commission on Non-Ionizing Radiation Protection (ICNIRP) and the Institute for Electrical and Electronic Engineers (IEEE) issue guidelines and standards recommending exposure limits that protect against such effects. As described later, they also publish recommendations for EMF.

Our electric power systems operate and produce EMF near the lowest end of the spectrum, 50 Hz in Europe and 60 Hz in North America (note the transmission line tower symbol in Figure 1 ). These frequencies are also referred to as ‘power frequencies’. EMF exposures at power frequencies neither directly damage cells nor produce tissue heating. This brochure focuses on the health research addressing exposure to 50 and 60 Hz EMF, with a greater emphasis on magnetic than electric fields. Although of comparatively greater concern from the 1970s through the mid-80s, the research into potential biological effects from exposure to electric fields did not reveal apparent health risks. The health issue and the associated scientific questions concerning the electrical power system evolved since that period to deal mainly with magnetic fields.

Basic Electricity and EMF

But first, what are voltage and current? Voltage may be visualized as electric ‘pressure’ similar to the pressure in a water hose. Current is the movement or flow of electricity like the flow of water in a hose. Electric fields are created by the voltage applied to an electrical cable or piece of equipment, whether or not current is flowing. A magnetic field is created by current, and disappears upon interruption of the current. Electric fields are readily shielded by objects and materials, such as houses, trees, wood, metal, animals and people. Magnetic fields, on the other hand, are not as easily shielded and pass freely through most objects (and people).

The unit of measure for electric fields is volts per meter (V/m), and directly beneath transmission lines where the field is typically in the thousands of V/m, kilovolts per meter (kV/m) is the unit most commonly used. In the U.S., the unit of measure for the magnetic field is the gauss (abbreviated as G), with exposure expressed often in milligauss or mG (1/1000th of a gauss). The international unit for magnetic field is the Tesla, with exposures usually expressed in units of microtesla (μT); one μT is equal to 10 mG. Most of the fields experienced in daily life are in the range of about 1 to 100 mG, but can be up to (and exceed) 1,000 mG near electrical appliances and equipment. By way of reference, and as described later, ICNIRP recommends a 50/60 Hz magnetic field exposure limit for the general public of 2.0 G (2,000 mG) and IEEE recommends 9.1 G (9,100 mG).