The following information on electromagnetic fields is from Electromagnetic fields, published by the World Health Organization Regional Office for Europe in 1999 (Local authorities, health and environment briefing pamphlet series; 32).

Electromagnetic Fields - Definitions and Sources

Electric fields are created by differences in voltage: the higher the voltage, the stronger will be the resultant field. Magnetic fields are created when electric current flows: the greater the current, the stronger the magnetic field. An electric field will exist even when there is no current flowing. If current does flow, the strength of the magnetic field will vary with power consumption but the electric field strength will be constant.

Natural Sources of Electromagnetic Fields

Electromagnetic fields are present everywhere in our environment but are invisible to the human eye. Electric fields are produced by the local build-up of electric charges in the atmosphere associated with thunderstorms. The earth's magnetic field causes a compass needle to orient in a North-South direction and is used by birds and fish for navigation.

Human-Made Sources of Electromagnetic Fields

Besides natural sources the electromagnetic spectrum also includes fields generated by human-made sources: X-rays are employed to diagnose a broken limb after a sport accident. The electricity that comes out of every power socket has associated low frequency electromagnetic fields. And various kinds of higher frequency radiowaves are used to transmit information - whether via TV antennas, radio stations or mobile phone base stations.

The Basics of Wavelength and Frequency

What makes the various forms of electromagnetic fields so different?

One of the main characteristics which defines an electromagnetic field (EMF) is its frequency or its corresponding wavelength. Fields of different frequencies interact with the body in different ways. One can imagine electromagnetic waves as series of very regular waves that travel at an enormous speed, the speed of light. The frequency simply describes the number of oscillations or cycles per second, while the term wavelength describes the distance between one wave and the next. Hence wavelength and frequency are inseparably intertwined: the higher the frequency the shorter the wavelength.

A simple analogy should help to illustrate the concept: Tie a long rope to a door handle and keep hold of the free end. Moving it up and then down slowly will generate a single big wave; more rapid motion will generate a whole series of small waves. The length of the rope remains constant, therefore, the more waves you generate (higher frequency) the smaller will be the distance between them (shorter wavelength).

1. The electromagnetic spectrum encompasses both natural and human-made sources of electromagnetic fields.
2. Frequency and wavelength describe an electromagnetic field. In an electromagnetic wave, these two characteristics are directly related to each other: the higher the frequency the shorter the wavelength.
3. Ionizing radiation such as X-ray and gamma-rays consists of photons which carry sufficient energy to break molecular bonds. Photons of electromagnetic waves at power and radio frequencies have much lower energy that do not have this ability.
4. Electric fields exist whenever charge is present and are measured in volts per metre (V/m). Magnetic fields arise from current flow. Their flux densities are measured in microtesla (µT) or millitesla (mT).
5. At radio and microwave frequencies, electric and magnetic fields are considered together as the two components of an electromagnetic wave. Power density, measured in watts per square metre (W/m2), describes the intensity of these fields.
6. Low frequency and high frequency electromagnetic waves affect the human body in different ways.
7. Electrical power supplies and appliances are the most common sources of low frequency electric and magnetic fields in our living environment. Everyday sources of radiofrequency electromagnetic fields are telecommunications, broadcasting antennas and microwave ovens.

Reports on Health Effects of EMFs by WHO

The "Sensitivity of Children to EMF Exposure" workshop was conducted by the World Health Organization (WHO) in 2004 in response to the following concerns: "There have been suggestions that exposure of young children to electromagnetic fields (EMF) may be detrimental to their health, especially during the development and maturation of the central nervous system, immune system and other critical organs. In addition children are exposed to EMF for a much greater part of their lifespan than adults. Use of mobile telephones by young children has been a concern expressed by the Stewart Committee report in the United Kingdom and others."

The following information on electromagnetic fields is from Electromagnetic fields, published by the World Health Organization Regional Office for Europe in 1999.

Biological effects

Electromagnetic fields in the frequency range of 300 Hz-300 GHz interact with human and other animal systems through direct and indirect pathways. Indirect interactions are important at frequencies below 100 MHz, but are specific to particular situations. When metallic objects (such as automobiles, fences) in an electromagnetic field have electrical charges induced in them, they can be discharged when a body comes into contact with the charged object. Such discharges can cause local current densities capable of shock and burns.

A major interaction mechanism is through the currents induced in tissues, so effects are dependent on frequency, wave shape, and intensity. For frequencies below approximately 100 kHz, the interactions with nervous system tissue are of interest, because of their increased sensitivity to induced currents. Above 100 kHz, the nervous tissue becomes less sensitive to direct stimulation by electromagnetic fields and the thermalization of energy becomes the major mechanism of interaction.

There is evidence from a number of studies that weak-field interactions also exist. Different mechanisms for such interactions have been postulated, but the precise mechanism(s) has not been elucidated. These weak-field interactions result from exposure to RF fields, amplitude modulated at lower frequencies.

Laboratory studies

Many of the biological effects of acute exposure to electromagnetic fields are consistent with responses to induced heating, resulting either in rises in tissue or body temperature of about 1°C or more, or in responses to minimizing the total heat load. Most responses have been reported at specific absorption rates (SARs) above about 1-2 W/kg in different animal species exposed under various environmental conditions. The animal (particularly primate) data indicate the types of responses that are likely to occur in humans subjected to a sufficient heat load. However, direct quantitative extrapolation to humans is difficult, given species differences in responses in general, and in thermoregulatory ability, in particular.

The most sensitive animal responses to heat loads are thermoregulatory adjustments, such as reduced metabolic heat production and vasodilation, with thresholds ranging between about 0.5-5 W/kg, depending on environmental conditions. However, these reactions form part of the natural repertoire of thermoregulatory responses that serve to maintain normal body temperatures.

Transient effects seen in exposed animals, which are consistent with responses to increases in body temperature of 1°C or more (and/or SARs in excess of about 2 W/kg in primates and rats), include reduced performance of learned tasks and increased plasma corticosteroid levels. Other heat-related effects include temporary haematopoietic and immune responses, possibly due to elevated corticosteroid levels. The most consistent effects observed are reduced levels of circulating lymphocytes, increased levels of neutrophils, and altered natural killer cell and macrophage function [emphasis added]. An increase in the primary antibody response of B-lymphocytes has also been reported. Cardiovascular changes consistent with increased heat load, such as an increased heart rate and cardiac output, have been observed, together with a reduction in the effect of drugs, such as barbiturates, the action of which can be altered by circulatory changes.

Most animal data indicate that implantation and the development of the embryo and fetus are unlikely to be affected by exposures that increase maternal body temperature by less than 1°C. Above these temperatures, adverse effects, such as growth retardation and post-natal changes in behaviour, may occur, with more severe effects occurring at higher maternal temperatures.

A safety factor of 10 is introduced, in order to allow for unfavourable, thermal, environmental, and possible long-term effects, and other variables, thus arriving at a basic limit of 0.4 W/kg. An additional safety factor should be introduced for the general population, which includes persons with different sensitivities to RF exposure. A basic limit of 0.08 W/kg, corresponding to a further safety factor of 5, is generally recommended for the public at large. Derived limits of exposure are given in Tables 34 and 35 of this publication.

At frequencies below about 1 MHz, exposure limits are selected that will prevent stimulation of nerve and muscle cells. Basic exposure limits refer to current densities induced within body tissues. Exposure limits should have a sufficiently large safety factor to restrict the current density to 10 mA/m2 at 300 Hz. This is the same order of magnitude as natural body currents. Above 300 Hz, the current density necessary for excitation of nervous tissue increases with frequency, until a frequency is reached at which thermal effects dominate. For frequencies around 2-3 MHz, the basic limit for current density is equivalent to the limit for the peak SAR of 1 W/100g. Since SAR or induced current density values cannot be measured easily in practical exposure situations, exposure limits in terms of conveniently measurable quantities must be derived from basic limits. These "derived limits" indicate the acceptable limits in terms of the measured and/or calculated field parameters that allow compliance with the basic limits.