In case of exposure to very high intensities of 50 Hz magnetic and/or electric fields, acute direct effects can occur. These effects are well studied in human volunteers and animals. Recommendations and guidelines are intended to protect us from these short term established effects.
Short term effects depend on the local intensity of the electric field in each tissue. This explains why the reference unit is the electric field expressed in Volt per meter (V/m).
There are a number of well established acute effects of exposure to low-frequency electric and magnetic fields on the nervous system:
- Direct stimulation of nerves and muscle tissues
- Induction of retinal phosphenes (*).
(*) Phosphenes are flashes of light that can be perceived in the eye. These flashes can be directly induced by mechanical, electrical, or magnetic stimulation of the retina. High magnetic fields can induced current at the retina level, that could lead to magnetophosphenes.
There is also indirect scientific evidence that brain functions such as visual processing and motor co-ordination can be transiently affected by induced electric fields.
All these effects have thresholds below which they do not occur and can be avoided by meeting appropriate basic restrictions on electric fields induced in the body.
Exposure to low-frequency electric fields causes well-defined biological responses through surface electric-charge effects. Prevention of the painful effects of surface electric charge induced on the body by such exposure are addressed by the reference levels in the standards.
One should also remind that electric currents do naturally occur in the human body:
- An electroencephalogram is the trace of electric activity of the brain.
- An electrocardiogram is the trace of electric activity of the heart.
The purpose of the guidelines is, among others, to avoid internal electric fields higher than those that occur naturally in the body.
ICNIRP, "International Commission on Non-Ionizing Radiation Protection", is an independant international
commission recognized by the World Health Organization (WHO)
The main objective of ICNIRP is to establish guidelines for limiting exposure to electric and magnetic fields (EMF) that will provide protection against all established adverse health effects.
Guidelines are based on the following:
- Well established effects (from perception of a slight tingling on the surface of the skin to annoyance) may be caused by exposure to low frequency electric fields.
- Induction of phosphenes in the retina (ml) by low frequency magnetic fields can be used as a model to put forward induced electric fields effects on the central nervous system
Further information in International Commission on Non-Ionizing Radiation Protection. Guidelines for limiting exposure to time-varying electric and magnetic fields (1 Hz to 100 kHz). Health Phys. 2010 Dec;99(6):818-36 (pdf).
In view of the uncertainty inherent in the scientific data, reduction factors have been applied in establishing the exposure guidelines. A reduction factor is applied for the exposure of the general public (in comparison with occupational exposure). It is intended to take into account its heterogeneity (age diversity, the health status of the public, environmental conditions, personal sensitivity).
Guidelines for workers and the public
Basic restrictions for 50 Hz fields are the values of the electric field strength considered as acceptable for people's well-being.
The maximal values of the internal electric field in the brain and retina are 0,1 V/m for occupational exposure and 0,02 V/m for general public exposure.
Values of induced electric fields are translated into electric quantities allowing direct measurement: external electric and magnetic field strength and magnetic flux density. They are reference levels.
The reference levels are obtained from the basic restrictions by mathematical modeling. The reference levels take in consideration the effects of the induced electric fields in the central nervous system (CNS), e.g the brain, and in non-CNS tissues, meaning anywhere in the body.
An additional reduction factor of 3 is applied to these calculated values to allow for dosimetric uncertainty.
The reference levels for exposure to 50 Hz electric and magnetic fields are:
- for workers (occupational exposure): 10 kV/m - 800 A/m - 1000 µT
- for the general public (everyday exposure): 5 kV/m - 160 A/m - 200 µT
As mentioned by ICNIRP, "compliance with the reference level will ensure compliance with the relevant basic restriction". If the measured or calculated value exceeds the reference level, it does not necessarily follow that the basic restriction will be exceeded.
International Commission on Non-Ionizing Radiation Protection. GUIDELINES FOR LIMITING EXPOSURE TO TIME-VARYING ELECTRIC AND MAGNETIC FIELDS (1 Hz TO 100 kHz). Health Phys 99(6):818-836; 2010.