Exposure Guidelines and Standards
As indicated, a mechanism through which low-level EMF could cause biological effects has not been identified. The absence of a validated biological effect in whole animals or humans at low levels is consistent with the absence of a mechanism. However, at much higher exposure levels magnetic and electric fields can produce immediate (or ‘acute’) effects through established mechanisms. Magnetic fields ‘couple’ to people causing currents to flow within the body. Above a threshold level these currents stimulate nerve tissue, a phenomenon referred to as ‘electrostimulation’. Electric fields also cause currents to flow in the body, but before an exposure threshold is reached that causes electrostimulation inside the body, electric fields can stimulate sensory receptors present on the surface of the body; this interaction is also grouped under the broader term of electrostimulation. At the levels where magnetic and electric fields reach their respective perception thresholds, that is, levels at which they are just perceived or sensed, the effect does not produce any apparent harm or injury and ends when exposure at those levels ceases. However, as the exposure level is raised past the perception threshold, the effect can become annoying and ultimately painful, though reversible when exposure ceases.
The European-based International Commission for Non-Ionizing Radiation Protection (ICNIRP) and the U.S.- based Institute for Electrical and Electronic Engineers (IEEE) have each published reports that recommend exposure limits to protect against electrostimulation. Both sets of power frequency field limits for the general public are shown in Table 1 . Though a bit different from one another, each build in adequate safety margins that protect against aversive electrostimulation. Less stringent limits exist for workplace personnel, because those who work in high field environments are trained to be aware of the electromagnetic factors present. One cannot assume that all members of the public have received such training and to compensate, the public limits are lower compared to those for workers. The magnetic fields listed in Table 2 are rarely, if ever, encountered by the general public. The only location with access to the general public where electric fields at levels near guideline limits would be present is on rights-of-way (ROW) of overhead transmission lines of 230- 345 kV or greater, with the maximum electric field found approximately beneath the outer conductors at the midpoint between two towers. Some individuals may feel a ‘tingling’ sensation when in such locations, with the effect disappearing upon moving away.
|Organization||Magnetic field (gauss)*||Electric field (kV/m)|
|ICNIRP||2.0||4.2 (60 Hz)/5.0 (50 Hz)|
|IEEE||9.1||5.0 (10.0 on ROW)|
* 1 gauss = 1,000 milligauss (mG)
With regard to acute effects and exposure limits, the 2007 WHO report (see above) concluded: “Acute biological effects have been established for exposure to ELF [extremely-low-frequency] electric and magnetic fields in the frequency range up to 100 kHz that may have adverse consequences on health. Therefore, exposure limits are needed. International guidelines exist that have addressed this issue. Compliance with these guidelines provides adequate protection for acute effects.”