Indirect effects in exposure limits

The main effect that EMF exposure limits protect against is the direct effect on the body of induced fields and currents.  But electric fields can also have indirect effects though the way they charge conducting objects which aren't earthed.  This includes the effect of charge on the surface of the body causing hairs to vibrate, and the phenomenon of microshocks.  We summarise here what the various exposure limits say about these indirect effects.

NRPB 1993

The 1993 NRPB exposure guidelines say:

"In many exposure conditions and for most people, the annoying effects caused by electric charge on the surface of the body will not occur at power-frequency electric field strengths below 12 kV/m."

ICNIRP 1998

The 1998 ICNIRP Guidelines set a reference level for electric fields for the public of 5 kV/m and for occupational exposure of 10 kV/m, then add:

"Provided that basic restrictions are met and adverse indirect effects can be excluded, field strength values can be exceeded."

When describing how the reference levels are calculated from the basic restrictions, they also say:

"For the specific case of occupational exposures at frequencies up to 100 kHz, the derived electric fields can be increased by a factor of 2 under conditions in which adverse indirect effects from contact with electrically charged conductors can be excluded."

EU 1999

The 1999 EU Recommendation on public exposure gives basic restrictions in Annex 2 (which make no mention of indirect effects at all), and reference levels in Annex 3.  Annex 3 says:

"Provided that adverse health impacts of indirect effects of exposure (such as microshocks) can be avoided, it is recognised that the general-public reference levels can be exceeded provided that the basic restriction on the current density is not surpassed."

But the main body of the Recommendation says Member States should "aim to achieve respect of the basic restrictions given in Annex II for public exposure" without mentioning Annex 3 at all, so the status of Annex 3 is unclear.

ICES 2002

The 2002 ICES guidelines give basic restrictions on the induced electric field in various tissue types.  It is possible to derive external electric fields that correspond to these basic restrictions.  But these levels, derived from the basic restriction, would be higher than the levels ICES sets to protect against indirect effects.  So in ICES, it is the indirect effects which determine the electric-field limits.

They are termed "maximum permissible exposures" and are

  • 5 kV/m for the public in general
  • 10 kV/m (under normal load conditions) for the public on the right-of-way of a power line
  • 20 kV/m for controlled exposures
  • For a controlled environment in which an exposed individual is not within reach of a grounded object, it may be acceptable to exceed 20 kV/m

The explanation given for the higher public limit on a power-line right-of-way is:

"Power line rights-of-way fall somewhere between the definitions of “controlled” and “uncontrolled” environments for the general public in that public activity can be circumscribed by the utility, but that public access is often allowed for the public benefit. Consequently, this standard specifies a limit of 5 kV/m for the general public in regions off the right-of-way, but allows an intermediate field of 10 kV/m within the rightof-way under normal load conditions."

NRPB 2004

The 2004 NRPB advice on limiting exposures says:

“When an ungrounded person is in an electric field and comes into contact with a grounded object there is the possibility of occurrence of a spark discharge at the point of contact between the person and the object. For fields external to the body greater than about 5 kV m-1, there is the likelihood of such discharges being painful. The extent to which this is a problem in practice is unclear and further investigation is merited.

“When a person is in an electric field and comes into contact with an ungrounded object there is the possibility of occurrence of a spark discharge at the point of contact between the object and the person. For such situations, the probability and the magnitude of the effect depend on the field strength and the size of the ungrounded object.”

Current position in the UK

The most definitive statement of the position on indirect effects in the UK is contained in the Code of Practice on compliance with exposure limits:

"While indirect effects are more tangible due to effects such as microshocks, they have historically given rise to less concerns than direct effects. For indirect effects, while the Guidelines give a cautionary reference level of 5 kV m-1 for the general public as a trigger to fuller assessment of compliance with the exposure guidelines, using that as a limit is not the most appropriate way of dealing with indirect effects. Rather, there is a suite of measures that may be called upon in particular situations, including provision of information, earthing, and screening, alongside limiting the field which should be used to reduce the risk to the public of indirect effects. In some situations, there may be no reasonable way of eliminating indirect effects, for instance where erecting screening would obstruct the intended use of the land. The approach to addressing indirect effects of electric fields will be the subject of a separate document to be developed between the industry and the Health Protection Agency."

See also