Guidance is based solely on scientifically established adverse health effects. Such effects are identified by the health risk assessment. In developing the guidelines, ICNIRP considers direct and indirect, acute and chronic health effects. Different adverse effects can be ranked according to the exposure level at which each becomes relevant. The critical effect is the established adverse health effect relevant at the lowest level of exposure. Protection against the critical effect means that protection is provided against all other adverse effects occurring at higher exposure levels. In principle, the ICNIRP guidelines are set to protect against critical effects, by limiting the related specific biologically effective quantity. The biologically effective quantity reflects the efficacy by which the external exposure causes a certain biological effect. This quantitative relationship between external measurable exposures and the target tissue biologically effective parameter is unique to a singleexposure condition. Reduction factors are included as a measure of caution, to account for quantitative uncertainties in the scientific database and biological variability in response. As a consequence, the guidelines will be set below the thresholds of the critical effects. There is no rigorous scientific basis for establishing reduction factors. They are not intended for compensating uncertainties in measurements performed to check compliance with exposure standards, nor do they incorporate social or political considerations, including precautionary approaches.
Restrictions on the effects of exposure based on established health effects are termed basic restrictions. It is the general strategy of ICNIRP to define a basic restriction in terms of the appropriate biologically effective quantity. Depending on frequency, the physical quantities used to specify the basic restrictions on exposure to EMF are current density, specific absorption rate, and power density. Protection against adverse health effects requires that the basic restrictions are not exceeded.
Additionally, reference levels of exposure are provided for comparison with measured values of physical quantities; compliance with reference values given in the guidelines will ensure compliance with basic restrictions. In general, the reference levels are more conservative than the basic restrictions because they have been developed for situations of optimum coupling conditions between the radiation or fields and the exposed person. If measured values are higher than reference levels, it does not necessarily follow that the basic restrictions have been exceeded, but a more detailed analysis is necessary to assess compliance with the basic restrictions. In some circumstances, it may be advisable to distinguish between members of the general public and individuals exposed because of or while performing their work tasks (occupational exposure).
Whereas ICNIRP provides general practical information on measurable levels that are derived from basic restrictions on exposure, it recognizes the need for further technical advice on special exposure situations. This requires physics and engineering expertise to develop practical measures to assess and/or to enable assessment of compliance with exposure guidelines. These measures include guidance on the principles and practice of measurements, design of equipment and/ or shielding to reduce exposure, and, where appropriate, setting emission limits for specific types of device (see Section 1.4.6).
The ICNIRP Guidelines (1998) have been adopted by more than 40 countries worldwide. The European Union, for example, has adopted a Directive on the minimum health and safety requirement regarding the exposure of workers to the risks arising from physical agents (electromagnetic fields), which is based largely on the ICNIRP Guidelines (EU, 2004).
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