6.4.3 Exposure Guidelines And Standards
International exposure guidelines provide a scientific basis for protecting both workers and the general public from established adverse effects of RF fields. Two of the principal frameworks are published by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) and the Institute of Electrical and Electronics Engineers (IEEE). The current ICNIRP RF guidelines were published in 2020 and cover frequencies from 100 kHz to 300 GHz. IEEE Std C95.1-2019 provides a related framework for human exposure to electric, magnetic, and electromagnetic fields from 0 Hz to 300 GHz.
The ICNIRP guidelines are intended to protect against scientifically substantiated adverse health effects arising from short- and long-duration, continuous, intermittent, and pulsed exposure. Within the RF range, the principal established interaction mechanisms are nerve stimulation at the lower frequencies and excessive tissue heating at higher frequencies. The guidelines incorporate conservative reduction factors to account for scientific uncertainty, differences among people, variations in environmental conditions, and uncertainty in relating external fields to effects within the body.
ICNIRP specifies two complementary types of exposure limit:
- Basic restrictions apply to quantities within or at the surface of the body that are closely related to the relevant biological effects. Depending on frequency and exposure duration, these include induced electric field strength, whole-body and localized specific absorption rate, specific energy absorption, absorbed power density, and absorbed energy density. These quantities cannot always be measured directly in a person.
- Reference levels are therefore provided in terms of quantities that can generally be measured or calculated more readily outside the body, including electric-field strength, magnetic-field strength, incident power density, energy density, and current in the limbs. Reference levels are derived conservatively from the basic restrictions. Exposure below the applicable reference level demonstrates compliance. Where a reference level is exceeded, this does not necessarily mean that the basic restriction has been exceeded; a more detailed assessment may instead be used to determine the quantities induced or absorbed within the body.
An exposure quantity is not completely specified by its magnitude and frequency. The spatial and temporal averaging conditions associated with the applicable limit must also be considered. Under the ICNIRP Guidelines and RPS S-1, sustained whole-body thermal exposure is assessed using a 30-minute average, sustained localized thermal exposure using a 6-minute average, and brief localized exposure above 400 MHz using additional energy-based restrictions integrated over intervals shorter than 6 minutes. The biological basis for these different timescales is discussed in Chapter 7, while their practical application in exposure assessment is considered in Chapters 8 and 9.
Different restrictions apply to occupationally exposed workers and members of the general public. General-public restrictions are more conservative because members of the public include children, older people, people with differing states of health, and individuals who may be unaware of or unable to control their exposure. Occupational restrictions apply only where the required training, supervision, risk controls, and ability to respond to changing conditions are present.
The ICNIRP and IEEE publications are guidelines or consensus standards and do not automatically have legal force. Governments and regulatory authorities may adopt them directly, incorporate their numerical limits into legislation, modify them to suit national policy, or use them as the scientific basis for product and installation standards. Regulatory requirements may therefore differ among jurisdictions and may not always reflect the latest edition of the underlying scientific guideline. Compliance must be assessed against the legislation and technical standards that apply in the location concerned.
Exposure assessment may involve direct field measurements, calculations based on source and antenna characteristics, computational modeling, or standardized laboratory tests. Where several frequencies or sources are present, their contributions must be combined using the summation rules specified by the applicable guideline. If an assessment indicates that a relevant limit may be exceeded, exposure is reduced through suitable controls such as increasing separation, limiting access or transmission time, reducing transmitter power, redirecting an antenna, installing shielding, or de-energizing the equipment before work begins.
The purpose of this section has been to establish the relationship between RF sources and the external fields experienced by people and to explain how compliance can be assessed using external field quantities. The next section introduces RF dosimetry, which relates those external fields to the electric fields induced and the power or energy absorbed within the body. Chapter 7 then examines the resulting interaction mechanisms and biological responses, together with the scientific basis used to develop RF exposure restrictions.