The basic restriction for human exposure is defined by Specific Absorption Rate (SAR) limits. Maximum Permissible Exposure (MPE) limits are derived from the SAR limits, in terms of free-space field strength and power density. SAR compliance is determined using tissue-equivalent media, at the applicable test frequencies. For devices that operate at larger distances from persons, where there is minimal RF coupling interaction between a device and the user or nearby persons, the more complex SAR evaluation can be forgone by evaluating RF exposure compliance using MPE limits. MPE calculations are used as acceptable justification that SAR testing does not apply when a transmitter falls below defined power limits.
The FCC has adopted guidelines and methods for evaluating the environmental effects of radiofrequency (RF) emissions from FCC-regulated transmitters. These rules are based on recommendations from federal health and safety agencies; Environmental Protection Agency (EPA) and the Food and Drug Administration (FDA). Other contributing bodies are the Institute for Electrical and Electronics Engineers, Inc. (IEEE), National Council on Radiation Protection and Measurement (NCRP) and the American National Standards Institute (ANSI). Specific guidance from ANSI and IEEE is found in ANSI/IEEE C95.1 covering the frequency range 3 kHz to 300 GHz.
The FCC has adopted MPE limits for electric and magnetic field strength and power density for transmitters operating at frequencies from 100 MHz to 6 GHz. The FCC’s guidance documents for SAR and MPE requirements are linked below. While these are specific to the United States, similar requirements exist throughout the world.