Answer to Question #679 Submitted to "Ask the Experts"

Q

What are current recommended IEEE EMF Occupational Exposure levels and guidelines for Radio Base Station Antennas?

A

The following answer assumes "radio Base Station Antennas" refers to base stations used for personal wireless communications services such as cellular telephone, Personnel Communications Services (PCS), and Enhanced Specialized Mobile radio (ESMR). In the United States, the frequency band allocated for base stations used for personal wireless communications is 869-894 MHz for cellular service, 851-866 MHz for ESMR, and 1930-1990 MHz for PCS. The IEEE exposure limits (maximum permissible exposure—MPE) for occupational exposure (controlled environments where exposures may be incurred by persons who are aware of the potential for exposure) are frequency dependent and are expressed in terms of incident power density. The appropriate value to compare with the MPE is the incident power density averaged over the vertical cross-section of the human body. The occupational exposure limits are also time-averaged over any six-minute interval. The IEEE C95.1-1991 MPE values are shown in the table below:

As indicated above, these values are for continuous exposure (t > 6 minutes) and are to be compared with the spatial average of the incident power density. all base stations used for personal wireless telecommunications services in the United States are required to comply with the FCC limits (as mandated by the Telecommunications Act of 1996). The FCC limits are based on a hybrid of the IEEE limits and the 1986 recommendations of the National Council on Radiation Protection and Measurements. The FCC MPEs are the same as the IEEE values for frequencies below 1500 MHz but are capped at 5 mW/cm² (50 W/cm²) for frequencies greater than 1500 MHz.

Both the FCC and IEEE MPEs for exposure of the public (uncontrolled environments) are one-fifth of the above values. As above, the value to compare with the MPE is the incident power density averaged over the vertical cross-section of the human body. The averaging time for the general public is 30 minutes. Thus, for transient exposures lasting six minutes or less, the MPEs for occupational exposure and exposure of the general public MPEs are the same. For cases of simultaneous exposure from antennas operating in different frequency bands, the ratio of the spatially averaged incident power density in each frequency band to the corresponding MPE for that band is determined and the sum of all such ratios must be less than unity in order to comply.

R. C. Petersen
Manager, Wireless and Optical
Technologies Safety Department