Answer to Question #11558 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
This question is in relation to your article on radiofrequency (RF) radiation exposure (http://hps.org/publicinformation/ate/faqs/radiofrequencyqa.html). I am familiar with the overall science, the U.S. Federal Communications Commission (FCC) calculations, and maximum permissible exposure (MPE) limits, as well as the use of personal protective equipment (PPE) when working in areas that exceed the MPE.
My question is this: Is there a certain recovery time that an individual who is not wearing PPE can wait so they can reenter an area where the MPE was exceeded? Is there a cooldown period for the body to dissipate the heat generated while within such an area?
For example, suppose a maintenance worker needs to repair something near a rooftop antenna, and the work area has an FCC MPE limit of six minutes. However, the job will take 10 minutes to complete. Can the worker stop the job at the six-minute mark, then go back into the area after a certain amount of time to finish the job?
After looking over many standards and publications, talking to local RF vendors/installers, and discussing this with our local Occupational Safety and Health Administration (OSHA) office, I cannot find an answer to this question. Is there some sort of standard or publication that addresses this?
The MPE values found in contemporary safety standards and guidelines for exposure to electromagnetic energy at radiofrequencies (e.g., FCC regulations, American National Standards Institute/Institute of Electrical and Electronics Engineers [ANSI/IEEE] standards, American Conference of Governmental Industrial Hygienists [ACGIH] threshold limit values [TLVs], etc.) ensure that the energy-absorption rate, spatially averaged over the entire body mass, is equivalent to about one-third of the resting metabolic heat production of an average human adult. As noted in IEEE Standard C95.1-2005,a this level is completely benign and will not increase the core body temperature by a measurable amount under almost all environmental conditions.
With respect to your question, in order to determine how long one must remain out of the field before returning, one would have to know exactly how the exposure compares with the MPE.
In instances where the exposure exceeds the MPE, perhaps for the example you cite (antenna work on the roof of a building), the requirements are met provided the exposure averaged over the whole body and over any six-minute period does not exceed the MPE. (The six-minute averaging time is based on cooling-time constants derived from the diathermy literature and from partial-body irradiation of test animals.)
For example, if the MPE is 5 milliwatts per centimeter2 (mW cm-2) and the exposure is 5 mW cm-2 or less, one can remain in the field indefinitely. If, however, the exposure is 10 mW cm-2 (two times the MPE), in order to comply one can only remain in the field for one-half the averaging time, i.e., a total of three minutes in any six-minute period. In a practical sense this means that one could remain in the field for three minutes, then stay out of the field for three minutes, and then return for three more minutes. If the exposure is 15 mW cm-2 (three times the MPE), one can only remain in the field for one-third the averaging time, i.e., a total of two minutes in any six-minute period. As above, in a practical sense, one could remain in the field for two minutes, then stay out of the field for four minutes, and then return for two more minutes. So from the measured power density and frequency, or power density expressed as a percentage of the MPE for the combined services, one can readily determine how long one can remain in the field during any six-minute period.
a Until 16 April 2016, the IEEE C95.1-2005 standard, which explains the origin and need for the averaging time, is available at no cost at http://standards.ieee.org/getieee/C95/download/C95.1-2005.pdf.
Ronald C. Petersen
IEEE Committee on Man and Radiation
Institute of Electrical and Electronics Engineers, Inc. IEEE standard for safety levels with respect to human exposure to radio frequency electromagnetic fields, 3 kHz to 300 GHz. New York, NY: IEEE; IEEE Standard C95.1-2005; 2006.