For the last several months, we've been discussing the FCC limits on public and occupational RF exposure. The FCC is using the ANSI C95.1-1982 Standard Safety Levels. Stations can determine compliance with the standard through measurement of the electric and magnetic fields at all locations people are to occupy, or by making predictions. Some prediction methods are available in the FCC Office of Science and Technology Bulletin 65 (available from NTIS as publication PB#86-127081, call 800 336 4700 to order). The predictions in OST65 are worst case, so it is possible for a station to have predicted fields above the ANSI limits but still actually be within the limits. In cases where the predictions show possible exposure above the limits, measurements should be made.
As part of station license renewal, each station must certify that it does not impose a significant environmental impact (FCC Form 303-S, question 7) or submit an environmental assessment. Consultants I've spoken with have suggested doing whatever is possible to meet the ANSI spec so that an environmental assessment need not be submitted.
OST65 should be reviewed. The appropriate calculations should be made to determine where the areas of predicted excess fields are. These locations should be fenced off (with a nonconductive fence to avoid reradiation) and appropriate signs posted. Based on OST65, these locations will be around the bases of AM and FM towers.
A report should be written demonstrating the method used to determine compliance and the location of the restricted areas. I'd probably answer the renewal form question 7 something like this, "Using the methods contained in FCC OST65, it has been determined that the station complies with FCC specified guidelines for human exposure to radiofrequency radiation. A copy of the analysis is available on request." The report should carefully document the procedures used to determine compliance and what measures are taken to limit public and worker access to areas of excessive fields.
AM stations often must allow worker access to areas that are above the radiation limits (typically for reading tower base currents). It is possible to do this and still comply with the specifications by limiting the exposure time. The ANSI specifications do not limit "peak" exposure, but instead limit the exposure as averaged or integrated over a six minute period. It would be permissable to go to six times the limit for a one minute period if there were no exposure for the six minutes immediately surrounding the exposure (such as 2.5 minutes before and 2.5 minutes after). If there is continuous but varying exposure (such as when the worker walks up to the tower base, takes the reading, then leaves), the level should be determined at frequent intervals in the six minute period (ideally, the interval size approaches zero), and the average exposure determined. Consulting figures 1 through 3 of OST65, it appears that the close in field (magnetic or electric) doubles each time the distance to the tower is halved (which would be the case for an isotropic radiator). Based on this, we could estimate that a worker would be exposed to twice the ANSI limit at 2.5 meters from the tower base (using this "doubling" concept and table 1) of a 5 KW station. 1.25 meters from the base, the worker would be exposed to four times the limit. Using such an approach (or better yet, actual measurements), the compliance report should include details as to how these base current measurements are going to be made while continuing to comply. Instruction on reading base currents (including time limits) should be posted at the transmitter site for other engineers who may not be familiar with the details of your station (such as an FCC inspector!).
Research is being done into safe RF current levels in a human body. These currents are induced when someone climbs a hot tower. Substantial currents are also induced when someone climbs a floating tower of a DA when other towers are driven. At this point, it is suggested that climbing of hot AM towers not be permitted. Climbing of FM towers while the transmitter is on may be permitted, depending upon the distance from the antenna, the antenna pattern and power.
I've been told that an FM station has been fined (perhaps by OSHA) for allowing a worker on the tower in the immediate vicinity of a radiating FM antenna. The worker complained of feeling sick. It's quite possible that there was significant internal heating before the worker felt the effects of the heating, possibly causing damage to internal tissue. I'll try to dig up the details of this OSHA case. There may be effects of electromagnetic radiation, in addition to heating, that may have an adversely affect health. At this point, however, it is subject to considerable research. Stations must comply with the current ANSI specification (or submit an environmental assessment).