As of Sept. 1, 2000, virtually all stations licensed by the Federal Communications Commission were required to comply with the human radio frequency exposure standards. Those stations not now in compliance risk enforcement action by the FCC, including the possibility of substantial fines. If you are the licensee of a radio facility and you have not given this topic much thought up to now, then now is the time.
The FCC’s rules are intended to limit human exposure to RF energy. Applicants for new stations must certify to compliance when filing the application with the FCC. Existing stations must certify to compliance at license renewal time or when there is a modification to the transmitting facility that requires filing a license modification application with the FCC. Stations are subject to being in compliance with the rules at all times they are in operation.
Unlike ionizing radiation (x-rays or nuclear energy), which can cause permanent molecular changes in cells, nonionizing radiation (RF energy) is believed only to heat cells. This is much like a cup of water being heated in a microwave oven. When the oven is on, RF energy heats the water in the cup. When the cup is removed from the oven, the water cools down. Its molecular structure remains unchanged in the process. Similarly, as long as the amount of heating of human cells is limited to safe levels, the radio-frequency energy does no known permanent damage.
The amount of cell heating depends on several factors: the intensity of the RF field; the wavelength of the energy (the body is most susceptible to heating from energy between 30 MHz and 300 MHz); and the duration of the exposure. The FCC has adopted exposure guidelines based on these three variables. When RF field levels exceed the FCC prescribed thresholds, human exposure must be limited in duration. The rules do not set maximum values of RF energy that can be encountered; rather, the rules merely limit the amount of time one can be in the field. The rules also only apply to accessible areas. By definition, a nonaccessible area is in compliance.
The FCC rules recognize different exposure levels for areas accessible to the general public vs. areas that are restricted to trained personnel. The FCC defines those two areas as:
Accessible/Uncontrolled Areas: Areas in which the general public may be exposed or in which people who are exposed as a consequence of their employment may not be fully aware of the potential for exposure or cannot exercise control over their exposure.
Occupational/Controlled Areas: Areas in which people are exposed as a consequence of their employment and in which those who are exposed have been made fully aware of the potential for exposure and can exercise control over their exposure.
The Maximum Permissible Exposure levels (MPEs) allowed by the FCC are based on exposure times of 30 minutes for uncontrolled areas and six minutes for controlled areas. Threshold RF field levels vary depending on the frequency (wavelength) of the field. RF threshold levels for uncontrolled areas are one-fifth those for controlled areas, except for frequencies below 3 MHz, where the levels gradually become equal. The FCC provides charts of permitted exposure levels vs. frequency in its rule section 1.1310.
The MPE concept can be applied very easily to any situation. For example, the MPE level for controlled areas at 100 MHz is 1.0 mW/cm2. A trained worker could stay continuously in a field of this level for any six-minute period, regardless of how long the worker has been in the field or when the six-minute count begins. However, if the worker were in a field of 2.0 mW/cm2, the exposure would have to be limited to only three minutes and then the worker would have to leave the field for three minutes. A worker could stay in a field of 3.0 mW/cm2 for only two minutes and then would have to leave the field for four minutes.
To apply the above tables, one must know the level of the fields in question. The FCC has provided guidance on how to predict RF fields in a publication titled, “Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields, OET Bulletin 65, Edition 97-01.” This publication is available on the FCC Web page under the Office of the Chief Engineer. OET 65 provides two very useful charts that allow users to determine instantly the distance to the MPE for a given effective radiated power (ERP). The first chart (OET 65, Figure 1, Page 24) assumes no reflections that could add to the RF field level. This would be the case much of the time, as antennas are normally intentionally mounted in locations that are open and not next to reflecting surfaces. The second chart (OET 65, Figure 2, Page 25) assumes reflections and should be used when additive reflections can be expected.
To determine whether a facility meets the FCC exposure limits, three steps should be followed:
Determine if the station is “categorically excluded” from a routine evaluation.
If not categorically excluded, perform a routine environmental evaluation (RE).
If the site is not in compliance based on the routine evaluation either (a) bring the site into compliance, or (b) perform a full environmental analysis (EA).
Many land mobile stations operate at relatively low power levels and are assumed to pose no health hazard. Such stations have been categorically excluded from performing a routine evaluation to determine compliance with the rules. For most paging and private land mobile stations, any antenna on a tower with the base of the antenna at least 10 meters above ground is categorically excluded from a routine evaluation. For a roof-mounted antenna or tower-mounted antenna at any height, a station having an effective radiated power of 1,000 watts or less is categorically excluded from the routine evaluation. In both cases, the licensee can certify categorical exclusion to the FCC. For SMR, PCS and cellular systems, the sum of all power on all channels must be considered when computing the 1,000-watt limit. For paging and other private land mobile systems, each channel is considered separately. (See FCC Rules and Regulations, Section 1.1307 for a complete list of categorical exclusions.)
Even though all stations at a multiple-transmitter site may be categorically excluded, the site may not actually be in compliance when the fields from all the antennas are considered together. In such cases, to assure sitewide compliance, a routine evaluation would be necessary. It is important to realize that categorical exclusion from routine evaluation does not relieve a licensee from being in compliance with the RF exposure rules. Even though your transmitter may be categorically excluded, if the combined effects of all transmitters at a site make the site noncompliant, then each station at the site is potentially noncompliant.
Licensees must perform always a routine evaluation of stations that do not fall under the categorical exclusion. This evaluation will require summing the contributions of all transmitters at a site at all accessible points around the site. Hot spots can be determined by measurements, manual calculations, or software modeling. By far, the easiest procedure is to use software modeling. For example, the software developed by Fox Ridge Communications Inc. requires only entry of the parameters for each antenna. The software then computes the values on the rooftop or around the tower and provides a graphical display of levels along with a table of predicted values. Once identified, hot spots can then be brought into compliance, often by restricting access to the area, and the licensee can so certify to the FCC. Similarly, measurements can be made at the site. If the measurement method is used, it is important to be sure all transmitters are on the air during the measurements. Otherwise, the measurements may not accurately paint the true picture at the site. Licensees, not site owners or managers,
are responsible to the FCC for compliance.
Although the cal
culations or measurements needed to show compliance can be very complex and time consuming, sites can often be brought into compliance quite easily by application of simple mitigation procedures. For example, warning signs and other restrictions to limit access to high RF field areas will generally be sufficient to bring a site into compliance. Once restricted, those allowed access to high RF field areas must have training in proper work procedures at such sites and follow those procedures. Licensees should also have RF exposure compliance plans for their companies and each site should have a designated safety officer. One of the best ways to help assure that workers are aware of the RF levels at a site is to provide them with personal RF monitors that continuously time-average the RF levels encountered.
If the site cannot be brought into compliance, the FCC must be notified by performing a complete environmental assessment that fully describes the potential hazards and mitigating reasons why the FCC should grant the application. Because of the complexity of the EA, it is far better to bring a site into compliance than try to justify why it is not in compliance. Licensees should do everything possible to avoid having levels above the uncontrolled MPE levels in areas that are generally accessible to the general public.
Compliance with these regulations is not optional.
Ralph A. Haller is the president of Fox Ridge Communications Inc., 122 Baltimore Street, Gettysburg, PA, 17325. Phone (717) 334-7991, fax (717) 334-5656, e-mail [email protected], Web site http: www.frci.com. Fox Ridge Communications provides a variety of regulatory and technical services for companies in the telecommunications industry, including RF exposure modeling, RF exposure measurements and personnel RF awareness training. Prior to forming Fox Ridge Communications, Haller was with the Federal Communications Commission for 25 years, including three years a chief of research at the FCC Laboratory and nine years as chief of the Private Radio Bureau. Haller holds a B.S. in electrical engineering from the University of Kansas and is a member of the IEEE.
