The Electromagnetic Spectrum
- The research on RF radiation is extensive, and is adequate for establishing safety standards.
- Exposure to RF radiation can be hazardous if the exposure is sufficiently intense. Possible injuries include cataracts, skin burns, deep burns, heat exhaustion and heat stroke. See Reeves for a discussion of the known effects of overexpose to RF radiation in humans.
- Biological effects of RF radiation depend on the rate of energy absorption; and within a broad range of frequencies (1 to 10,000 MHz), the frequency matters very little.
- Biological effects of RF radiation are proportional to the rate of energy absorption; and the duration of exposure matters very little No biological effects have been consistently shown below a certain rate of whole body energy absorption (this rate is called the specific absorption rate or SAR)
Standards for Mobile Phone Base Stations
Now continued questions:
- Are there safety standards for mobile phone base station antennas?
ANS:Yes. There are national and international safety guidelines for exposure of the public to the RF radiation produced by mobile phone base station antennas. The most widely accepted standards are those developed by the Institute of Electrical and Electronics Engineers and American National Standards Institute (ANSI/IEEE), the International Commission on Non-Ionizing Radiation Protection (ICNIRP), and the National Council on Radiation Protection and Measurements (NCRP) These radio frequency standards are expressed in "plane wave power density", which is measured in mW/cm-sq (milliwatts per square centimeter) For PCS (about 1800-2000 MHz) antennas, the 1992 ANSI/IEEE exposure standard for the general public is 1.2 mW/cm-sq. For analog cellular phones (about 900 MHz), the ANSI/IEEE exposure standard for the general public is 0.57 mW/cm-sq The ICNIRP standards are slightly lower and the NCRP standards are essentially identical In 1996 the U.S. Federal Communications Commission (FCC) released radio frequency guidelines for the frequencies and devices they regulate, including mobile phone base station antennas. The FCC standards for mobile phone base station antennas are essentially identical to the ANSI/IEEE standard
The public exposure standards apply to power densities averaged over relatively short periods to time, 30 minutes in the case of the ANSI/IEEE, NCRP, and FCC standards (at mobile phone frequencies). Where there are multiple antennas, these standards apply to the total power produced by all antennas
A number of countries have their own regulations for public exposure to RF radiation from mobile phone base station antennas. While most of these regulation follow the same patterns and rationales used by ANSI/IEEE and ICNIRP they do differ.
With proper design, mobile phone base station antennas can meet all safety standards by a wide margin.
A mobile phone base station antenna, mounted 10 meters (33 ft) off the ground and operated at the maximum possible intensity, might produce a power density as high as 0.01 mW/cm-sq on the ground near the antenna site; but ground level power densities will more often be in the 0.00001 to 0.0005 mW/cm-sq range, These power densities are far below all the safety standards, and the standards themselves are set far below the level where potentially hazardous effects have been seen.
Within about 200 meters (650 ft) of the base of the antenna site, the power density may be greater at elevations above the base of the antenna site (for example, at the second floor of a building or on a hill). Even with multiple antennas on the same tower, power densities will be less than 5% of the FCC guidelines at all heights and at all distances of more than 55 meters (180 ft) from an antenna site.
Further than about 200 meters (650 ft) from the antenna site power density does not rise with increased elevation.
Power density inside a building will be lower by a factor of 3 to 20 than outside Petersen et al measured power densities around cell phone base stations. The measurements were for 1600 W (ERP) antennas on towers that ranged from 40 to 83 meters (130 to 275 ft) in height. The maximum power density on the ground was 0.002 mW/cm-sq, and the maximum was at 20 to 80 meters (65-265 feet) from the base of the towers. Within 100 meters (330) feet of the base of the towers, the average power density was less than 0.001 mW/cm-sq. These maximum RF power densities are all less than 1% of the FCC, ANSI/IEEE, NRPB and ICNIRP standards for public exposure.
- Are there circumstances where mobile phone base station antennas could fail to meet the safety guidelines?
ANS:Yes. There are some circumstances under which an improperly designed (or inadequately secured) mobile phone base station antennas could fail to meet safety standards.
Safety guidelines for uncontrolled (public) exposure could be exceeded if antennas were mounted in such a way that the public could gain access to areas within 6 meters/20 feet (horizontal) of the antennas themselves. This could arise for antennas mounted on or near the roofs of buildings. Petersen et al, for example, found that 2-3 feet (1 meter) from a 1600 W (ERP) roof-top antenna, the power density was as high as 2 mW/cm-sq (compared to the ANSI public exposure standard of 1.2 to 0.57 mW/cm-sq). For antennas mounted on towers, it is very difficult to imagine a situation that would not meet the safety guidelines.
- What siting criteria are required to ensure that a mobile phone base station antenna will meet safety standards?
- 10) What are some general siting criteria?
- Antenna sites should be designed so that the public cannot access areas that exceed the 1992 ANSI or FCC guidelines for public exposure. As a general rule, the uncontrolled (public) exposure guideline cannot be exceeded more than 6 meters (20 feet) from an antenna.
- If there are areas accessible to workers that exceed the 1992 ANSI or FCC guidelines for uncontrolled (public) exposure, make sure workers know where the areas are, and what precautions need to be taken when entering these areas. In general, this would be areas less than 6 meters (20 feet) from the antennas.
- If there are areas that exceed the 1992 ANSI or FCC guidelines for controlled (occupational) exposure, make sure that workers know where these areas are, and that they can (and do) power-down (or shut down) the transmitters when entering these areas. Such areas may not exist; but if they do, they will be confined to areas within 3 meters (10 feet) of the antennas.
The FCC guidelines require detailed calculations and/or measurement of radio frequency radiation for some high-power rooftop transmitters, and some high-power transmitters whose antennas are mounted on low towers In general, the above guidelines will always be met when antennas are placed on their own towers. Problems, when they exist, are generally confined to:
- Antennas placed on the roofs of buildings; particularly where multiple base station antennas for different carriers are mounted on the same building;
- Antennas placed on structures that require access by workers (both for regular maintenance, and for uncommon events such as painting or roofing).
- For roof-mounted antennas, elevate the transmitting antennas above the height of people who may have to be on the roof.
- For roof-mounted antennas, keep the transmitting antennas away from the areas where people are most likely to be (e.g., roof access points, telephone service points, HVAC equipment).
- For roof-mounted directional antennas, place the antennas near the periphery and point them away from the building.
- Consider the trade off between large aperture antennas (lower maximum RF) and small aperture antennas (lower visual impact).
- Remember that RF standards are stricter for lower-frequency antennas (e.g., 900 Mhz) than for higher-frequency antennas (e.g., 1800 MHz).
- Take special precautions to keep higher-power antennas away from accessible areas.
- Keep antennas at a site as far apart as possible; although this may run contrary to local zoning requirements.
- Take special precautions when designing "co-location" sites, where multiple antennas owned by different companies are on the same structure. This applies particularly to sites that include high-power broadcast (FM/TV) antennas. Local zoning often favors co-location, but co-location can provide "challenging" RF safety problems.
Work Practices for Reducing Radio-frequency Radiation Exposure (from Tell Individuals working at antenna sites should be informed about the presence of RF radiation, the potential for exposure and the steps they can take to reduce their exposure.
- "If radio frequency radiation at a site can exceed the FCC standard for general public/uncontrolled exposures, then the site should be posted with appropriate signs." [Per Richard Tell, personal communication, Feb 2000]
- Radio-frequency radiation levels at a site should modeled before the site is built.
- Radio-frequency radiation levels at a site should measured.
- Assume that all antennas are active at all times.
- Disable (lock out) all attached transmitters before working on an antenna.
- Use personal monitors to ensure that all transmitters have actually been shut down.
- Keep a safe distance from antennas. "As a practical guide for keeping [radio-frequency radiation] exposures low, maintain a 3-4 ft [1-1.2 m] distance from any [telecommunications] antenna."
- "Keep on moving" and "avoid unnecessary and prolonged exposure in close proximity to antennas".
- At some site (e.g., multiple antennas in a restricted space where some antennas cannot be shut down) it may be necessary to use protective clothing.
- Remember that there are many non-RF hazards at most sites (e.g., dangerous machinery, electric shock hazard, falling hazard), so allow only authorized, trained personnel at a site.
- How do you assess compliance with radio-frequency radiation guidelines for mobile phone base stations?
ANS:Compliance can be assessed through measurements or calculations. Both methods require a solid understanding of the physics of RF radiation, and measurements require access to sophisticated and expensive equipment.
Calculation: If the effective radiated power (ERP), the antenna pattern and the height of the base station antenna is known then "worst case" calculations of ground level power density can be made. However, the calculation method is not simple and the ERP and antenna pattern are often unknown.
Measurement: Actual measurement of power density from mobile phone base stations requires sophisticated and expensive equipment and considerable technical knowledge. The instruments designed to measure power line fields and the instruments designed to test microwave ovens are not suitable for measuring base stations. Determining that base stations meet ANSI/IEEE, FCC, or ICNIRP guidelines is "relatively easy", but the instruments required cost well over US$ 2000. Actual measurement of the power-density from a base station antenna is much more difficult, as there are many other sources of RF radiation at a typical site The U.S. Food and Drug Administrations (FDA) appears to have seen the WTR genotoxicity studies, and published the following comments on 20-Oct-99].
"Researchers conducted a large battery of laboratory tests to assess the effects of exposure to mobile phone RF on genetic material. These included tests for several kinds of abnormalities, including mutations, chromosomal aberrations, DNA strand breaks, and structural changes in the genetic material of blood cells called lymphocytes. None of the tests showed any effect of the RF except for the micronucleus assay, which detects structural effects on the genetic material. The cells in this assay showed changes after exposure to simulated cell phone radiation, but only after 24 hours of exposure. It is possible that exposing the test cells to radiation for this long resulted in heating. Since this assay is known to be sensitive to heating, heat alone could have caused the abnormalities to occur. The data already in the literature on the response of the micronucleus assay to RF are conflicting. Thus, follow-up research is necessary. [Tice et al. Tests of mobile phone signals for activity in genotoxicity and other laboratory assays. In: Annual Meeting of the Environmental Mutagen Society; 29 March 1999, Washington, D.C.; and personal communication, unpublished results.]."
Are there epidemiological studies showing that RF exposure from base stations is safe?
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