Radio Frequency Emissions StudyWhenever a tradesman is confronted with either a roof deck bristling with antenna arrays or a single radio mast he should contact his employer or supervisor immediately. Safety inquiries should begin with the building owner to determine if the broadcast operators who are leasing rooftop antenna anchorage from him have: 1) conducted any radio frequency site modeling to verify compliance to the FCC exposure limits; 2) produced any hardcopy maps of the high-power near fields and low-power far fields associated with the antennas; and 3) have developed a written radio frequency safety plan for potentially exposed personnel (occupational and general public).
Hiring a professional electromagnetic radiation specialist or qualified personnel from the International Electrical and Electronics Engineers to survey, sample, analyze and map the roof for the incidence and field strengths of radiation would be cost-prohibitive and impractical for the typical small roofing contractor.
Exposure levels may be evaluated by characterization zoning using either theoretical modeling and/or physical site testing. They may also develop a comprehensive site safety plan calling for maximum permissible exposure limits for operations that may be conducted on the roof, such as repairs and replacements. Most emission studies provide a tower emission certification letter, acknowledging the calculated results and demonstrating site compliance and standard safety recommendations.
There are a number of expensive technical instruments that can accurately survey these radiation fields. Make sure you have seen their original and most current calibration certifications to verify that their readings are dependable. If abatement measures are taken, the reasonable protocol would be to continue the testing schedule until the job is complete and the roof worksite returned to its original state of transmissions. In any case, the contractor assigned to work on a roof where antennas are installed should always follow any host employer's (property owner) written site guidelines (if available) and accepted safe work practices listed in his radio frequency safety plan.
Employee TrainingAppropriate safety training is always a requirement of administrative hazard controls. It has been proven that giving your employees information about their worksite health and safety risks is the best way to minimize injury, illness and death. Everyone should have a basic understanding of: electromagnetic radiation awareness; steps to reduce exposure; understanding of the regulatory standards; and the most current equipment, hardware and technology. Workers who enter the electromagnetic radiation field infrequently (i.e, roofers) may additionally be given a "standard operating guidelines for working in electromagnetic environments." Property owners (host employer) may require the contractor's current verification of both site-specific and awareness training.
Personal MonitorsAfter awareness training for trades people exposed to roof top antennas, the next most practical level of protection would be the personal radio frequency monitor. The units are small, portable, and expensive ($1,500 and up). They have precalibrated alarm levels to indicate when the wearer has entered a radio frequency field in excess of the recommended maximum exposure limit of 1 milliwatt per square centimeter. Work around antennas should be conducted only after all possible signal emissions have been reduced or eliminated.
In large single antenna arrays, it is difficult to confirm zero energy state, and therefore work around these yellow and red zones will require the use of personal monitors to measure "real time" radiation exposure. Remember that wearing a monitor on your belt will not indicate if your head or hands are being irradiated, giving you a false sense of security.
Barricades and SignsTemporarily fence or barricade those areas of hazardous radiation detected in the radiation survey. Prominently post the appropriate NEMA/ANSI -Z535.3-1991 warning sign on all sides of the exposed work areas. These signs may list guidelines for safe work practices. Others may indicate: "Notice" as you enter the green zone; "Caution" as you enter the yellow zone; and "Danger" as you enter the red zone.
Try to maintain at least 3 feet of safety clearance from any antenna. Clearly identify and post a high-visibility warning line between the "near field" and the "far field" areas. It is at this junction where some studies suggest the "resonance factor" may maximize the body's absorption rate of radio frequency and microwave pollution. Know where that area of the roof exists at all times. Enter the "near field" briefly and only when necessary and under tightly monitored conditions. In some high dosage cases, wearing a personal dosimeter or monitory would certainly be recommended. Competent site monitors posted at the edge of the green zone should control access to ensure that only permitted, qualified personnel enter the yellow and red zones.
Alternative AntennasSometimes in some locations it is feasible to divert transmission, reception or relay power to an auxiliary antenna for the duration of the rooftop work. This is an undoubtedly expensive protection method and may not be feasible due to the lack of acceptable tower sites available. Remember that if power is transferred from one antenna to another in a cluster, the total power of all the transmitting antennas at the site is concentrated in the antenna that is closest to the areas in question. Any antenna's total transmission power is diminished as the distance from the transmitter is decreased.
Angle of IncidenceThere are relatively few high-power radio frequency and microwave satellite transmission antennas mounted on roofs. They are mostly government and industry-owned. The electromagnetic radiation from satellite and other communication antennas is highly directional and has a relatively narrow beam except for receiver antennas. Their power levels are generally 1,000 times below most recommended radiation guidelines. They are used for transmitting telephone and telegraph signals and linking broadcast and cable TV studios and their distant antennas. Although satellite transmission towers require high power levels, their high angle of incidence to reach their particular communication satellite usually eliminates most roofers' exposure. If the satellite is not geo-synchronous and the roofer does experience some exposure symptoms, work could be curtailed until the line of site to the relay satellite is periodically lost.
Power ReductionIn some instances lowering the power levels of the transmitter or increasing the filter on the relay station may be possible during certain times of the day. This will significantly reduce the roofer's level of exposure as the "near field" limit will be greatly reduced under low-power-transmission conditions.
Shutdown PeriodsTransmission shutdowns may often be scheduled on an annual basis in order to work on structural as well as electronic components of the transmitter, relay or tower itself. This administrative hazard control is an opportune time for the roofer to get his repairs done without any exposure to radiation. It may prove to be difficult, since other trades may also occupy a small roof area, but with adequate pre-shutdown scheduling and some creative problem solving it might be the most practical solution.
Work PositioningDirectional radio frequency antennas are typically mounted on the sides of masts about 50 meters high or on shorter masts on the rooftop of the taller buildings. Their energy discharge resembles a spotlight's beam and travels parallel to the ground. It may be a simple case of performing as much work as practical outside of its energy discharge path. The broadcast array of microwave antennas leaves a lot larger, conical pathway of energy. There is something called the "resonance factor" also at work here. That's when the amount of electromagnetic radiation that the human body may potentially absorb is maximized when the body is positioned several wavelengths from the radiation source. The effective frequency range for this resonance factor is 30-300 MHz.
Radiation ShieldingThis method of engineering hazard control is very effective during certain applications, such as maintenance periods where technicians are in close proximity to base station antennas, transmitter equipment rooms and adjacent areas. Appropriate shielding materials may absorb or reflect the radiation depending on their design. Their installation and usage are often problematic for rooftop workers located in the open environment around a cluster of antennas which tend to re-radiate signal energy.
Personal Protective EquipmentPersonal protective equipment should only be considered when all other engineering and administrative hazard controls have been exhausted. The clothing most utilized by exposed employers is made of Naptex®. It is a fabric made of polyester yarn wound coaxially around stainless steel fibers. It has been laboratory tested to provide between 10-12 dB of reduction in electromagnetic energy absorption at virtually any frequency over the radio and microwave frequencies. It is rated for use in environments 100 percent over the FCC occupational/controlled maximum permitted exposure values. The suit usually consists of coveralls, gloves, boots and visor hood. Employees should be properly fit tested and trained in donning/doffing prior to use.
Duration of ExposureThere are several studies currently being conducted on the supposition that just like the effects of an electrocution, the health effects of electromagnetic radiation are often dominated by the length and sequence of physical exposures. Remember, like any toxic effect on the human organism, dosage = concentration X length of exposure. Similar to exposing workers to ionizing radiation, carefully monitoring and logging your workers by measuring the concentration and length of exposure at that level will give the employer a reproducible and dependable means of administrative hazard control.
A workforce may be divided into smaller teams who may then work in shorter electromagnetic exposure duration. While their total exposure duration may be the same or even longer than the undivided workforce, the accumulation of the thermal effect is either eliminated or greatly reduced. The issue of cumulative exposures is not accurately addressed by many of the electromagnetic regulations drawn up by OSHA, ANSI and FCC. Scientists have yet to calculate an accumulated dosage that can be endured without ill effects.
ConclusionIn preparation for this article I spoke at length with Rick Tell of Richard Tell Associates Inc., Las Vegas (www.radhaz.com). He has 35 years of extensive radiofrequency experience in the private sector as well as the federal government (EPA). His firm is well known and respected and has many cellular and pager operators as well as large and small broadcasters throughout the United States as clients.
Tell's mission is to solve radio frequency antenna technical compliance issues. He agrees that the issue of electromagnetic radiation exposure, even to the experienced antenna worker, can be technically challenging. There are also many complicated regulatory standards to be carefully considered. The problem for any trade when they approach a roof full of antennas is that there may be no effective hazard communication program being implemented by anyone. The roofer who contracts with a building owner (controlling employer) to repair a roof may be dealing with a host employer who has very little (if any) technical data from the broadcaster who is leasing antenna space on his roof.
Tell suggests that after receiving site-specific radio frequency hazard training, the contractor should include in his pre-bid documents "requests for information" including several inquiries: 1. Does the building owner have the operator's radio frequency safety plan and exposure study (if any)? 2. Does the building owner have any standard operating guidelines for work to be performed on the roof (including worker awareness training)? 3. Is there a standard lock-out/tag-out policy for controlled potential energy sources at roof level to which the roofer may be exposed? 4. Are there any site-specific maps indicating roof areas to be avoided? 5. Is the radio frequency hazard information, which the building owner does possess, up to date?
Tell expresses a real concern for the intermittently exposed, inexperienced roofer, painter or electrician who is confronted with potential radio frequency hazards for a number of reasons. The most disturbing issue by far is how little the building owner actually knows (or cares to know) about the broadcast operators who are leasing his rooftop. Tell suggests that many, if not most, of the owners have little or no expertise in radio frequency technology, are predominantly ignorant of the FCC and OSHA regulations, and understand little of the hazards their own property may create for their hired contractors.
Tell was once doing a radio frequency survey for a broadcast operator, when he noticed an exterminator nonchalantly spraying for bugs closely around the masts. He obviously had no idea that he was in any sort of physical danger from hazardous radio frequency exposure. This example, he believes, is more the rule than the exception.
Sidebar: Definition of TermsElectromagnetic Radiation: Interconnected waves of electrical and magnetic energy moving together through space, generated by the movement of electrically charged particles with magnetic fields.
Far Field: A "free space" region of the antenna field where the electromagnetic radiation distribution is independent of the distance from the antenna; characterized by a plane-wave shape that spreads uniform shape of electric and magnetic fields across the direction of the beam.
Frequency: This is the number of signal wavelengths (cycles) produced by a transmitter in a measured length of time (seconds). This number indicates the frequency of energy in inverse proportion to the wavelength (strength) of the energy. Green Zone: This is a posted area with the lowest electromagnetic radiation exposure on site, where the work time and spatial-average exposure is below 20 percent of the occupational/controlled maximum permitted exposure. Workers in this area are not time-monitored but should be trained to the awareness level. Hertz: A hertz equals one cycle of the wavelength generated per second. A kilohertz is 1,000 hertz (cycles per second); a megahertz is 1 million hertz; a gigahertz is 1 billion hertz. Ionizing Radiation: This form of radiation generates intense, short wavelength energy (high frequency) sufficient to free ions. These rays exist above the visible light spectrum and include ultraviolet light rays, X-rays and gamma rays Microwave Radiation: A high frequency form of radio frequency radiation in the 300 MHz to 300GHz range.
Near Field: A region generally in proximity to an antenna or other radiation source, in which the electric and magnetic fields do not have a plane-wave character, but vary point-to-point.
Non-Ionizing Radiation: This category of radiation is located at the longer wavelength (low frequency) end of the electromagnetic spectrum below visible light and includes lower energy infrared, microwave and radio frequencies (AM, FM, UHF, VHF). This low frequency energy is unable to dislodge electrons and therefore cannot cause ionization or radioactivity in the body. Normal Magnetic Field: This is the magnetic field level in areas where human beings can be expected to be repeatedly present for a considerable length of time, such as housing, schools or work places. OSHA's Radiation Protection Guideline: For normal environmental conditions and for incidental electromagnetic energy of frequencies from 10 MHz to 100GHz, the radiation protection guideline is 10 milliwatts per square centimeter as average over any possible 0.1-hour period, whether the radiation is continuous or intermittent and applies to both whole or partial body irradiation. Power Density: A term associated with any electromagnetic wave indicating the power of the wave per unit area of a surface toward which it is projected (human body). Usually expressed in units of microwatts per square centimeter or milliwatts per square centimeter.
Radio Frequencies: This form of non-ionizing transmission is a type of electromagnetic energy with frequencies in the range of 3 kilohertz to 300 gigahertz. Radio frequency radiation is commonly absorbed in deep body organs. Red Zone: This is an exposure area requiring special work practices and engineering controls including. Specific Absorption Rate: This is the rate of energy absorbed by a unit mass (kilogram of human tissue) of the object over a specific period of exposure, expressed as watts per kilogram. Whole Body Irradiation: Pertains to the case in which the entire body is exposed to the incident eletromagnetic energy or in which the cross section of the body is smaller than the cross section of the incident radiation beam. Yellow Zone: While radiation fields in this area may be within acceptable limits, caution is required to define its limits and monitor for any changes in field strength or configuration. There would not be a yellow zone without a red zone in the vicinity.