Understanding Radio Frequency Energy Exposure
Are you concerned about cellular antennas? Decades of research on cellphones and cancer have not found a link between the two, but that hasn’t stopped some communities from creating laws and public service campaigns regarding protection of the public from cellular system threats. What these actions have done is created a sense that the risk exists, leading to much concern and confusion for the public. There are risks, and they are not to be ignored, but many of them are misunderstood.
As communications technology continues to develop, its next iteration – 5G – is already here. The idea of 5G is better coverage using smaller, low-power, overlapping range with multiple antennas. This is the same technology used in large offices and hospitals to overcome the cellphone signal shielding caused by buildings. The buildings have numerous low-power, overlapping antennas that ensure cellular signal communications. The communications industry needs more mounting locations, and utility poles are the obvious answer. 5G is more of a physical hazard than a radio frequency (RF) hazard because it includes a powered cabinet on the pole wired to the antenna above, creating more congestion on the structure for climbers. I receive lots of questions and rightly so because line personnel are finding themselves looking at antenna installations where they have never seen them before. 5G is very low energy compared to other RF emitters but should not be ignored. Most of the 5G hazard is the antenna at the top of the pole, which can be anything from a 30-foot light pole to a 60-foot transmission pole. The obvious precaution, as with any antenna, is to not put yourself in the antenna beam. So, a 360-degree 5G antenna is like any 360-degree antenna: Don’t put your body in the beam.
Ionizing and Non-Ionizing
Since we can’t address all types of energy or antennas in this article, let’s look at some general information and precautions. Panel antennas are easier to avoid because they have a directional focus. The tubular 5G antenna is a little more difficult to avoid because it operates at 360 degrees.
Radiation that’s non-ionizing is of little risk. It is too weak to affect chemical bonds in the human body. That includes ultraviolet, visible light, infrared and everything with a lower frequency, like radio waves. Technologies like power lines, FM radio and Wi-Fi also fall into this range. Microwave falls into this class, too, but is an exception. It is the only non-ionizing radiation that can cause tissue damage. That’s because microwave is precisely tuned to resonate with water molecules. Frequencies above ultraviolet, like X-rays and gamma rays, are ionizing.
Microwave, radio, cellular tower and television tower antenna signals are ionizing RF energy. RF radiation will blind workers and quickly burn and destroy human tissue – both external and internal – depending on the frequency and strength of the RF energy field. You can’t see RF energy nor can you hear it. There are personal RF energy exposure detectors available. But even with a detector, you can boom into a high-energy field and be injured before the detector can warn you. You must understand what RF hazards are and be able to recognize where the energy is to avoid a hazardous exposure.
RF Health Effects
The Health Physics Society explains that the quantity used to measure how much RF energy is absorbed in a body is called the “specific absorption rate” (see https://hps.org/hpspublications/articles/rfradiation.html). According to the HPS, “It is usually expressed in units of watts per kilogram (W/kg) or milliwatts per gram (mW/g). In the case of whole-body exposure, a standing human adult can absorb RF energy at a maximum rate when the frequency of the RF radiation is in the range of about 80 to 100 MHz.”
In the same article, the HPS also states that “[b]iological effects that result from heating of tissue by RF energy often are referred to as ‘thermal’ effects. It has been known for many years that exposure to very high levels of RF radiation can be harmful due to the ability of RF energy to rapidly heat biological tissue. This is the principle by which microwave ovens cook food. Tissue damage in humans could occur during exposure to high RF levels because of the body’s inability to cope with or dissipate the excessive heat that could be generated. Two areas of the body, the eyes and the testes, are particularly vulnerable to RF heating because of the relative lack of available blood flow to dissipate the excessive heat load.”
RF Hazard Training
All employees working in the vicinity of RF sites shall be trained to recognize and be aware of the presence of RF/microwave exposures. In 29 CFR 1910.268(p)(2), “Hazardous area,” OSHA requires placement of warnings, stating the following: “Accessible areas associated with microwave communication systems where the electromagnetic radiation level exceeds the radiation protection guide given in §1910.97 shall be posted as described in that section.” The issue is that the rule is for “areas associated,” which are the microwave facilities themselves. There is no requirement for the owner of the antenna to look downstream and post warnings. That leaves the task of warning up to the employers of potentially exposed employees. OSHA requires that all employees exposed to such hazards have awareness training prior to working in the vicinity of transmitting antennas. Specifically, rule 1926.967(k)(1)(iii) states the following: “When an employee works in an area where the electromagnetic radiation could exceed the radiation protection guide, the employer shall institute measures that ensure that the employee’s exposure is not greater than that permitted by that guide. Such measures may include administrative and engineering controls and personal protective equipment.” Of course, this is not always easy to accomplish when the antenna is a foreign installation on a pole or structure. It is often the case that the utility department that approved the installation is not in touch with the people that work on the structures – thus the OSHA rules. Part of the exposure issue is that when workers are on a transmission structure, that structure may be remote from an antenna, but it may still be within the hazard range for workers even over a short period of time.
What to Look for in the Field
Antennas used for cellular and paging/personal communications service (PCS) transmissions are typically located on towers, water tanks and other elevated structures, including rooftops and the sides of buildings. A cellular base station may utilize several omnidirectional tube antennas that are 4 or more inches in diameter and 10 to 15 feet in length.
In urban and suburban areas, cellular service and PCS providers commonly use sector antennas for their base stations. These antennas are rectangular panels, about 1 foot by 4 feet in size, typically mounted on a rooftop or other structure, but they also are mounted on towers and poles. Panel antennas are usually arranged in three groups of three each. It is common that not all antennas are used for the transmission of RF energy; some antennas may be receive-only.
The RF emissions from cellular or PCS base station antennas generally are directed toward the horizon in a relatively narrow pattern in the vertical plane. In the case of sector (panel) antennas, the pattern is fan-shaped, like a wedge cut from a pie. As with all forms of electromagnetic energy, the power density from the antenna decreases rapidly as one moves away from the antenna.
Radio and television broadcast stations are always antennas mounted on purpose-built towers 100 to 500 feet tall. Broadcast stations transmit their signals via RF electromagnetic waves at various RF frequencies, depending on the channel, ranging from about 540 kHz for AM radio up to about 700 MHz for UHF television stations. Frequencies for FM radio and VHF television range from less than a watt to more than 100,000 watts. Some of these transmission systems can be a significant source of RF energy within the antenna beam for hundreds of yards. That is usually where lineworkers become exposed.
The level of exposure depends on several factors, including the type of station, design characteristics of the antenna, power transmitted to the antenna, height of the antenna and distance from the antenna. Since energy at some frequencies is absorbed by the human body more readily than at other frequencies, both the frequency of the transmitted signal and its intensity are important.
The antenna’s wave signal is pie- or cone-shaped and aimed at the horizon. If you are near an antenna, imagine a line from the center of the antenna to the earth’s horizon. That will be the beam center. You cannot know how wide the beam is. It widens as it leaves the antenna, but it also weakens.
RF Warning Signs
Look for RF warning signs at antenna facilities. The warning sign symbol for RF radiation hazards shall consist of a red isosceles triangle above an inverted black isosceles triangle, separated and outlined by an aluminum color border. The words “Warning: Radio Frequency Radiation Hazard” appear in the upper triangle. There is no standard requirement, but safety information is expected in the lower black panel and can include wattage output, safe clearances or contact information for the operator. You will most likely find operator contact information.
Where lineworkers may be exposed above safe limits, cell transmitters should be locked out to protect workers from radiation energy hazards. Quality of service can be diminished for the cellular customer, but in metropolitan areas, cellular system operators usually can continue service to customers even if one tower in a local area is offline. Remote areas often relay in a hopscotch mode, so one antenna offline may have more impact, especially for the crew working in that area, so keep that in mind if your rescue plan relies on cellphones.
When you know their contact information, cellular transmitter owners/operators can provide energy hazard information, approach angle and minimum approach proximity to prevent radiation hazards to workers.
Pre-job hazard analysis near cellular sites shall include recognition of radiation energy hazards, the safety practices to be employed, the limits to radiation exposure and how to protect exposed workers from radiation hazards.
Where RF warning signs are present, look for information on the signs advising of the nature of the potential hazard, how to avoid the potential hazard and who to contact for additional information. Warning signs also may provide instructions that direct individuals as to how to work safely in the RF environment of concern. RF energy warning signs typically are located prominently in areas that will be readily seen by persons who may have access to an area where high RF fields are present.
General Antenna Clearance Guidelines
Following are some general antenna clearance guidelines that readers should familiarize themselves with:
- Unless the operational characteristics of a particular antenna are understood and exposures are known to be below the appropriate maximum permissible exposures, personnel should remain at distances greater than the standoff distances shown below:
- Within 10 feet or less directly in front of a directional (sector or square-face, panel-type) antenna.
- Out of the center beam and more than 50 feet from the main beam of a horn-reflector or parabolic-reflector (dish) antenna.
- Leave the area if you sense unusual heating of the skin from the direction of an antenna.
- Check Federal Communications Commission warning signs at antenna facilities to see if site modeling or monitoring has been performed and the clearances listed.
- Use personal monitors if work is to be performed where the possibility of exposure exists.
- Assume all antennas are active unless you have contacted the operator and performed a lockout and tagout of the antenna power supply.
- Use of a personal monitor can detect the presence of strong electromagnetic energy fields.
- The FCC’s OET Bulletin 65 establishes human exposure limits for various types of RF energy (see www.fcc.gov/general/oet-bulletins-line#65). Many personal RF monitors are calibrated to these standards.
- Do not rely on field-strength personal RF monitors to determine if an antenna is active. If not locked out, an antenna can begin transmissions at any time without warning.
- If a personal RF monitor issues an alarm, the affected employees shall immediately remove themselves from the vicinity of the antenna.
OSHA Rules on Microwave Energy
OSHA has several rules regarding microwave energy that readers should be aware of.
Paragraph 1926.967(k)(1)(i) regarding microwave transmission states the following: “The employer shall ensure that no employee looks into an open waveguide or antenna connected to an energized microwave source.”
Paragraph 1910.268(p)(3), “Protective measures,” states, “When an employee works in an area where the electromagnetic radiation exceeds the radiation protection guide, the employer shall institute measures that insure that the employee’s exposure is not greater than that permitted by the radiation guide. Such measures shall include, but not be limited to those of an administrative or engineering nature or those involving personal protective equipment.”
Maximum permissible exposures are measured by centimeters of skin exposure in milliwatts squared. That is body or skin exposure. OSHA has published a rudimentary guide in 1910.97. These limits may not be enough to protect the eyes of a worker looking toward a transmitter while in the antenna’s transmitting beam.
In 1910.97(a)(2)(i), which focuses on a radiation protection guide, OSHA states the following:
“For normal environmental conditions and for incident electromagnetic energy of frequencies from 10 MHz to 100 GHz, the radiation protection guide is 10 mW/cm.2 (milliwatt per square centimeter) as averaged over any possible 0.1-hour period. This means the following:
- “Power density: 10 mW./cm.2 for periods of 0.1-hour or more.
- “Energy density: 1 mW.-hr./cm.2 (milliwatt hour per square centimeter) during any 0.1-hour period.
- “This guide applies whether the radiation is continuous or intermittent.”
Paragraph 1910.97(a)(2)(ii) states, “These formulated recommendations pertain to both whole body irradiation and partial body irradiation. Partial body irradiation must be included since it has been shown that some parts of the human body (e.g., eyes, testicles) may be harmed if exposed to incident radiation levels significantly in excess of the recommended levels.”
There is a hazard to power-line workers from RF energy. It may not be at every location, but workers must be able to recognize where it is. Just like all worker safety programs, the employer must have a defensible training plan in place proportional to the exposure of the employees. If you are an employer or hold the employer’s safety role, survey your system for RF hazards. Work with antenna owners to identify the nature of the hazards and establish training and protections for workers who may be exposed.
About the Author: After 25 years as a transmission-distribution lineman and foreman, Jim Vaughn, CUSP, has devoted the last 22 years to safety and training. A noted author, trainer and lecturer, he is a senior consultant for the Institute for Safety in Powerline Construction. He can be reached at [email protected].