Radio Frequency Safety: An Update with a Recent Perspective

On many Vertical Access projects, we work on roofs or the sides of buildings where there are radio frequency (RF) antennas.  Part of our site-specific safety check always involves assessing the risk posed by RF antennas, and we have discussed Radio Frequency Safety in a previous newsletter article.  In most of our projects the small number, low power or location of the antennas relative to where we perform our work means that there is little risk from the existing antennas.  However, during a site visit a couple months prior to the scheduled field work for the hands-on investigation of the LeVeque Tower in Columbus, Ohio, we noticed a concentration of large antennas beyond what we typically encounter.  Below are some of the RF issues that were thoroughly discussed and reviewed both internally and with the project team prior to the inspection.  The information presented in this article, intended for those who may come in close proximity to antennas as part of their work but who do not necessarily have training in RF safety, is provided only as general background.  Further information is available from some of the links included in the article.  In addition, training including basic RF awareness is recommended for those who may be exposed to radio frequencies as an indirect consequence of their work near RF antennas.

RF antennas at top of the LeVeque Tower

The Federal Communications Commission (FCC) is the government body that is responsible for evaluating the effects of FCC-controlled transmitters on the human environment and for developing regulations for Radio Frequency Safety.  The FCC’s Rules and Regulations (Title 47 CFR) incorporate recommendations from organizations such as the American National Standards Institute (ANSI), the Institute of Electrical and Electronics Engineers, Inc. (IEEE), and the National Council on Radiation Protection and Measurements (NCRP).  In addition, the FCC’s Office of Engineering and Technology publishes information bulletins, such as OET Bulletin No. 56, “Questions and Answers About the Biological Effects and Potential Hazards of Radiofrequency Electromagnetic Fields.”

The FCC requires licensees to assure that that people are not exposed to RF power densities in excess of the applicable Maximum Permissible Exposure (MPE) limit.  The FCC defines two tiers of permissible exposures differentiated by the situation in which the exposure takes place and/or the status of the individuals who are subject to exposure.  General Population/uncontrolled exposure limits apply to those situations in which persons may not be aware of the presence of electromagnetic energy, where exposure is not employment-related, or where persons cannot exercise control over their exposure.  Occupational / controlled exposure limits apply to situations in which persons are exposed as a consequence of their employment, have been made fully aware of the potential for exposure, and can exercise control over their exposure. For an area in excess of 100% Occupational MPE, access controls such as locked doors, signage and administrative policies must be instituted.[1]

Electromagnetic energy includes radio frequency radiation, the portion of the electromagnetic spectrum in the 30 kHZ to 300 GHz range, and is present everywhere.  Occupational RF exposure limits relate to the hazard of bodily heating.  The eyes and testes are the most susceptible to heating due to low blood flow.  A secondary hazard of RF energy is the energization of conductive structures by strong electromagnetic fields.  Without any loose wires or stray currents, it is possible to get shocked by an energized structure.   RF energy may be dangerous if there is direct contact of an energized material, if the power density is high (such as from multiple sources) or if the exposure time is long.  Symptoms of RF exposure are similar to altitude sickness and can include listless or confused behavior, sore joints, dizziness, headaches, bad taste in mouth, blurred vision or nausea.

If working on a building where RF safety is a concern, it is prudent to request a compliance report.  The FCC mandates that RF emission levels for any site with an FCC-controlled transmitter be calculated and kept on file.  This file must be updated if there are changes to the RF environment.  An RF compliance report or emissions study provides additional information.  It is a comprehensive evaluation of the emitted electromagnetic energy and RF transmissions, with the findings compared to FCC guidelines for human exposure.

If and RF emissions study or compliance report identifies potential RF hazards, several steps can be taken to mitigate RF exposure when working near antennas.  The most straightforward action is to turn off the antennas.  Depending on the purpose of the antennas, this cannot always be achieved.  Understanding what types of antennas are present in a work area and the specific exposure associated with each antenna, it may be possible to create a temporary controlled access area to avoid entering an RF field that exceeds the MPE.  Another mitigation measure is for personnel working near transmitting antennas to wear RF suits.  These head-to-toe suits, composed of flameproof Nomex and stainless steel, attenuate the RF signals to reduce their effect and may be appropriate where there are strong RF fields.

Another measure to consider is the use of personal RF monitors.  Although personal monitors do not directly mitigate the RF exposure, they do indicate what the exposure at a particular location is.  When performing the exterior condition survey of the LeVeque Tower, each technician used a Nardarlert XT A8862 personal RF monitor, capable of detecting frequencies from 100 kHz to 100 GHz .  These monitors give an audible and vibratory signal when the field levels above a preset limit are detected.  If levels above a certain limit are detected, the user then knows that she should move to another area immediately or limit the amount of time in that area, depending on the reading.

It is important to know that the transmitting signals from two-way radios may exceed 100% MPE as detected by personal RF monitors.  We quickly discovered this at the LeVeque Tower when using our Motorola radios, which we always use for on-site communication between technicians.  In the case of our radios, this does not become a health concern unless the radio is used in transmission (talk) mode, as posed to the default receiving (listening) mode, more than 50% of the time it is on.  As a corollary, the antennas on the building may also interfere with two-way radio signals, creating static or complete disruption of the radio signal.

In the case of the LeVeque Tower, Turner Construction and the property manager shared our concern over RF issues and were extremely cooperative.  Prior to the scheduled fieldwork they commissioned an updated RF Compliance Report that focused on the areas where we anticipated working.  Most importantly, they turned off the most powerful antennas during our working hours to greatly reduce the RF exposure.  During the five days of fieldwork, with each technician wearing a personal RF monitor, we did not encounter RF fields above 100% MPE from the existing antennas.  The comfort gained by learning about general and site-specific RF issues before the inspection and the assurance provided from the personal RF monitors used during the inspection were key components to the successful completion of the project.

[1] Waterford Consultants, LLC, “On-Site RF Emissions Compliance Report,” pepared for LeVeque Tower, dated March 28, 2012.