Keep your eye on RF risks
By Richard A. Quinnell, Contributing Technical Editor -- Test & Measurement World, 4/1/2006
It seems as if everything is going wireless: Wi-Fi has blossomed, Bluetooth has begun to catch on, and ZigBee and wireless USB are coming on strong. Along with this rise in the popularity of wireless communications has come an increased concern for controlling the safety hazards of RF energy—especially for handheld and wearable transmitters. Safety standards are now trying to catch up with the technology, and greater regulatory enforcement will likely follow.
The main safety concerns for human exposure to RF energy are the bulk effects of that energy on tissue, including the stimulation of peripheral nerves and muscles, the shocks and burns that can come from touching conductive parts, and the heating of tissue caused by the absorption of energy. RF energy may also have a negative effect on cellular DNA, resulting in an increased risk of cancer, but as yet there is insufficient data to prove the existence of such effects, and they are not addressed in current standards or regulations.
Determining the risksThe FCC has established two guidelines for guarding against the risks of RF energy. The first is maximum permissible exposure (MPE), which sets limits on the radiated fields to which people may be exposed (figure). These limits vary with frequency and assume different levels for controlled and uncontrolled exposure situations. The MPE covers exposure that occurs in the RF source's far field, and manufacturers must assess their products for compliance with these limits.
 |
| Maximum permissible exposure (MPE) limits help protect users against excessive RF exposure from fixed transmitters. Courtesy of ETS Lindgren. |
The second guideline involves evaluating human tissue's absorption of RF energy in near-field conditions (less than 20 cm from the antenna) to determine the body's specific absorption rate (SAR) for the transmitter under test. SAR measurement techniques include the use of a human model, and results are expressed in terms of watts absorbed per kilogram of tissue mass. This method is the one to use for assessing the safety of portable devices that a person holds or wears.
The SAR method has received most of the recent attention at the regulatory and standards level. In October 2005, for instance, the IEEE issued an amendment (IEEE 1528a) to its standard for determining the peak spatial-average SAR in the human head from wireless communications devices held next to the ear. The amendment includes a CAD file for a human head model (known as SAM Phantom) and resolves ambiguities in the original standard. The IEEE is also working on an amendment that extends the frequency range of interest to 6 GHz and defines additional procedures for measuring SAR.
A new standards project now underway, IEEE P1528.1, addresses the use of the Finite Difference Time Domain (FDTD) method for simulating spatial-peak SAR anywhere on the human body when SAR cannot be easily measured. The standard will allow for compliance assessment using FDTD and will describe FDTD concepts, anatomical models, techniques, validation procedures, uncertainties, and limitations. A second new standard under development, IEEE P1528.2, will describe the use of FDTD when the exposure is from vehicle-mounted antennas.
Enforcing the regulationsAlong with these new standards may come increased regulatory enforcement. Speaking at the January 2006 meeting of the Northeast Product Safety Society, David Seabury, senior project manager for ETS-Lindgren, pointed out that the FCC has increased its funding and test capability for performing on-site reviews of MPE compliance over the last several years. Seabury also noted that the Occupational Safety and Health Administration (OSHA) is now adopting the FCC's OET Bulletin 65 guidelines for MPE and SAR limits.
The FCC controls licensed transmitter installations with an eye toward public safety, while OSHA has jurisdiction over the workplace to protect individual workers. OSHA's involvement thus extends the application of SAR techniques beyond communications devices to include all RF sources, including induction heating, plasma processing, and welding.
Similar activities are underway worldwide. The International Electrotechnical Commission is developing IEC 62209-2 to extend its SAR limits as well as to cover the case of multiple, body-mounted transmitters. China is in the process of drafting its own, more restrictive, standard. And as wireless communications continues to grow and evolve, the standards and regulations are sure to follow suit.
