New developments in SAR test methods are bringing stricter limits and requirements, but more-accurate results.
Concern about human exposure to radio frequencies (RF) is not new. Ensuring the safety of RF devices is the primary motivation for new standards and test methods. The concept of specific absorption rate (SAR) has been around for many years, but recent developments have improved test methods. This article provides an overview of the current limits and test methods for SAR. Standards, specifications, and requirements are also discussed.
The heating effect from RF devices causes the most concern from an RF safety point of view. The human body counters local heating by thermoregulation (blood flow through the affected organs). The eyes and male testes are particularly susceptible to RF heating because these organs have no direct blood supply and, hence, no way of dissipating heat. The heating effects in biological tissue escalate with the increase in frequency, although the heat’s penetration depth decreases.
With the proliferation of cellular phones, most RF safety concerns have focused on RF absorption by the head, particularly from mobile handsets. The dose of RF exposure is linked to exposure time: maximum SAR is normally averaged over a 6-minute period during the 24-hour day.
Some concerns have focused on other effects of RF exposure. Most communications systems are pulse-like in nature, and their effects on brain function have been discussed recently. For example, the global system for mobile communications (GSM) frame rate, at 8.33 Hz, is close to that characteristic of alpha waves in the brain. Although there is no conclusive proof of such effects, considerable research is currently examining the effects of RF. Much of the research in this area was sparked by a report published by the Independent Expert Group on Mobile Phones, chaired by Sir William Stewart. The report, released in April 2000, is also known as the Stewart Report.
In the UK, nearly £7.4 million ($11.7 million) has been allocated from both government and industry sources to research the effects of RF. The LINK Mobile Telecommunications and Health Research (MTHR) Programme will be funded over a three-year period. The Programme Management Committee (PMC) was set up to advise on this research program. To date, PMC has published two calls for research proposals, and the first group of the projects is now under way. PMC has decided to issue a third call for research proposals. Much of this program’s research addresses the biological effects of RF on the human body. Currently, widely reproducible studies of RF effects on biological cells are not available.
The SAR Index
SAR is an index that quantifies the rate of energy absorption in biological tissue. SAR is expressed in watts per kilogram (W/kg¬1) of biological tissue. SAR is generally quoted as a figure averaged over a volume corresponding to either 1 g or 10 g of body tissue. The SAR of a wireless product can be measured in two ways. It can be measured directly using body phantoms, robot arms, and associated test equipment, or it can be mathematically modeled. Mathematical modeling of a product for SAR can be costly, and it can take as long as several months. Using conventional SAR test methods, a dual-band GSM 900 and GSM 1800 handset takes about one day to test to current standards.
Several organizations have set exposure limits for acceptable RF safety via SAR levels. The International Commission on Non-Ionizing Radiation Protection (ICNIRP) was launched as an independent commission in May 1992. This group publishes guidelines and recommendations related to human RF exposure.
For the American National Standards Institute (ANSI), the RF safety sections now operate as part of the Institute of Electrical and Electronic Engineers (IEEE). IEEE recently wrote one of the most important publications for SAR test methods.1
In the UK, the National Radiological Protection Board (NRPB) sets SAR limits. SAR limits are expressed for two different classes of people: workers (occupational/controlled exposure) and the general population (uncontrolled exposure). Because the general-population exposure is considered to be uncontrolled, the limit for this group is five times more stringent than the limit for the workers, whose environment and exposure can be monitored and controlled.
The limits are defined for exposure of the whole body, partial body (e.g., head and trunk), and hands, feet, wrists, and ankles. SAR limits are based on whole-body exposure levels of 0.4 W/kg¬1 for workers and 0.08 W/kg¬1 for the general population. Limits are less stringent for exposure to hands, wrists, feet, and ankles. There are also considerable problems with the practicalities of measuring SAR in such body areas, because they are not normally modeled. In practice, measurements are made against a flat phantom, providing a conservative result.
Most SAR testing concerns exposure to the head. For Europe, the current limit is 2 W/kg¬1 for 10-g volume-averaged SAR. For the United States and a number of other countries, the limit is 1.6 W/kg¬1 for 1-g volume-averaged SAR. The lower U.S. limit is more stringent because it is volume-averaged over a smaller amount of tissue. Australia, Canada, and New Zealand have adopted the more-stringent U.S. limits of 1.6 W/kg¬1 for 1-g volume-averaged SAR. Japan and Korea have adopted 2 W/kg¬1 for 10-g volume-averaged SAR, as used in Europe.
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