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A free electronic newsletter covering news and other topics for those interested in RF safety issues.
LIVE, Web-Based RF Safety Training
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Contents
What are medical devices? For the purposes of this area of the web site in the context of the issues that are described on this page, "medical devices" are electronic devices implanted in the human body. Pacemakers were the first implanted medical devices. When pacemakers first came into use, there were concerns that they would malfunction in the presence of significant electromagnetic fields. At the same time, the first microwave ovens were appearing in homes. And these early microwave ovens tended to leak energy around the edges of the door when food got trapped in the electronic gasket. Thus, it was common to see signs warning people with pacemakers that a microwave oven was in use.Susceptibility of medical devices to RF fields Today, microwave ovens rarely leak. And the device manufacturers have made the pacemakers more immune to interference from electromagnetic energy. But the problem with medical device interference has grown far more complicated for two reasons:
Over the years, many incidents of suspected electromagnetic interference (EMI) with medical devices have been documented. There is heightened concern for the safe and effective use of devices in the presence of electromagnetic energy. For medical devices the environment has become crowded with potential sources of EMI. Because of its concern for the public health and safety, the Center for Devices and Radiological Health (CDRH) part of the Food and Drug Administration (FDA), has been in the vanguard of examining medical device EMI and providing solutions. Extensive laboratory testing by CDRH and others has revealed that many devices can be susceptible to problems caused by EMI. The CDRH began investigating incidents with the advent of cardiac pacemakers in the the late 1960's. Quoting the CDRH "The key to addressing EMI is the recognition that it involves not only the device itself but also the environment in which it is used, and anything that may come into that environment. More than anything else, the concern with EMI must be viewed as a systems problem requiring a systems approach. In this case the solution requires the involvement of the device industry, the EM source industry (e.g., power industry, telecommunications industry), and the clinical user and patient. The public must also play a part in the overall approach to recognizing and dealing with EMI. Electromagnetic compatibility, or EMC, is essentially the opposite of EMI. EMC means that the device is compatible with (i.e., no interference caused by) its EM environment, and it does not emit levels of EM energy that cause EMI in other devices in the vicinity. The wide variation of medical devices and use environments makes them vulnerable to different forms of EM energy which can cause EMI: conducted, radiated, and electrostatic discharge (ESD). Further, EMI problems with medical devices can be very complex, not only from the technical standpoint but also from the view of public health issues and solutions." The strength of an electromagnetic field at a given distance from the source is proportionate to the radiated power from the source and inversely proportionate to the distance. In many cases the field strength falls off inversely with the square of the distance from the source. The CDRH gives some good examples to illustrate the point. Note that an electric field strength of 3 V/m (Volts per meter) is often referenced because it is the most common standard that is referenced for medical device immunity.
It is important to note that device susceptibility can be very frequency dependent. This makes it much more difficult and expensive to test for. For example, a particular device might have good immunity at all but one or two narrow frequency ranges. Unless tests are made in small increments of frequency, the problem might not be detected. Conversely, the problem might not occur in the real world because there is nothing operating at the problem frequencies. Regulations and standards
Issues While the major focus of the CDRH and the IEC has been in hospital settings where there is a great deal of sensitive medical devices (not just implanted medical devices) there is also a concern for people that work in or visit RFR "sites". Consider that even if a site is fully compliant with any of the major standards, these standards are designed to protect people from the biological effects of RF radiation—not from EMI with their medical implants. For example, most of the major standards have limits in the human resonant region of 1.0 mW/cm² for Occupational exposures and one-fifth of that level (0.2 mW/cm²) for the General Population. An equivalent power density of 0.2 mW/cm² is equal to an electric field strength of 27.4 V/m. Looking at it the other way, a 3 V/m field is equivalent to an equivalent power density of 0.0024 mW/cm². This is about one percent of the public limits and 0.2 percent of the occupational limits! |
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