Answer to Question #541 Submitted to "Ask the Experts"

Q

I am working in a laboratory that has many pieces of equipment that deal with sputtering, ion milling, plasma etching, etc. I know that there is RF radiation from these kinds of machines and would like to know if it will affect my health. I am trying to conceive a baby now and wish to know the effect of RF radiation and what are the things that I should be careful of. How can we measure RF radiation and what is the limit that is considered unsafe for a pregnant woman? Please advise. Thanks!

A

Although the devices of which you speak use RF energy, e.g., for generating plasmas, the exposure of people working with these devices is normally very low. Devices such as these normally operate in what are called ISM (Industrial, Scientific, and Medical) frequency bands. These bands are assigned by the Federal Communications Commission to ensure that leakage from devices operating at these frequencies will not interfere with sensitive radio and navigation equipment. The ISM frequencies most commonly used in the United States for industrial applications such as you describe are 13.56, 27.12, and 2450 MHz. Surveys of the RF leakage from devices used in semiconductor clean-rooms and laboratories demonstrates that the RF energy to which employees are exposed is far below the corresponding maximum permissible exposure values (MPE) for both occupational and exposure of the public. Shielding, including metalization of viewing ports on plasma etchers and similar devices, efficiently contains the energy within the reaction area. When RF fields can be found that approach the MPEs, it is usually only at isolated locations within a few centimeters of some portions of the equipment. Based on typical exposure patterns this is not considered hazardous. Exposure to RF energy below the MPE is considered safe—not only for the healthy worker but for pregnant women and the fetus as well. The overwhelming weight of scientific evidence unequivocally demonstrates that biological effects associated with exposure to RF energy are threshold effects, i.e., unless the exposure level is sufficiently high an effect will not occur regardless of exposure duration. This is quite different from ionizing radiation, e.g., x rays and nuclear radiation. Unlike exposure to ionizing radiation, repeated exposures to low levels of RF energy (or nonionizing radiation) are not cumulative. Thus, it is relatively straightforward to derive safety limits. By adding safety factors to the threshold level at which the most sensitive reproducible effect occurs, conservative exposure guidelines have been developed to ensure safety. At present, there are about 20,000 reports in the scientific literature that address the subject of RF bioeffects. These reports, most of which describe the results of animal, cell-culture, and epidemiology studies, have been critically reviewed by leading researchers in the field and all new studies are continuously being reviewed and interpreted by various groups and organizations whose interest is developing safety and health standards. The National Council on Radiation Protection and Measurements (NCRP), the standards committees sponsored by the Institute of Electrical and Electronics Engineers (IEEE Standards Coordinating Committee 28), the International Commission on Non-Ionizing Radiation Protection (ICNIRP—which is endorsed by the World Health Organization), the American Conference of Governmental Industrial Hygienists (ACGIH), and the National Radiological Protection Board of the UK are but a few examples. Most of these groups have recently either reaffirmed existing RF safety standards, developed and adopted new RF safety standards, or proposed similar RF safety standards. Despite the many thousands of studies that have been reported on all aspects of the subject since the first safety criteria were proposed in 1953, the exposure limits have not changed significantly. The exposure limits are biologically based and reflect a consensus interpretation of relevant studies from the bioelectromagnetics literature by qualified scientists, physicians, and engineers. Although the scientific consensus is that the RF-biological interaction mechanism is thermal in nature (effects result from tissue heating), the database evaluated by the expert panels includes some speculative studies reporting "nonthermal" effects. Classifications of findings are made without prejudgement of the mechanism of effects. The intent is to protect all exposed human beings (including pregnant women and the fetus) from harm by any mechanism, including those arising from excessive elevations of body temperature. Although there are small differences between the limits proposed by the various panels, the differences are engineering related—not philosophical regarding the biology. All contemporary western-world exposure limits are based on the exact same threshold levels, i.e., there is scientific consensus. The exposure to personnel working with these devices can be measured with commercially available portable devices called broadband RF field-strength meters, survey meters, or hazard probes. These devices simultaneously measure the total RF field across the entire portion of the spectrum over which they are calibrated, e.g., 300 kHz to 6000 MHz or much higher. Depending on the manufacturer, e.g., Holaday Industries or Narda, the display reads in "RF Power Density," "Field Strength Squared," (either of which can be compared directly with the MPE if the source frequency is known) or "percent of the MPE." The readings represent the total combined RF environment at the point of measurement. Finally, with respect to the last part of your question, the ACGIH and the IEEE MPEs for exposure in the workplace at the ISM frequencies normally used for plasma etching, ion milling, etc., are approximately 5, 1.2, and 8.2 milliwatts per square centimeter (mW/cm2) at 13.56, 27.12, and 2450 MHz, respectively. The corresponding IEEE MPEs for exposure of the public are one-fifth the above values. The MPEs are for continuous long-term, whole-body exposure. For short duration exposures the limits for the workplace are averaged over a six-minute interval; the limits for the public are averaged over a 30-minute interval. This means that for exposure durations less than the averaging time, exposures are permitted that are higher than the above values. Also, the MPEs are for whole-body exposure. The MPEs for the hands, wrists, feet, etc., are higher. Based on the above and anticipated exposure patterns based on experience with similar operations, there should be no cause for concern about working in this environment if pregnant. Because of the potential for any leakage to interfere with other sensitive or computer-controlled equipment in the laboratory, commercial equipment such as what you describe is well shielded. The RF levels that could cause interference are normally well below the MPEs. Ronald C. Petersen, Manager Wireless & Optical Technologies Safety Department