Microwaves Ä What are they, where do they come from, and are they a hazard? John Mills c. 1993 John Mills Clarisa Moore is 12 years old and has an enquiring mind. She noticed the students in her high school gathered around the microwave oven to warm their lunches.She wondered if the oven was leaking microwaves. She also was interested in knowing if microwave radiation is dangerous. This was to become her science project and, in order to help her, I came up with the coffee "Cantanna" microwave leakage detector. Just what are Microwaves? Just what are microwaves? They are non-ionizing electromagnetic radiation similar to light, heat or radio waves. They were originally discovered by Heinrich Hertz in 1888. Professor Hertz built the first radio transmitter and receiver which just happened to work in the microwave band. At first microwaves were thought to be useless, but, just before the second world war, scientists realized that microwave pulses could be used to detect aircraft. The subsequent development of radar in the 1940s and 50s was largely responsible for bringing high power microwaves into our environment. Several generations back our ancestors lived in an environment almost free of microwaves. True, minute amounts come from the sun, the stars, and warm objects but this level is so incredibly small that it is almost, but not quite, nonexistent. Many types of radiation bombard us daily from natural sources and it is curious that so little occurs in the microwave region. For some reason we have been relatively sheltered from this type of energy throughout human history. Early radar researchers discovered that microwaves could penetrate nonmetallic substances and heat them from the inside. Dr. Percy Spencer, while working for the Raytheon Corporation in the early 1940s, noticed that a candy bar melted in his pocket when he was near a microwave source. In 1954 Raytheon Corporation produced the first practical microwave oven. In 1967 Amana introduced the Radarangetm. Consumers liked the idea enough to make this a successful product. In response to safety concerns in the late 1960s the industry and the U.S. government instituted design changes and testing programs to improve things. Interlock switches were designed to insure that the oven would not operate with the door ajar. New hinges, latching mechanisms and door gasket designs improved long-term reliability. All ovens were tested at the factory for excess leakage. Today the food and drug administration assures us, "In F.D.A.'s experience most ovens tested show little or no detectable microwave leakage."(1) This sounds great. It All Depends on How You Look at It Before we make a safety judgement, however, we need a frame of reference. The question of how little is a little or how much is a lot is important. In testing microwave ovens the F.D.A. and industry use instruments that are very accurate but very insensitive. These meters respond to the r.m.s. or average heating value of the leakage being tested. Our "Cantanna" is a different kind of animal. Instead of measuring averages it measures peaks and is about 1000 times more sensitive than professional instruments commonly used to measure oven leakage. If you measured a number of ovens with the "Cantanna" you would find that virtually all would be leaking. So here we see that the type of meter one uses can affect the test results and our interpretation of them. To illustrate this important difference in another way, suppose we measured the average depth of a river and found it to be 2 feet. That sounds quite safe. If, on the other hand, we measured the maximum places we might find treacherous spots 20 feet deep. Both systems of measurement are accurate and honest but they might well lead us to different conclusions about the safety of the river. Microwave measurements have this same problem. It's quite easy to visualize a 20 foot deep spot in a river but few people can visualize the significance of a leakage measurement. Microwaves are normally measured in milliwatts or microwatts per square centimeter at a 5 centimeter distance. Let's say your oven is leaking 100 microwatts. Is that a lot, a little, or what? Looking at it one way it is 50 times lower than the current U.S. government safety limit. That's reassuring. Looking at it another way it is perhaps 100 billion times higher than the natural microwave level of times past. That's scary. Microwave Health Effects Just how dangerous are microwaves? What are the health effects? Anything that can cook a meatloaf can surely be regarded as a potential health hazard. The cornea of the eye and the testes seem to be especially sensitive to damage by heating. If a candy bar in your pocket ever starts melting from microwaves you might be in immediate danger, but this is not likely to occur. The exposure that you would receive from being within 10 feet or so of an operating microwave oven meeting current leakage standards would cause no detectable heating. It would, however, be perhaps 100 million times higher than the natural level. This is considered "low level" exposure. Has low level exposure been associated with health problems? Yes. It has been known since the 1950s that low level pulsed microwaves can be sensed or felt by humans and animals. In the late 1950s the Russians examined workers in their microwave industry and concluded that low level microwave exposure: can effect the sense of smell, the stamina of rats, has an exhausting effect on the central nervous system, was associated with hypotension, alterations in hearing, and had various effects on the thyroid gland.(2) The U.S. F.D.A. notes that some researchers have found low level exposure associated with: genetic changes, an immune response, and decreased ability to perform certain tasks. A well-known American researcher, Allan H. Frey, recorded brain wave changes on microwave exposure and noted that humans can hear microwave pulses. Recent media reports have linked microwaves from hand-held cellular telephone units to brain cancer. (3) Cellular phones emit microwaves at a frequency of 870-890 MHz while microwave ovens operate at 2450 MHz. It has been suggested that microwave energy does not cause cancer directly but rather promotes its growth. The Lid on the Can of Worms Other scientists are more cautious. James Jauchem, a research physiologist working for the U.S. Air Force concludes, "scientists should attempt to place potential health risks in their proper prospective and present a balanced account of the data." (4) One researcher who seems to have aimed at a balanced account is Nicholas Steneck. He is a professor of history at the University of Michigan and has examined the microwave problem with grants from the National Science Foundation and the National Endowment for the Humanities. Professor Steneck is concerned that industry and military control of microwave bioeffects research in the U.S. has resulted in too little study of low level exposure. He writes, "The scientific community has allowed social, economic, and political pressures to influence its activities thereby destroying the credibility of its product." (5) If this is true, it is not surprising. Government and industry are heavily committed to microwave technology. Any new studies that might question the safety of our present products or systems would most likely be seen as a can of worms. You, or Miss Moore standing in front of a leaking microwave oven, "Cantanna" in hand, might see things in a different light. You might wonder if pulsed microwave exposure at school could effect young people's memories. Could microwave exposure of sick people in hospitals or the elderly alter the healing process? How much microwave energy would an unborn child receive from a counter top microwave oven and would this have any effect? Do microwave chefs have an impaired sense of smell? Good questions all, and deserving of honest answers. What Can We Do? We could try not to worry and hope for the best, or we could build a "Cantanna" and check our homes and work places for microwave leakage. The "Cantanna" can check cellular phones and also identify "hot spots" in your kitchen that have higher levels of leakage from your oven. It's a great project that can help young people become aware of microwaves in our environment. It is sensitive enough to detect most ovens at a distance and perhaps even from another room. Remember that microwaves can pass through most walls with little loss in power. When you move twice as far away from an oven the microwave leakage becomes one quarter as strong. This suggests that standing away from an oven while it is operating is one of the simplest ways to minimize personal exposure. While preparing material for this article I found that there are quite a few concerned and conscientious people in industry, the government, and the military. Many of them welcome our comments, so we might consider making some calls or writing some thoughtful letters. What about improved microwave ovens? Yes, it is possible. Clarisa and her dad were able to accomplish just that. They built a cage out of aluminum window screening and wood. It had a tight-fitting, latching door and some small microwave absorbers made from salt water soaked paper towels or fine steel wool placed inside plastic sandwich bags. These absorbers were placed outside the oven but inside the wire cage around the door area. By using these simple low cost techniques they were able to reduce the oven leakage to less than one tenth of what it was originally. If you are able to shield your microwave oven so that no leakage is detectable with the "Cantanna", give yourself a pat on the back. Your unit will be about 5000 times lower in leakage than the average. Some final words of caution: you should never make any changes or modifications to the oven itself that might interfere with its safety systems. If you suspect that your oven is damaged or leaking excess microwave radiation the safe thing to do is to have it checked out by a service facility with a calibrated meter. Good luck, have fun, and stay safe. Access Author: John Mills, 10475 Vineland Rd., Ben Lomond, CA 95005 The author is grateful to the following for providing material and information related to this article. ù C.D.R.H., Food and Drug Administration, Department of Health and Human Services, Rockville, MD 20857 USA ù R. Matthes, Federal Office of Radiation Protection, Department of Radiation Hygiene, Ingolst„dter Landstrasse I, D-8042, Neuherberg, Germany ù Bureau of Radiation and Medical Devices, Room 238A, 775 Brookfield Road, Ottawa, Ontario K1A 1C1, Canada ù Australian Radiation Laboratory, Lower Plenty Road, Yallambie, VIC. 3085, Australia ù Department of the Air Force, Headquarters Human Systems Center (AFMC) Brooks Air Force Base, Texas, USA ù Naval Aerospace Medical Research Laboratory, Naval Air Station, Pensacola, FL 32508 - 5700 USA ù General Microwave Company, 5500 New Horizons Blvd., Amityville, NY 11701 USA ù Holaday Industries Inc., 14825 Martin Drive, Eden Prairie, MN. 55344 USA ù Loral Microwave - Narda, 435 Moreland Road Hauppauge, NY 11788 USA ù Amana - A Raytheon Company, Amana, Iowa 52204 USA ù 319-622-2142 ù General Electric Company, Appliance Park, Louisville, KY 40225 USA ù 502-452-4557 ù Frigidaire Company, POB 7181, Dublin, OH 43017-0781 USA ù Maytag Company, One Dependability Square, Newton, IA 50208 USA ù 515-791-8588 References 1. Microwave Oven Radiation, F.D.A. HHS Publication No (FDA) 86-8120-1986 2. The Biological Action of Ultra High Frequencies, A.A. Letavet and Z.V. Gordon, Institute of Labor Hygiene and Occupational Diseases of the Ams USSR, Academy of Medial Sciences, USSR, Moscow 1960 3. "Cellular Phone Industry Rocked by Cancer Scare", San Jose Mercury News, January 31st, 1993 4. "Alleged Health Effects of Electromagnetic Fields; Misconceptions in the Scientific Literature", J. Jauchem, Journal of Microwave Power and Electromagnetic Energy, Vol. 26, No. 4, 1991 5. The Microwave Debate, Nicholas H. Steneck, MIT Press, 1984. Other points of view and sources of information: ù Cross Currents, Robert Becker, Jeremy P. Tarcher Inc., 1990 ù The Body Electric, Electromagnetism and the Foundation of Life, Robert Becker and Gary Selden, William Morrow and Company, 1985. ù The ARRL UHF/ Microwave Experimenter's Manual, American Radio Relay League, 1990 ù Pocket Guide to Non-Ionizing Radiation, General Microwave Company, Amityville, NY 1992 ù The Zapping of America, Paul Brodeur, W.W. Norton, 1977 ù Practical Microwave Oven Repair, Homer L. Davidson, Tab Books, 1st Ed., 1984. Sidebar Some Technical Notes About Microwaves and Their Measurement Microwaves are a non-ionizing radiation comprising part of the electromagnetic spectrum from about 0.3 GHz to 300 GHz. GHz stands for gigahertz or one billion vibrations per second. All electromagnetic waves travel through free space at a speed of about 300 million meters per second. The length of any given wave in space (LAMBDA) is the speed of propagation divided by its frequency (Ÿ), Or: LAMBDA = 30/Ÿ Ÿ = 30/ LAMBDA for wavelengths in centimeters and frequency in gigahertz For a microwave oven: Ÿ = 2.45 GHz, LAMBDA = 30/Ÿ, LAMBDA = 30/2.45, LAMBDA = 12.24 cm or 4.82 inches Microwave power is measured in units, and in units per area space. For example, a typical microwave oven will generate about 700 total watts of power and the leakage, measured 5 cm from the unit might be 0.1 milliwatt per square centimeter (cm2), or 100 microwatts per cm2. One milliwatt = 1/1000 watt and microwatt = 1/1,000,000 watt. Ideally microwaves radiate from a source in spherical waves the power density of which is proportional to the inverse square of the distance to the source. If the distance is doubled (D = 2) power is 1/2^2 or X1/4. If the distance is halved, (D = 0.5) power is 1/(0.5)^2 or X4. Power measurements made within one wavelength or so of a source (near field) will exhibit irregularities due to power storage and exchange in the elements of the radiating part. The natural microwave power level It is challenging to compare a single frequency modulated signal such as a microwave oven leakage to the complex sum of microwave power exposure from nature. For this article I have assumed that the power of natural microwave radiation at 2.45 GHz can be estimated as a an audio bandwidth modulation of 290øK [THE POWER OF NATURAL MICROWAVE RADIATION AT 2.45 GHZ CAN BE ESTIMATED AS] an audio bandwidth modulation of 290øK microwave noise centered on 2.45 GHz is analogous. This may or may not be true. Natural microwave noise power = PN = kTB where: PN = noise power (watts) k = Boltzmann's constant (J/K) B = bandwidth (Hz) T = temperature (K) Given T = 290øK, B = 20 kHz, PN = 8 x 10-17 watts. The "Cantanna" is a circular waveguide to coaxial transition with a point contact diode mounted as a shunt element. The extended wire serves as an E (electrostatic) field probe. For the TEll mode the circular guide exhibits cutoff at a wavelength equal to 1.7 times the diameter. The next mode will exist at 1.3 times the diameter. The coffee can is optimized for frequencies near 2.45 GHz but is sensitive to others as well. It has a polarization axis parallel to the E field probe and should be rotated during measurement for the strongest detected signal.