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Information Directory

Reference Directory

Microwave Radiation

What is a microwave?
A microwave is a form of energy that falls within the electromagnetic spectrum. That means that microwaves are in the same family as visible light, radio waves and gamma rays.

So does that mean that microwaves come from things that are radioactive?
No. Microwaves that we use routinely in industry and at home come from machines and devices that can generate microwaves. No radioactivity is necessary.

Wait a minute . . .you mean there other sources of microwaves besides the oven in my kitchen?
Absolutely. Microwave generators are commonly used in industry to cure chemical reactions, heat a part or seal a plastic seam. There are some estimates as many as five million people using some type of microwave source every day. Therefore, you are likely to encounter microwaves in just about every industry and manufacturing location you can imagine.

Just industry?
No. Microwaves are useful in the research and medical industries also. In fact, the first laser that was assembled in 1954 using a source of microwaves to energize or stimulate the electrons in a cloud of ammonia and form a coherent beam of light. It was called a MASER or microwave amplified by stimulated emission of radiation.

How do you make a microwave beam?
You can create a beam of microwaves by vibrating a molecule in a crystal or other solid mass. This vibration may create a broad range of radio frequencies, such as alternating current, radio waves or microwaves. There is no clear differences between these forms of generated wave energies other than the frequency or other words the speed that wave moves. As the frequency increases there is more energy to distribute.

Are there limits to the frequency?
Well, microwaves are defined as radiation with a frequency between 10 to 300,000 megahertz. That means the wave oscillates up to 300,000 million times each second . . . darn fast. If the frequency is outside of the 10 to 300,000 megahertz range, we’re no longer talking about microwaves.

How do microwaves get around?
This form of radiation is normally transmitted into the atmosphere from antennas like televison antennas, FM radio antennas, and radar transmitters. Specific frequencies are used depending in the application.

Can you give me a couple of examples?
Yes. The microwave oven in your kitchen probably uses a frequency of 2,450 megahertz. On the other hand, the radar gun that the police use in one of your local speed traps usually operates in the frequency range of 10,500 to 34,700 megahertz.

Why are the frequencies different?
Well, you select the frequency depending on how the microwave energy will be used. Each frequency is absorbed or reflected differently depending upon the surface it strikes. The goal is bit different for the kitchen oven and the radar gun. In the first case, we want microwave energy to be absorbed into a container of food, so that it will heat up. In the latter case, we want the microwaves to bounce off of a moving vehicle and travel back.

Can microwaves hurt you?
The primary effect upon the human body when exposed to microwave energy appears to be thermal effects. In general, the higher the frequency, the lower potential health hazard.

I’m not sure I follow you.
Well, frequencies less than 3,000 megahertz can penetrate the outer layers of skin and be absorbed in underlying tissues. These tissues have little or no sensation to a change in temperature resulting in damage to these tissues because of a rise in the tissue temperature. The damage is directly related to the intensity and duration of exposure. That is, the longer you are exposed that more likely you are to experience damage to the body,. Such damage can include, burns, cataracts in the eyes, and may be fatal if the deposition of energy is not stopped. Cataracts have been reported with a power density in excess of 100 milliwatt per/square centimeter; the symptoms are likely to be worse when the exposed skin or organ has a low capacity to remove the heat generated. This may be related low blood flow in the tissue.

Are there any agencies that regulate how much exposure is too much?
There are no regulations by the Occupational Safety and Health Administration (OSHA) for limits on microwave radiation or other radio-frequency waves. However, OSHA is allowed to enforce consensus standards through the General Duty Clause of the Act when there is no standard for a specific hazard.

What kinds of consensus standards might OSHA refer to?
There are industry standards or consensus standards that are used by the manufacturing and research facilities, including International Commission on Non-Ionizing Radiation Protection, “Guidelines on Limits of Exposure to Static Magnetic Fields.”; American Conference of Governmental Industrial Hygienists, “Threshold Limit Values for Physical Agents in the Work Environment.”; and The Institute of Electrical and Electronic Engineers, “American National Standard Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3 kilohertz to 300 gigahertz” (IEEE/ANSI C95.1-1991). This standard addresses exposures under controlled situations where people are trained and informed of the potential hazards of radio-frequency/microwave radiation and fields, and exposures under uncontrolled situations where such awareness does not exist.

What do these standards tell us?
Each of these standards summarize acceptable exposures to specific frequencies and list intensity and durations. The ACGIH has established a TLV for frequencies from 3,000 megahertz to 15,000 megahertz to be less than 10 milliwatts per square centimeter averaged over an eight hour day. The field strength changes with the frequency. It is assumed that the energy is received over the entire body; peak values of the field strength may exceed the TLV if the spatial average is below the limit.

How do I know that I am being exposed to a field strength greater than the TLV? Can I feel the radiation or do I see some kind of a glow?
No. At these levels, you would have no measurable sensation of the energy striking your body. The only way to evaluate your exposure is to measure the field strength with a calibrated instrument. Fortunately, many of the instruments available for these types of measurements are small enough to be held into the hand, and are simple to use.

How do I know that a microwave generator is being used?
Many of the microwave devices used today are shielded and do not represent a potential for exposure during operation. Even devices in manufacturing, research and medical fields operate in the same way that the kitchen microwave oven operates; That is when the door is closed and the source of microwaves is shielded. Typically sources of microwaves are posted with warning others about the presence of a potential source of microwaves. Persons operating industrial microwave equipment are trained in its correct operation and the consequences of exposure. They are also typically taught how to reduce or minimize any exposure.

That’s well and good for ovens, but what about the radar gun that the police officer uses to measure the speed of a passing car? Those microwaves aren’t shielded inside of a box. Are the occupants of the car exposed to hazardous microwaves?
The microwaves being emitted by the radar gun in the hand of your local police offer are reflected off of the surface of your car, and returned to the radar gun. It is the principles of reflection and the time it takes microwave radiation to travel the round trip that the computer in the radar gun uses to measure the speed of the on coming car. Because the waves are reflected rather than absorbed, the occupants of the car are not exposed.

What about the police officer holding the radar gun?
The police officer is instructed to point the radar gun away from his or her body and into traffic. As a result, there is no real hazard associated with the correct use of this device.

And what about the microwave oven in my kitchen? Why is there just a piece of glass in the door? Seems like kind of a flimsy barrier to me. Don’t the microwaves leak through the glass?

Actually, the glass has nothing to do with microwave leakage.  Its just there so you can “watch the pot boil”.  If you look closely, you will see that there is a screen with small holes mounted inside of the glass. The holes are smaller than the wavelength of the microwaves. To the microwaves, the screen is a solid wall. On the other hand, visible light has a much smaller wavelength, which can easily pass through the holes. So what you have are microwaves that bounce into the screen and are reflected back into the cooking chamber, but visible light from the light bulb escaping through the screen and the glass window.

You said that OSHA does not have standards for exposure to persons who work with microwaves. Does that mean that there are no standards for microwaves at home?
Absolutely not. The Food and Drug Administration promulgated their regulations in Title 21 Code of Federal Regulations, Part 1030.10, “Performance Standards for Microwave and Radiofrequency Emitting Products”. These standards not only limit the amount of radiation permitted outside of the cooking chamber but also give the limits on the amount that may leak around intact seals and gaskets.

What are those limits?
The measured radiation must be less than 1 milliwatt per square centimeter at a distance of up to five centimeters away from the door.

How can I be sure that is what my home microwave is doing?
The manufacturer of your microwave oven is required to demonstrate that the particular oven meets the emission limits if it is being operated properly. This means that the latch to the door of the oven must be closed during operation, and that microwave generation ceases when the latch is opened.

But what if my door latch is broken? Or what if I dropped by microwave oven and the door becomes bent?
Then you no longer have a properly operating microwave oven. It is your job to make sure that the door to your oven is not bent and that the latch works properly. If it does, you and your family are safe from any microwave-related health hazards. (Although you will need to instruct your children about handling food heated in a microwave. Thermal burns can occur just as easily with a microwave oven as with a conventional oven.) If your microwave is broken, do not use it until you have taken it back to your vendor for repair.

Can a broken microwave be fixed?
Most of the time it can. However, what is most important is that the vendor who repairs microwave ovens must perform measurements of leakage and emissions, just like the manufacturer of a new oven. If the vendor says its “good to go”, then you and your family can use the repaired oven with confidence.