Microwave Ovens and Food Safety

The Issue

Many Canadians use microwave ovens as a convenient way to thaw, cook and reheat food. A number of people have concerns, however, about the effect of microwaves on their health and on the health and safety of their foods.


Many people who use microwave ovens say they are going to “nuke” their food. This reference to nuclear energy is incorrect and misleading. Microwaves are a form of radiofrequency electromagnetic energy. They are generated electronically. They do not come from radioactive sources and they do not cause food or the oven itself to become radioactive.

When microwaves penetrate food, they cause water molecules in the food to rotate. The rotation causes friction between the molecules and the result is a rapid rise in temperature. This is why the cooking time with microwave ovens is shorter than with conventional ovens. When you shut the microwave oven off, the microwaves disappear.

Health and Safety Concerns Associated With Microwave Cooking

Some microwave energy may leak from your oven while you are using it, but this would pose no known health risks, as long as the oven is properly maintained. The Need More Info? section below refers to an It’s Your Health article called Radiation Safety of Microwave Ovens. The article features safety tips to minimize your exposure to microwave energy when using a microwave oven.

Microwaves do not change the chemical components of in food and so the formation of new compounds, like carcinogens, is not expected. Some studies have been conducted to investigate any possible negative health effects of microwaving foods. These studies, which have been reviewed by Health Canada scientists, have found no evidence of toxicity or carcinogenicity.

In general, the health and safety concerns associated with microwave cooking are similar to the issues involved with other cooking methods, such as conventional ovens, stove-top cooking and grilling. For example, all cooking methods have some effect on the nutrients in food. The effect is worse if you over-cook the food. Microwave cooking tends to be less harsh on nutrients than conventional cooking methods, because the cooking times are shorter and less water is used. To help preserve nutrients when microwaving food, use techniques that promote the even distribution of heat. This will help prevent the formation of “hot spots” where portions of the food could be over-cooked. Steps to promote even heating are outlined in the Minimizing Your Risk section below.

There is no simple answer to questions about which cooking method is best for retaining nutrients. Research into the subject is ongoing. From a health perspective, there is no reason to use any one cooking method exclusively.

Other concerns associated with all methods of cooking, including microwave cooking, are foodborne illness and burns.

Minimizing Your Risk

Foodborne Illness

Raw food of animal origin, such as meats, seafood, poultry and eggs (including juices and drippings) may carry disease-causing bacteria. No matter which cooking method you use, the risk that bacteria will multiply and cause foodborne illness increases when foods are allowed to sit at temperatures in the “danger zone” between 4o C and 60o C (40o F to 140o F) for more than 2 hours. To minimize the risk of foodborne illness:

* When handling raw foods of animal origin, always chill promptly.
* Clean your hands and kitchen surfaces often.
* Keep foods separate and do not cross-contaminate.
* Cook food thoroughly.
* If you use the microwave oven to defrost or partially cook food, be sure to refrigerate or finish cooking the food by some other method right away. Do not let perishable foods linger in the "danger zone" for longer than 2 hours.
* Take steps to ensure that the microwave oven heats food evenly and does not leave underheated areas (cold spots) where bacteria might multiply and cause food poisoning. You can promote even heating in the microwave oven by:
      * cutting food into small pieces for uniform cooking
      * arranging items in a uniform manner
      * adding a liquid (such as water, juice or gravy) to solid foods
      * stopping part way through cooking to stir foods or rotate trays or containers
      * covering food with a microwave-safe lid or with microwave-safe plastic wrap to trap steam
      * following directions for "standing times". This helps ensure that heat is distributed uniformly, even after cooking.
* Use a food thermometer to check that your food has reached a safe internal temperature. Take the temperature at several locations, especially in the thickest area of the meat, to ensure that the food is cooked all the way through. Make sure that the thermometer is inserted away from bone, fat or gristle. For example:
      * all ground beef products should be cooked to 71oC (160oF).
      * food mixtures containing poultry, eggs, meat and fish should be cooked to 74oC (165oF).
      * leftovers should be heated to 74oC (165oF).
      * Never cook whole poultry, including turkey in the microwave.


It is always important to be careful when handling or eating hot food. With conventional cooking methods, there are often warning signs that you are dealing with high temperatures. However, your microwave oven is enclosed and you cannot see the source of heat, so you may get a false sense of how hot the food and containers may be.

Therefore, with microwave ovens, there are specific concerns about potential burns related to:

* "superheated" liquids: These liquids are at or above the boiling point, but look harmless and show no sign (such as bubbling) that the liquid has boiled. When you remove superheated liquids from the microwave oven, they can erupt suddenly and cause serious skin burns.
* heat transfer from food to containers: Many microwave-safe containers and dishware are not heated directly by microwave energy. However, parts of these containers may become very hot due to heat transfer from the food being cooked.
* heating formula in baby bottles: When you heat baby formula in a microwave oven, the outer container (or baby bottle) may feel cool to the touch even though the formula inside is very hot. This can pose a risk of serious burns to the baby.

To minimize these risks when using the microwave oven:

* Be careful when heating liquids and removing them from the oven. Avoid superheating liquids by stopping the microwave oven part way through the heating process to stir the liquid.
* Always use containers labelled as microwave-safe.
* Use protective oven mitts or pot holders when you remove containers/dishware from the oven.
* If you heat a bottle of baby formula in the microwave, be sure to shake the contents and test the temperature of the formula on your own skin before you go ahead with feeding the baby. Health Canada recommends using conventional methods, such as a baby bottle warmer, to heat baby formula. With these methods, you can use the warmth of the bottle to measure the temperature of the inner liquid more accurately.

Other Concerns regarding Containers and Food Wraps

Some foods come wrapped in materials (e.g., styrofoam) that may not be suitable for use in the microwave. These materials could leach chemicals into your food or cause burns if they melt during microwave cooking. To minimize risks when using your microwave oven:

* Do not re-use trays or containers that come with conv
* enience foods. These are designed for one-time use only.
* Do not use containers intended for cold storage (e.g., margarine tubs) or wrappings that come with packaged foods.
* Make sure the plastic wraps and containers you use are labelled as microwave-safe.

As a general safety precaution, always supervise young children when they use the microwave oven (or any other cooking appliance). Finally, read and follow the manufacturer’s directions for using the oven, and keep the oven in good working order.

Health Canada’s Role

Health Canada carries out many different activities to minimize risks related to the use of microwave ovens. For example, Health Canada has established a regulation under the Radiation Emitting Devices Act to govern the design, construction and functioning of microwave ovens that are imported, sold or leased in Canada. This regulation specifies limits for microwave leakage from ovens.

In addition, Health Canada assesses risks, sets standards and monitors the safety record of such products as microwave cooking containers and food packaging materials, and establishes policies on the safety and nutritional value of food. As a founding member of the Canadian Partnership for Consumer Food Safety Education, Health Canada also participates in public awareness campaigns about safe food practices. One example is CanFight BAC?, a program that encourages Canadian consumers to think of food safety at every step of the food handling process, from shopping for groceries to reheating leftovers.
Need More Info?

For information on Food Safety
Or contact: food-aliment@hc-sc.gc.ca

For more information on microwave radiation safety, contact:
The Consumer and Clinical Radiation Protection Bureau
775 Brookfield Road
Ottawa, Ontario K1A 1C1
Telephone: (613) 954-6699
Fax: (613) 952-7584
E-mail: CCRPB-PCRPCC@hc-sc.gc.ca

For additional information, visit the following Health Canada Web sites:

It’s Your Health: Radiation Safety of Microwave Ovens

For information on food packaging

Additional information can also be located at:

The Canadian Food Inspection Agency Web site, Food Safety Facts on Microwave Ovens

CanFight BAC?


Radiofrequency and Microwave Radiation

* Health Effects

Radiofrequency (RF) and microwave (MW) radiation are electromagnetic radiation in the frequency ranges 3 kilohertz (kHz) - 300 Megahertz (MHz), and 300 MHz - 300 gigahertz (GHz), respectively. Research continues on possible biological effects of exposure to RF/MW radiation from radios, cellular phones, the processing and cooking of foods, heat sealers, vinyl welders, high frequency welders, induction heaters, flow solder machines, communications transmitters, radar transmitters, ion implant equipment, microwave drying equipment, sputtering equipment and glue curing.



If microwaves in an oven can cook a piece of beef, they will also have the same effect on human tissue if exposed to high enough intensities for a long enough period of time. Certain body organs are particularly sensitive to this thermal effect. Thermal means heat. Just as it is the heat produced by a hot stove that causes the careless cook to voice a sudden unsavory expletive, so too, it is the heat generated by the microwaves that creates the hazard in this case. For example, if the lens of the eye were exposed to excessive heat from microwaves, its circulatory system would be unable to provide sufficient cooling, and it would cook like the white of an egg. Exposure to high levels of microwaves can cause cataracts. Also, the stomach, intestines and bladder are especially sensitive to thermal damage from high levels of microwaves. Likewise, the testes are very sensitive to changes in temperature, since sperm can be formed only at temperatures lower than that of the body itself. Thus, accidental exposure to high levels of microwave energy can alter or kill sperm, producing temporary sterility. The question is: How intense would levels of microwave energy have to be to create such a danger?


The power density of microwaves is determined by measuring the amount of energy that flows through one square centimeter (a square centimeter is about the size of an aspirin tablet) of space in one second. Western scientists believed that serious injuries could result only at levels of 100 Milliwatts per square Centimeter (mw/ cm2 ) or higher. It was theorized that a built in safety factor of 10 times would be a safe margin. So, in the mid-1950?s a voluntary industry standard of 10 mw/cm2 (or, one-tenth of 100 mw/cm2) was adopted.

In 1971, due to the concern of the Department of Health, Education and Welfare (now the Department of Health and Human Services), the standard for allowable leakage from microwave ovens in the United States was set by law to the present, more stringent, levels of 1mw/cm2 (at a distance of five centimeters–see Section 15.7) prior to acquisition by a purchaser, and 5mw/cm2 thereafter. These safety standards were based on the belief that the only danger from exposure to high-intensity microwave energy was a thermal or overheating effect.


No one really knows for sure how to interpret the emerging results as painstaking experimentation continues. One thing they do know, however, is that there is a non-thermal effect from microwaves at levels that many people may be exposed to on a daily basis. What degree of danger does this non-thermal effect represent? The answer to that has to do with the controversial difference between a simple biological effect and a serious biological hazard. For example, a reduction in the ability to perform certain tasks may be the effect, but at what point does that effect constitute a hazard?

So, what are safe levels of exposure to microwaves? While vigorously warning of the invisible dangers involved with non-ionizing radiation, Dr. Milton M. Zaret, a professor of ophthalmology, and a long time student of the biological effects of microwaves, answers: “I have no idea what a safe level is, I don?t think anyone in the world knows what a safe level is.”

The effects of long term exposure to low levels of microwaves, and their significance to human health, will become clear only after large numbers of people who are being exposed to microwaves are studied for many years. Studies are being done with animals, but it is difficult to translate the effects of microwaves on animals to possible effects on humans. For example, researchers find it quite difficult to simulate the conditions (with animals) under which people use microwave ovens. Since no one can say with certainty what levels of exposure are safe, the course of wisdom would be, as a U.S. government spokesman pointed out, to avoid "exposure to any unneeded radiation.??


One pertinent characteristic of microwaves is that they disperse and dissipate very quickly in the atmosphere. For example, the maximum allowable leakage from a microwave oven (after the sale) is 5 milliwatts of microwave radiation per square centimeter at approximately 2 inches from the oven surface. However, as Figure 3-1 illustrates, as you move away from the oven, the level of exposure to any energy that may be leaking decreases dramatically. This may be likened to holding your hand immediately above a burning candle as opposed to holding it 4, 8 or 12 inches away. Say you are standing 2 inches away from a microwave oven, and are being “zapped” by 5mw/cm2 of microwave energy, then you wisely step back to a distance of 20 inches or roughly an arm?s length. Your level of exposure would drop by a factor of 100, (the square of the distance) to .05mw/cm2, a level compatible even with stringent Soviet standards, (present Soviet occupational standard allows up to 0.1mw/cm in no more than two hours). However, it must be noted that Czech scientists have reported some effects even at these infinitesimal levels. This, combined with the opinion of Russian scientists that microwave effects are cumulative, certainly underscores the need for consumers and servicers alike to observe certain common sense precautions.

? Stay at least an arm?s length away from the front of an operating oven. This is especially so with pregnant women according to a U.S. government agency, which states that the human fetus is “probably the most sensitive segment of the population potentially exposed to microwave radiation.” Children represent another sensitive segment of the population. Never should anyone, and especially children, stand gazing into, or directly against an operating microwave oven.

? If the door of an oven will not close properly, is bent, warped, tampered with, or otherwise damaged in any way, DO NOT OPERATE the oven unless you are a qualified servicer with an approved RF survey meter in hand.

? Never operate an oven when it is empty. This creates a no-load condition, which can damage the oven and cause excess leakage.

? Never inactivate, interfere with, or try to adjust the built-in safety interlock system of an oven, unless you are properly equipped and qualified to do so. Tampering with safety interlocks would be as foolish as disconnecting the brakes on a car.

? The Food and Drug Administration recommends that microwave ovens not be used in home canning. It is believed that they do not produce or maintain temperatures high enough to kill harmful bacteria.

Observing these safety suggestions, as well as others that will be presented in subsequent chapters of this book, will help to minimize exposure levels and the risk of serious accidents.


It has been a subject of great concern, especially for many heart patients, that stray leakage from microwave ovens could interfere with the proper operation of their cardiac pacemakers. The fact is, there are at least 20 other known sources of electromagnetic interference that could also cause a pacemaker to malfunction if it were non-shielded. RF interference is generated by such common items as: electric shavers, auto ignition systems, walkie-talkie radios, fluorescent lights, and dial telephones. Many more of these electronic interference-emitting products are commonplace items even in hospitals; diathermy, electro-surgical units, electric bed motors, elevators, personnel pagers, electric heaters and heating pads, to mention a few.

The problem has been resolved, for the most part, with the development of a new shielded pacemaker. Since microwaves, or any other type of electronic interference, cannot penetrate their stainless steel casing, the possibility of harm to people who wear these modern heart pacemakers is extremely remote. In an effort to determine the overall susceptibility of these units to electromagnetic interference, U.S. government agencies contacted all known U.S. manufacturers of cardiac pacemakers. Their findings indicate that less than 1% of all pacemakers are sensitive to electronic interference and this number is rapidly decreasing. Apparently, the external demand type of pacemaker continues to be a particularly sensitive device, so wearers of this type of pacemaker should avoid all possible sources of electronic interference. In fact, all patients with pacemakers would be well advised to contact the manufacturer of the unit and consult with their physicians for the final word on this matter.

While signs that warn “MICROWAVE IN USE” are not a federal requirement, local administrations or establishment owners may prefer to display such signs for various reasons. For example, some may display warning signs for their own protection (like a “watch your step” sign), to avoid the possible psychological trauma that could be suffered by an unwary pacemaker patient who suddenly discovers that he is sitting next to an operating microwave oven.