Can Microwaves Cause Cancer?

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In order to determine if microwave ovens can cause cancer, there are several factors to consider, including:

  • Whether electromagnetic radiation in the microwave range could cause cancer (such as by standing near a microwave oven)
  • Whether microwave cooking can create carcinogens in foods
  • How microwave cooking may affect the nutrients in foods that may protect against cancer

We'll look at each of these questions separately, discuss how the effects may differ with different foods, and share some pointers on when microwave heating could actually be dangerous.

Asian mother and child using a microwave oven

Prapat Aowsakorn / iStock / Getty Images Plus

Microwaves and Cancer Risk

On the electromagnetic spectrum, microwaves are a type of low-energy radiation classified as non-ionizing radiation. Other forms of non-ionizing radiation include:

  • Radio waves (slightly lower energy/frequency radiation)
  • Infrared/visible light waves (slightly higher energy/frequency radiation)

The type of radiation that has been associated with cancer, in contrast, is ionizing radiation. This includes higher energy/frequency radiation such as:

  • Ultraviolet light
  • X-rays
  • Gamma rays
  • Cosmic rays

Ionizing radiation has the ability to knock electrons off of atoms in molecules (which can damage DNA and potentially lead to cancer), whereas non-ionizing radiation (such as microwaves) do not. In addition, microwaves cannot make food radioactive.

How They Work and Potential Exposure

A microwave oven is powered by a device known as a magnetron. The magnetron converts electricity to microwave energy. When food is placed in the microwave oven, this energy causes water molecules within food to vibrate. These vibrations, in turn, cause the release of heat. During microwave heating, the actual structure of the food is not changed in any way.

When a microwave oven is working properly (and the door has a good seal), very little microwave energy can leak out. According to the Food and Drug Administration (FDA), the amount that does so is considered to be well below the level that could cause harm to humans.

Microwave Cooking and Carcinogens

If radiation from a microwave is not a concern, what about the potential for carcinogens being formed in some foods, or instead, be leached into foods from cooking containers?

Carcinogens That May Be Formed During Cooking

In the process of heating, chemical reactions may occur in some foods that result in the formation of carcinogens.

Acrylamides are chemicals that are formed when sugars and starches are heated at high temperatures and are classified as group 2A carcinogens according to the International Agency for Research on Cancer (IARC). While a few studies have found a link between acrylamide intake and cancers of the kidneys, uterus, and ovaries, most studies have not.

Whether microwave heating is more or less likely to result in acrylamide formation is uncertain at this time. Some studies suggest that microwave heating on high power may result in greater acrylamide formation, but the opposite may be true when microwaves are used to blanch or thaw foods on low power.

What About Microwave Cooking Prior to Grilling?

Microwave cooking prior to grilling may lower the concentration of carcinogens such as polycyclic aromatic hydrocarbons (PAHs) and heterocyclic amines, which are chemicals formed when muscle meat, including beef, pork, fish, or poultry, is cooked using high-temperature methods, such as pan-frying or grilling directly over an open flame.

Plastics in the Microwave and Cancer Risk

While there are no direct studies linking plastics in the microwave to cancer, it is important to use only plastics that are considered "microwave safe" according to the FDA. For many people, this isn't a major concern, as plastics wouldn't be recommended in a conventional oven either. There is concern that some plastics could melt to some degree, and chemicals present in the plastic could leach into foods.

Some plastics should be avoided due to potential exposure to endocrine-disrupting chemicals (EDCs). EDCs are chemicals that can mimic or alter hormonal processes in the body and have been linked to cancers of the thyroid, breast, and prostate.

Two of these, in particular, are best avoided:

  • Bisphenol A (BPA): A substance used to make plastics hard (and clear).
  • Phthalates: These are added to some plastics in order to make them softer and flexible.

When Microwave Cooking May Lower Carcinogens

There are some situations in which microwaving foods may actually reduce your exposure to carcinogens, and hence, the risk of cancer.

When meat is grilled or cooked by other methods (with the heat over 300 degrees F), substances known as heterocyclic amines and polycyclic aromatic hydrocarbons can form.

  • Heterocyclic amines (HCAs) are formed when amino acids, creatine (found in muscle), and sugars react while heating at high temperatures.
  • Polycyclic aromatic hydrocarbons (PAHs) are formed from smoke that sticks to meat after juices and fats drip onto the flames beneath the meat.

In addition to being known to cause the kind of changes in DNA that can lead to cancer (being mutagenic), the consumption of grilled meats has been linked to an increased risk of colorectal, pancreatic, and prostate cancers in some studies (but not all).

Since vegetables do not contain creatine or animal fats, grilling vegetables is generally safe from these carcinogens.

Effects of Microwave Cooking on Foods

Any method of heating can alter food. Therefore, when looking at changes in nutrient content, it's important to look not only at raw foods prior to cooking, but other cooking methods. Common cooking methods in addition to microwave heating include:

  • Baking
  • Boiling
  • Blanching
  • Steaming
  • Frying
  • Air frying
  • Pressure cooking
  • Grilling

In addition to the cooking method, nutrient content may vary with cooking time, temperature, and moisture levels. Of these, the moisture level is often most important as water can leach out nutrients from many foods. With steaming, the food is not in direct contact with the water so the nutrients are often less likely to leach out into the water.

Many vegetables already have a significant water concentration so that water does not need to be added during microwave heating (and hence, nutrients are less likely to be leached out).

In addition, the effect of heating on different foods may vary based on:

  • The water content of the food
  • Structure of the food matrix (softening of the food matrix of some foods via cooking can make the nutrients more or less accessible for digestion)
  • Chemical nature of the nutrient (phytochemical) being evaluated
  • Food storage time after preparation (nutrients may either decrease or, as with cooked carrots, increase with storage)

Foods contain macronutrients—such as proteins—carbohydrates and fats, phytonutrients, vitamins, and minerals. Cooking methods may vary in how they affect these nutrients.

Phytonutrients

Phytonutrients are plant chemicals that have a wide range of functions in the body. Some potentially play a role in the prevention and/or treatment of cancer. They may do this by performing as antioxidants, by enhancing immunity, by blocking the formation of carcinogens, and other mechanisms.

Some major categories of phytonutrients include:

  • Polyphenols (such as flavonoids)
  • Terpenoids (such as the carotenoid beta-carotene)
  • Thiols (such as glucosinolates)

Phytonutrients often give color and flavor to fruits and vegetables, and are the reason that some oncologists recommend eating a "rainbow of foods."

An older (2003) study raised concern about microwave cooking as it was found that microwaving broccoli removed 97.2% of the flavonoids (vs. 66% with boiling and only 11.1% with steaming). In this study, steaming resulted in the best retention of flavonoids. The problem with this study, however, is that the broccoli was cooked in the microwave with added water. Since broccoli (and most vegetables) have a significant natural moisture content, microwave cooking can be done without adding water.

A different study looking at a another type of phytochemical in broccoli (glucosinolates) found—in contrast—that microwave cooking fell in the middle of the spectrum. Glucosinolates were best preserved with steaming and least preserved with boiling or stir-frying.

More recent studies suggest that microwaves may actually be one of the better ways to cook vegetables (as long as water is not added). A 2019 study looking at the polyphenol content in broccoli following three cooking methods demonstrated that microwave heating resulted in the best retention of polyphenols. This was followed by steaming.

Since the food matrix of different foods varies, the effect of microwave cooking on broccoli may not translate to other vegetables.

A 2020 study looked at the effect of boiling, steaming, and microwaving on the nutrient content of cauliflower, carrots, and sweet potatoes. The stud found that:

  • Boiling reduced phenolic concentration and antioxidant activity, but enhanced the availability of carotene.
  • Steaming increased both phenolics and carotenoids.
  • Microwave cooking preserved carotenoids and increased total phenolics.

In this case, both microwave and steaming had no detrimental effect on the phytonutrients measured, and actually increased activity in some cases.

What About Garlic?

Few studies have been done looking at the effect of cooking on garlic alone, but it's noteworthy that the preparation of garlic prior to cooking may be equally or more important than the cooking method used.

In one older study, either 60 seconds of microwave cooking or 45 minutes of oven cooking blocked most of garlic's anti-cancer effects (by a specific measurement). When the garlic was first prepared (such as in a garlic press) and allowed to sit for 10 minutes before heating, the effect of cooking had much less impact. (This is one reason why some chefs first prepare the garlic and allow it to stand while preparing other ingredients.)

Vitamins and Minerals

The effects of heating on vitamins and minerals can be different than effects on phytochemicals, and in the case of vitamin C, microwave cooking may be the best choice.

The 2020 study looking at cauliflower, carrots, and sweet potatoes also evaluated vitamin C content with boiling, steaming, and microwaving. Boiling and steaming both significantly reduced vitamin C content. In contrast, microwaving best preserved the vitamin, possibly due to less leaching into water than boiling and a shorter cooking time than steaming.

Reducing Carcinogens When Grilling/Heating Meats

Preheating meats in the microwave prior to cooking may reduce the formation of both categories of carcinogens. By preheating meats in the microwave (for 60 to 90 seconds), the surface of the meat does not get as hot, and therefore there is reduced formation of HCAs. In addition, precooking and throwing away juices released may reduce the formation of PAHs. Decreasing the cooking time due to preheating can reduce the formation of both.

Risks of Microwave Cooking

There are some risks associated with microwave cooking that may differ from other methods of food preparation.

Radiation Injuries

While very rare, a few cases of radiation injury (not cancer, but burns or cataracts) have been documented related to improper repair of a broken microwave or unusual circumstances. In these cases, a large amount of microwave radiation was allowed to leak through the oven seals. Fortunately, the FDA regulates the design and manufacture of microwave ovens so that this should not occur.

Microwave ovens should not be used if the door seal is broken, or if the light/turntable remains on with the door open.

Uneven/Irregular Heating

Perhaps the most commonly encountered risk of microwave food preparation is that of uneven heating. Microwave rays penetrate food only to a depth of one or so inches. For this reason, foods may be quite hot on the inside while cold (or even frozen) on the inside. Some foods like breast milk may burn on the outside even though they remain cold on the inside. Uneven heating, in turn, can cause more than one problem.

Burns may occur if a person tests one part of a meal thinking it is the correct temperature but another part is very hot. This is one of the primary concerns behind the recommendation that baby bottles not be heated in the microwave.

Another significant concern, however, is that of food poisoning. Some foods—especially undercooked meats—could result in food poisoning due to inadequate heating of part of the meal. The exact incidence is unknown, but outbreaks of foodborne illness have now been clearly documented due to inadequate microwave heating.

When cooking raw meats in the microwave, it's important to stir the food frequently and make sure it is adequately heated throughout. Checking with a food thermometer is a good option (as long as it is done after a thorough stirring and the innermost part of the meat is tested).

Explosion of Liquids

Ordinarily, when heating water and other liquids on the stove it's possible to determine if it's boiling based on visible cues (such as bubbling). In the microwave, however, superheated water may be created that does not appear to boil. When removed from the microwave this can actually result in an explosion (and the potential for severe burns). Adding solids to water, such as sugar or instant coffee, reduces the chance that superheating will occur.

While uncommon, there have been several cases of severe facial burns due to exploding microwaved foods. In addition to superheated water, this commonly occurs with foods such as eggs and potatoes. In general, the risk is very low if you follow recommendations on heating times for common foods. In addition, it's best to limit the cooking time at first, and complete the full-time recommendation only if needed.

Metals in the Microwave

Accidentally placing metal objects in the microwave may confer more risk for the microwave than people, but is still unwise. It's important to avoid placing any metal or aluminum foil in the microwave. This includes metallic painting on cups or bowls, twist ties, etc. that may not immediately be obvious.

Microwave Oven Safety

A few simple pointers can help ensure your microwave oven is safe and effective in preparing the foods you eat. These include:

  • Making sure the door is properly shut
  • Using microwave-safe containers
  • Covering your bowls with paper plates or napkins
  • Standing back a few feet and not directly in front of the microwave
  • Being aware of the potential fo superheated water
  • Using the turntable and stir food often while cooking
  • Not exceeding recommended cooking times
  • Letting hot food sit for some time before removing it from the microwave
  • Using a potholder when removing hot bowls

A Word From Verywell

While we certainly do not have a comprehensive understanding of the best cooking method for each specific type of food, microwave cooking may be a good option in many cases. Fortunately, studies in recent years suggest that nutrients such as phytochemicals and vitamins are retained better than was thought in the past. And the convenience of microwave cooking might even give you time to eat those cancer-fighting foods like broccoli you might otherwise forego.

While other cooking methods such as steaming might be ideal in some cases, the stress of trying to cook everything perfectly might just outweigh any small differences in nutrient content. Until we know more, using a variety of cooking methods, eating a wide variety of colorful foods, and practicing the old adage "moderation in everything" remains a wise choice.

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