Chances are you’ve received an urgent “PLEASE READ THIS!” e-mail message about the dangers of microwaving food in plastic containers or plastic wrap. The message warns that chemicals can leach out of the plastic and into the food, causing cancer, reproductive problems, and other ills. Is there any truth to this, or is it just another Internet-fueled “urban legend”? As is often the case with alarmist e-mails, this one contains a small kernel of truth — and a lot of misinformation.

Migrating chemicals

When food is wrapped in plastic or placed in a plastic container and microwaved, substances used in manufacturing the plastic (plasticizers) may leak into the food. In particular, fatty foods such as meats and cheeses cause a softening agent called diethylhexyl adipate to leach out. This certainly sounds scary, so it’s little wonder that a warning is making its way across the Web.

But here’s what the e-mails don’t mention. The FDA, recognizing the potential for small amounts of plasticizers to migrate, closely regulates plastic containers and materials that come into contact with food. Before approving a container, the FDA conducts tests to make sure that it doesn’t leak unsafe amounts of any substance into food.

According to Dr. George Pauli, a retired associate director in the Office of Food Additive Safety at the FDA, these tests measure the migration of chemicals at temperatures that the container or wrap is likely to encounter during ordinary use. For microwave approval, the agency estimates the ratio of plastic surface area to food, how long the container is likely to be in the microwave, how often a person is likely to eat from the container, and how hot the food can be expected to get during microwaving. Because microwaves heat the water in food, the peak temperature is the boiling point of water — 212º F, or 100º C. The only exception is microwave popcorn and other packages that come with the instruction, “this side down.” Such packages, says Pauli, are made with small amounts of metal to create a “frying pan effect.” They get hotter than the boiling point of water and are tested accordingly.

The scientists then measure the chemicals that leach out and the extent to which they migrate to different kinds of foods. The maximum allowable amount is 100–1,000 times less per pound of body weight than the amount shown to harm laboratory animals over a lifetime of use. According to the FDA, this limit takes into account differences between laboratory animals and humans as well as individual variations in the use of plastic for microwaving. Only containers that pass this test can display a microwave-safe icon, the words “microwave safe,” or words to the effect that they’re approved for use in microwave ovens.

What about containers without a microwave-safe label? The FDA tests all containers that come in contact with food, but only those labeled microwave safe have been tested and found safe for that purpose. A container that’s not labeled safe for microwave use isn’t necessarily unsafe; the FDA simply hasn’t determined whether it is or not. According to the American Plastics Council, some unlabeled containers are made of the same kind of plastic as microwave-safe containers, but they may not be safe because their walls are thinner and could melt in the microwave.

The bottom line

Here are some things to keep in mind when using the microwave:

• Most takeout containers, water bottles, and plastic tubs or jars made to hold margarine, yogurt, whipped topping, and foods such as cream cheese, mayonnaise, and mustard are not microwave-safe.
• Microwavable takeout dinner trays are formulated for one-time use only and will say so on the package.
• Don’t microwave plastic storage bags or plastic bags from the grocery store.
• A recycle symbol does not mean a container is safe to use or reuse in the microwave oven. Only a microwave-safe icon or wording to that effect does.
• Before microwaving food, be sure to vent the container: Leave the lid ajar, or lift the edge of the cover.
• Don’t allow plastic wrap to touch food during microwaving because it may melt. Wax paper, kitchen parchment paper, or white paper towels are alternatives.
• If you’re concerned about plastic wraps or containers in the microwave, transfer food to glass or ceramic containers labeled for microwave oven use.

Stop that hoax

If you receive an e-mailed “health alert,” check out the claims before you forward it to friends and relatives or make any changes in your life. Click here to find out how.

For information on treating common and not-so-common digestive problems from heartburn to irritable bowel syndrome, order our special health report The Sensitive Gut at www.health.harvard.edu/SG.

Q: After a week of scorching summer temperatures, I recently found myself in the hospital for dehydration. What can I do to keep this from happening again?

A: It may come as a surprise, but your body is 50%–60% water. Each of your trillions of cells is about half water, and you also store water outside your cells — in your blood and in the spaces between your cells.

Dehydration is when the amount of water in your body gets too low. We get water from food and drinks, and we lose it through sweat, exhaled breath, urine, and feces. Many things can cause you to lose more water than usual, such as exercising in the heat, a bout of vomiting or diarrhea, a high fever that results in a heavy sweat (which can happen when you have the flu or something like it), or diuretic medicines. A mild case causes a little dizziness when standing up, weakness, and fatigue. More severe dehydration can cause seriously low blood pressure or even loss of consciousness.

Estimating how much water you’ve lost when you’re sick is difficult, and it’s often hard to eat and drink enough. Sometimes intravenous fluids are necessary. So if you’ve got a condition that could be causing you to lose water, just do your best to keep up with the fluids. And if you think you’re getting behind, call your doctor.

— Anthony L. Komaroff, M.D.
Harvard Health Letter Editor in Chief