Medical Edge Newspaper Column

Plastic Containers: Use In Microwave Only As Directed

May 7, 2007
Dear Mayo Clinic:
Is it true that you shouldn't microwave food in plastic containers? I have read that high microwave heat causes dioxins from the plastic to be released into food, which may increase the risk of cancer. -- Rochester, Minn.

Answer:
We can relieve you of your concern about dioxins from plastic containers, as they don't have any. Moreover, these plastics do not have the constituents, nor do they come close to reaching the temperatures, that would enable dioxins to form during product use.

However, it is possible for small amounts of other chemicals to migrate into food as a result of microwaving, particularly if the container is used improperly or is not meant for microwaving in the first place. Such chemicals are more likely to be additives to the plastics, such as the plasticizers that ensure flexibility, than the plastics themselves, which are quite stable.

In any case, the whole substance, plastic plus plasticizer (and any other additive), is subject to government regulation. Under the federal Food, Drug and Cosmetic Act, new materials used to make plastic products for containing food must undergo safety reviews -- reflecting the conditions of intended use -- to be approved for marketing. Toxicological testing should demonstrate that the cumulative dietary concentration of the substance migrating to food is well below (by at least 100-fold) the highest level that causes no observed adverse effects in animal studies.

Note that while the U.S. Food and Drug Administration reviews a new material, it does not regulate the finished product. Because manufacturers have an economic incentive to do so, they are likely to do their own testing of their products before labeling them "microwave-safe" -- provided that the products are used for their intended purposes and in accordance with the directions provided.

Key elements of safe use are these:

• Utilize containers and materials that are specifically labeled as being microwave-safe. Products such as margarine tubs and carryout containers from restaurants should not be used in the microwave. Discard containers that come with prepared microwaveable meals after you've used them, as they are designed for one-time use.
• Check the label on foods packaged in plastic containers. If you don't find instructions for microwave use, choose a different container -- one that you know is microwave-safe.
• If you cover food in the microwave, use glass, commercial ceramic, or microwave-safe plastic covers; microwave-safe plastic wraps; or white microwave-safe paper towels. Be sure to loosen the lid or wrap to allow steam to escape; otherwise, the container may warp, melt, or release a very hot rush of steam or food when opened.
• Microwave-safe plastic containers that become rough or scratched should be discarded.
• Microwave-safe plastic wrap should not directly touch the food. The labels on some plastic wraps recommend that there be at least a one-inch space between the plastic and the food during microwave heating.
• Don't use thin plastic storage bags or plastic grocery bags in the microwave.

All in all, the risk of using microwave-safe plastics is extremely low. But if you're still concerned, it might be worth investing in glass or commercial ceramic containers in order to ensure peace of mind.

-- Philip T. Hagen, M.D., Preventive, Occupational, and Aerospace Medicine, Mayo Clinic, Rochester, Minn.