Q. What is the current thinking about the safety of microwaving food covered by plastic? I see vegetables wrapped in plastic or in plastic bags with directions to microwave right in the plastic. I thought plastic when heated released carcinogens?

A. Samantha Salazar Ordonez, a dietetic intern at Tufts Frances Stern Nutrition Center, replies: The US Food and Drug Administration (FDA) recognizes that substances used to make plastic wraps and containers can leach into food. But these plastics undergo a strict approval process by the FDA before they can be found safe for their intended use and subsequently marketed.

In the FDA safety-approval process, both the toxicity of a particular chemical and the amount of the substance expected to migrate into the foods are examined. Internet-fueled claims have asserted that plastics contain dioxins, a group of contaminants that the Environmental Protection Agency (EPA) has labeled as a likely human carcinogen. These claims argue that plastic containers pose dangers to health by releasing carcinogens when heated in the microwave. There is no evidence, however, that plastic wraps or containers contain dioxins.

In microwave-safe containers, the migration levels and toxicological concerns are well within the margin of safety. Only containers and plastic wraps that pass this test are made to withstand high temperatures and permitted to display a microwave-safe icon or the words microwave safe.

Here are some tips to keep in mind when using the microwave, Ordonez adds:

– Check the label. Use cookware labeled for safe microwave oven use. Things like dinner trays are formulated for one-time use only, and will say so on the packaging.
– Avoid microwaving takeout or recycled containers. Most takeout containers, water bottles and containers that hold cream cheese, margarine and yogurt are not microwave-safe.
– Vent the container. When using containers, leave the lid ajar or lift the edge of the cover. If using microwave-safe plastic wrap, wrap loosely so steam can escape. If microwaving food in a microwave-safe plastic bag, poke holes into the bag with a fork.
– Do not allow plastic wrap to touch food. When plastic wrap touches food, it may melt. To avoid this from occurring, wrap food carefully or use alternatives such as plain white paper towels, microwave-safe wax paper, or kitchen parchment paper.
– When in doubt, use a different container. If you cannot find a microwave-safe label or are uncomfortable using plastic in the microwave, you can always transfer the food to a glass or ceramic container labeled for microwave oven use.