While you can't use plastic containers in the oven to heat your foods, they are commonly used to warm foods in the microwave. Unfortunately, some containers are made with a controversial ingredient known as bisphenol A, or BPA. The chemical can leach into your foods, especially when the containers are heated. Prevent this from happening by using glass or other BPA-free containers to warm or cook your foods.

BPA is a chemical found in polycarbonate plastic containers; sealants; epoxy resins that line canned foods; and thermal paper used in cash registers. Polycarbonate plastic containers are hard, shatter-resistant and clear. Plastic items containing BPA are usually marked with the No. 7 or the letters "PC" near the recycling symbol. Small amounts of BPA leach into the food and liquids stored inside polycarbonate plastic containers and cans lined with BPA-containing resins. While there is some concern about the possible negative health effects BPA can have when ingested, the U.S. Food and Drug Administration classifies BPA as safe for use in food containers and packaging.

When BPA-containing polycarbonate plastic containers are heated in the microwave, the amount of BPA they leach into foods increases. Polycarbonate containers leach up to three times more BPA into foods and liquids when heated, according to a 2009 study published in the "Journal of Toxicology and Environmental Health." This same effect occurs when boiling water and hot foods are added to polycarbonate plastic containers, without heating them. The type of food heated can also make a difference. In general, foods containing high-fat ingredients, such as heavy cream and butter, absorb more BPA than other types of foods heated in plastic containers.

Not all plastic containers are safe to heat in the microwave because they can melt. In addition to BPA, some plastic containers also contain chemicals called phthalates, which are used to make them more flexible and durable. Like BPA, phthalates leach into foods, especially when the containers are heated, warns the United States Environmental Protection Agency. These chemicals can negatively affect our endocrine systems and disrupt the way our hormones function, just as BPA can. Because the type of plastic that contains phthalates is polyvinyl chloride, avoid using containers made from this material to heat your food. They are usually marked with the No. 3 or the letters "PVC."

When heating foods and storing hot foods, use glass or ceramic containers rather than plastic ones if you are unsure of the type of plastic they are made of. The FDA recommends that you also keep plastic wrap from directly touching your foods when heating them and never reuse single-use plastic containers. This will help you to avoid excess chemicals from winding up in your food. Plastic containers labeled with a 1, 2 or 5 don't contain BPA or phthalates. Wash BPA-containing items by hand to avoid scratching them in the dishwasher, which can cause them to release more BPA into food.