7 Things That Are Not Microwave Safe

On one end, microwaves are a gift. They cook and reheat food quickly, and they can turn a sad meal into a happy one. At the same time, microwaves can completely ruin a meal if you don't understand what materials are safe and unsafe to zap.

"Undeniably, microwaves are one of the most useful and popular kitchen appliances in history," says Mary Jones, editor at CookyMom.com. "While these appliances have proven so beneficial, there is a tendency that they may cause a problem like fire, especially when you place the wrong thing inside," she says.

The good news is that you can easily avoid a kitchen fiasco by taking a moment to consider the food containers you are using. From styrofoam to metal, here are things you should think twice about before putting in the microwave.

Expanded polystyrene foam — more commonly known as "styrofoam" — is type of plastic that comes in many forms, from takeout containers to packing peanuts. While the FDA deems expanded polystyrene foam safe for food contact, and there are some polystyrene foam containers that are marked as microwave-safe, it's best to avoid putting styrofoam in the microwave altogether.

"Even though these types of containers are commonly used to keep your food and drinks warm due to the insulation provided, Styrofoam doesn't react well to the high temperatures emitted by a microwave," says Ron Shimek, President, Mr. Appliance. "This could lead the material to break, melt, or release toxic chemicals that could mingle with your food.

Microwaving plastics can cause chemicals to leach into your food or drink. Unless it's clearly labeled safe for the microwave, you should avoid microwaving any food in plastic.

The two main chemicals that could leach out are phthalates and BPA, which are commonly added to plastic to make it flexible and pliable. Moreover, thin, single-use plastics and cold-storage containers — like condiment cups, butter tubs, and yogurt cartons — can melt in the microwave.

It's worth noting that most plastic products have a number associated with them, typically displayed in a small recycling triangle emblem. You can technically use those numbers as a guide to understand what plastics are microwave-safe. As a quick guide, avoid putting plastics with numbers 1, 3, 6, or 7 in the microwave. Safer options are labeled 2, 4, and 5.

Even still, it's better to be safe. Microwave-safe plastics usually have a marking indicating they are safe to use, so make it a habit to check for those symbols, Jones says. A microwave-safe symbol usually features three wavy lines, which indicate heat.

To understand why it's not a good idea to put metal in a microwave, you need to have a basic understanding of how microwaves work.

When you press the start button, an electron tube inside the microwave — known as a magnetron — generates electromagnetic waves. Those waves are absorbed by the water molecules in the food, producing the the high temperatures that heats everything up.

Metal is unsafe to put in the microwave because it does not absorb electromagnetic waves. Metals are conductive, so with enough heat, those waves become concentrated on the surface.

"When microwaved, all metals, including utensils, cookware, and travel mugs can spark a fire," says Shimek. This also includes any objects with metal in it, like Chinese takeout containers with metal handles, dishware with metallic paint or trim, or any packaging with metal twist ties or staples.

It might seem like a good idea to heat up your coffee or tea straight in your travel mug, but this definitely should be avoided. Most travel mugs are made out of stainless steel or plastic. Travel mugs made out of ceramic are the only kind of mug that's safe to reheat.

As a thin form of metal, aluminum foil should not be put in the microwave. Aluminum foil will reflect energy, not absorb it, says Jones. Just like metal utensils, foil can spark, resulting in damage or a fire.

It also doesn't make sense to cover food in aluminum foil. Remember: Metal reflects microwaves away from foods, and generally would slow their heating. As the Department of Agriculture's Food and Safety Inspection Service (FSIS) explains it, food can't absorb microwaves if it's covered by aluminum foil.

That said, the FSIS says aluminum foil in small amounts, for short times, could be safe. For example, you may want to use a small amount of foil to shield a portion of a food item to prevent it from overcooking. If you do this, make sure the foil is as smooth as possible and keep close tabs on cooking.

Plain cardboard takeout containers should be OK to microwave — check for the microwave-safe label. However, if the container is lined with waterproof coating of wax or plastic, or the box contains metal, skip the microwave. Provided it truly is plain cardboard—again, check for that microwave-safe symbol—you can microwave it for about 60 to 90 seconds only; otherwise, it will dry out and pose a fire hazard.

Some paper products like towels, plates, and bowls are microwave safe. Shimek says you should always use one of these to cover a container in the microwave to keep the contents from splattering.

However, paper bags are a different story. "Paper bags can't withstand the same level of heat and can emit toxic fumes or even catch fire, so avoid putting them in the microwave," says Shimek.

As mentioned, glass, ceramic, and plastics that are safe to use in a microwave will be labeled clearly. Here are some common materials you can use in the microwave:

We test and recommend the best food storage containers. Several of our favorites are microwave safe. Check out our picks:

Nothing beats the convenience of a microwave when it comes to heating up food. However, you'll have a much bigger problem on your hands than cold leftovers if you use cookware not suited for microwave use. Be sure to check for the microwave-safe mark on all containers and exercise caution when taking heated items out of the microwave.

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