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Is It Safe To Microwave Styrofoam?

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Last night, you treated yourself to delivery from your favorite restaurant. The amount of food was so much that you couldn’t finish, so you got a Styrofoam to go container and stashed the food in the fridge. Now you’re ready to reheat the meal in the microwave oven, but you have your concerns. Can you safely microwave food in Styrofoam takeout containers?

If your Styrofoam container has a label denoting it as microwave-safe Styrofoam, then you can reheat it in the microwave. Without that label though, harmful chemicals, such as styrene, could release through the heating process. Styrene is a known carcinogen.

In this article, we’ll talk more about what Styrofoam is, whether it’s toxic when melted, and if it’s safe to reheat these foamy containers. You’re certainly not going to want to miss this!

Is Melted Styrofoam Toxic?

The Dow Chemical Company created the brand name Styrofoam, which is a foam product made of extruded polystyrene.

Polystyrene is a monomer and hydrocarbon polymer product that comes in foam form or in a more rigid resin that’s brittle and clear (this type is used in toys, hair dryers, etc).

Food containers and coffee cups use foam polystyrene due to its excellent insulation properties.

If you stuff a Styrofoam cup or container into a microwave for too long, it can begin to melt.

The soft, foam-like consistency of Styrofoam becomes sticky and soft, akin to a marshmallow roasted over an open flame.

Just like marshmallows, cleaning all that Styrofoam gunk out of your microwave is not going to be fun.

Does Microwaving Styrofoam Release Toxins?

As a matter of fact the answer is yes!

Polystyrene, as the name suggests, contains styrene, a compound. Although it’s used to produce synthetic rubber and latex, styrene is not as safe as it sounds.

One 2009 study, by B. L. Momani at the Worcester Polytechnic Institute, notes that, “Polystyrene has been found to leach styrene into water and food. Styrene is a possible carcinogen and endocrine disruptor.”

Endocrine disruptors have been linked to problems in the reproductive system of animals, so it is pretty certain that they also affect human health.

Along the same lines, you’ve probably heard of bisphenol a. This is another endocrine disruptor that is found in certain plastic products used to coat the inside of some types of containers, such as some water bottles and canned foods.

Another study was published in 2018 in the journal Epidemiology. In the long-term study, the researchers followed 73,036 participants who worked in reinforced plastics companies in Denmark between 1968 and 2011. They were all regularly exposed to styrene.

The researchers found 665 instances of lymphohematopoietic malignancies, with more than 20 kinds of these malignancies identified.

This doesn’t necessarily mean the participants had cancer, but the researchers did note that “accounting for time since exposure showed a trend of cumulative styrene exposure…and a doubled risk…of acute myeloid leukemia following estimated high compared with estimated low cumulative exposure during the prior 15-29 years.

Is It Safe To Reheat Food In Styrofoam?

If reading the last section made you nervous, that’s fair. For a little bit of good news, many polystyrene containers are microwave-safe, as we mentioned in the intro.

We’ll talk later about how to tell whether these plastic containers can go in the microwave when heating food, so make sure you check that out.

For the containers that aren’t microwave-safe though, putting Styrofoam in the microwave can be disastrous.

You already know that the compound in Styrofoam – styrene – will be released under high temperatures.

You’re now also aware that prolonged exposure to styrene could potentially cause cancer, although it’s not clear how much exposure would be required.

On top of all that, another risk is present.

That’s the risk of the substances and ingredients that go into Styrofoam leaching into your food during the microwaving process.

This doesn’t exclusively involve styrene, but other compounds as well.

The fattier the foods you’re reheating, the greater the risk of substances leaching into your food. That means cheeses and meats are at the top of the list.

Can You Microwave Styrofoam For 30 Seconds?

Okay, you went from feeling nervous to being freaked out. I get it!

You don’t want styrene or other compounds in your food, especially such tasty foods as cheese and meat.

But, what if you only microwave Styrofoam products like Styrofoam plates or a polystyrene cup for small amounts of time, like 30 seconds or so? Is that okay?

That’s hard to say. Not all microwaves are the same, which means they’re operating on different power levels.

It takes temperatures of 392 to 435 degrees Fahrenheit for styrene to leach out, and microwaves usually only reach temperatures of 212 degrees.

Even still, and despite that 30 seconds isn’t a long time to reheat food, we would tell you that you should not microwave a Styrofoam container unless it is marked as a microwave-safe container.

That doesn’t mean you have to throw your leftovers away, though. You can always use safer materials to reheat cold foods.

Simply transfer the food from the Styrofoam container to a baking tray lined with aluminum foil and reheat your meal in the oven. This will give the hot foods a nice crispness that makes it taste fresher, anyhow.

You can also warm up your meal on the stove top or put it in the microwave in a microwave-safe dish or on paper plates.

What kinds of materials are heat-safe? Here are your options.


Created in 1915 by Corning Inc., Pyrex is borosilicate glass with a low rate of thermal expansion. As both kitchenware and glassware, Pyrex is safe for microwave use.

It is also safe for your stove and your oven. It’s made of potassium (0.3 percent), silicon (37.7 percent), aluminum (1.1 percent), sodium (2.8 percent), and boron (4.0 percent).

Pyrex can withstand temperatures of at least 450 degrees.


High heat is no match for ceramic cookware, which is lighter weight than stainless steel and more appealing to most people as well.

Ceramic can handle higher temperatures into the thousands of degrees, so you won’t have to ever worry about using a ceramic container to heat food.


Wood baskets for reheating food are another popular option that we’d recommend over non-food-safe Styrofoam.

You don’t have nearly as high of a temperature range with wooden food containers as you do with the other materials. If temps get up to 700 degrees, the wood will begin burning.

Between 450 and 500 degrees, your wood basket can get a little charred, but it would take hours for it to set alight.

Read about How To Put Out A Fire Without A Fire Extinguisher.

How To Tell If Styrofoam Is Microwave-Safe

We’ve mentioned it enough times, so let’s finally discuss how you can tell whether your Styrofoam or plastic containers are microwave safe.

Underneath the container, you’ll find a small triangle symbol. This is the microwave-safe symbol.

It will contain a digit between 1 and 7. Depending on which digit you see, the container can go in the micro.

The types of plastics labeled with the numbers 1, 2, 4, and 5 are all microwave-safe.

However, 3, 6, and 7 are not. The number 3 means the container has polyvinyl chloride or PVC. This is more common in peanut butter jars and plastic bottles than polystyrene.

The number 6 refers to polystyrene plastic and indicates that the Styrofoam cannot be safely reheated in the microwave.

If you’re curious, the number 7 means the plastic is polycarbonate or another non-recyclable material.

By the way, if your Styrofoam containers aren’t microwave-safe, then it’s not a good idea to heat them using any other means.

Besides the microwave, that means you shouldn’t put the polystyrene in an oven or a stove. That’s a surefire way to melt the Styrofoam!

Read our Kitchen Fire Safety Tips.

The Food and Drug Administration or FDA created the numbering system for safe plastics. Before any plastic product for food packaging earns one of the above numbers, it’s tested rigorously by the FDA.

That includes not only Styrofoam containers, but disposable plates and cups as well.

Even if the microwaveable symbol shows your Styrofoam food container is safe, you can still take some extra measures to make it even safer.

Before you set the timer on your microwave, inspect the polystyrene container. Is it cracked, scratched up, or damaged in any other way?

Then don’t use it. A partial breakdown of the Styrofoam means styrene or other chemicals could still leach out.

You should also open the container and then close it again. If the Styrofoam container has vents, open them.

These measures prevent pressure from building up that could lead to a nasty leftover food explosion in your microwave.


Whether it’s safe to microwave Styrofoam depends on if the container has the microwave-safe label. If it doesn’t, please don’t risk it anyway. Polystyrene contains a compound known as styrene that’s a potential carcinogen. Styrene can leach into your food, making it all the more dangerous.

Be sure to pass on this information to your family and friends, especially those who eat a lot of takeout! It could just save their lives.

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