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Is it safe to microwave plastic containers? The science of reheating leftovers, explained

Ditch the little black box.

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Close-up of woman enjoying takeout pasta and chicken dish
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Takeout leftovers are one of the great simple pleasures in life. Knowing you get to heat up a little pad thai for lunch is a source of joy and calm. But if you’re microwaving your food in the plastic container it came in, you’re also eating plastic along with your noodles. Is that dangerous? Before you scream “yes,” the science is actually more complex than you might think.

Plastic is convenient and a top choice for food storage. That’s why it is such a popular product: It’s cheap, strong, and some kinds are also relatively heat resistant, making them seemingly perfect to hold food in the microwave without spills.

At the same time, humans are constantly ingesting microplastics — tiny bits of plastic that measure less than 5 millimeters across (less than the width of a pencil eraser). Microplastics infiltrate the seafood supply chain, and studies on human poop show plastic passes through our digestive systems all the time. But none of these truths tell us whether plastic is actively damaging our health.

How does plastic get into food?

Plastic is everywhere, literally. Fish and other sea creatures also consume vast amounts of microplastics because the ocean’s churning with them, and of course, we eat seafood. Microplastics also flake off from plastic containers.

When heated, these containers can shed microplastics, as can other common food storage and reheating practices.

“If you dishwash plastics, if you heat them up in the microwave, if you’ve used them for many years, and they're worn down, all of those will release more microplastics,” Bruce Lanphear, a health sciences professor at Simon Frasier University in Vancouver, tells Inverse.

Other research backs this up: A 2017 study found plastics in table salt, and a 2019 study found that teabags packaged in plastic brewed a microplastic-dense cup of tea.

Is it dangerous to eat microplastics?

Here’s where the science isn’t so conclusive. Plastic is a recent invention and there aren’t any long-term studies on the effects of microplastic consumption in humans. There are short-term studies and some done on animals, but nothing definitive to tell us how eating plastic affects our health over the longterm.

There are however stablished thresholds at which many substances become toxic for humans if ingested. Arsenic is a good example; there’s a limit to how much arsenic is allowed in public drinking water before it becomes unsafe for human health. In the U.S., that’s 10 parts per billion, which means that in a ratio that’s one billion parts water, the amount of arsenic cannot exceed 10 parts.

Thing is, there isn’t a threshold like this for microplastics, and no clear information on what the consequences are if a person crosses that threshold.

In one sense, our current behavior is fine until proven to be harmful. This interpretation says it’s okay to keep eating food in plastic containers because as far as we know right now, it’s not particularly harmful. Another way is “better safe than sorry.”

We don’t know how harmful consuming microplastics is, so it’s best to limit consumption until we know more. Use non-plastic containers whenever possible.

We also know exposure to high amounts of chemicals typically in plastic, like phthalates and bisphenol A (BPA), can be dangerous to human health, even though they’re ubiquitous in everyday objects. Lanphear says that if someone who’s pregnant consumes phthalates and BPA, it could affect development of the embryo. Particularly, in developing males it can affect their fertility because BPA is anti-androgenic, meaning it blocks hormones like testosterone.

Over the next few decades, as we continue to refine plastic formulas and study long-term microplastic consumption, a clear answer may emerge.

Microwaving your leftovers in plastic containers might allow some microplastics to reach your food, though there’s no conclusive evidence that this is harmful to human health.

BRETT STEVENS/Image Source/Getty Images

How much plastic do I eat?

The amount of microplastics you consume depends on what food is available to you and how you prepare it.

One 2019 study from the University of Victoria in British Columbia, published in the journal Environmental Science & Technology, estimates a person will consume anywhere between 39,000 to 52,000 particles a year and that’s not even factoring inhalation. People who favor drinking bottled water could be drinking an extra 90,000 microplastics per year, while those who drink tap water may consume about 4,000 microplastics.

Like water bottles, takeout containers are a substantial microplastic source. A 2020 study from East China Normal University in Shanghai published in the Journal of Hazardous Materials looked specifically at takeout containers, and concluded: “[b]ased on the microplastic abundance in take-out containers, people who order take-out food 4 to 7 times weekly may ingest 12–203 pieces of microplastics through containers.” These bits often flake off from the sides or top of the container and are too small to detect easily.

Another study from 2016 found a correlation between frequently eating fast food and increased levels of phthalates, substances often added to plastics to increase their durability. Those who ate fast food frequently had phthalate levels up to 40 percent higher than those who rarely ate fast food.

Should I microwave food differently?

If you follow the “better safe than sorry” approach the next time you get takeout, you can put your food on a plate and refrigerate leftovers in a non-plastic container (even though it’s so easy to just eat right from the little box you stuck in the fridge when you got home). When you gift yourself leftovers the next day, microwave the food in a microwave-safe bowl or container, like one made of glass, ceramic, or porcelain. (Remember: No metal in your science oven.)

This doesn’t mean you need to swear off your favorite frozen dinners or exclusively do takeout from spots that offer cardboard vessels. Cutting back on plastic any amount lowers your exposure to microplastics and whatever risks that may come with consuming them.

CHECK, PLEASE is an Inverse series that uses biology, chemistry, and physics to debunk the biggest food myths and assumptions.

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