Can you microwave cardboard?

Can You Put Cardboard In the Microwave?

Microwaves are a fantastic time-saver in the modern kitchen, but even the best and most convenient inventions can come with hidden hazards. Microwaves are no exception to this rule, as there’s a long list of materials that should never be microwaved (each with a specific reason it would be dangerous to ignore the warnings). If you’re wondering whether cardboard makes the list, you’ve come to the right place!

Can Cardboard Go In the Microwave?

The answer isn’t quite as simple as yes or no, unfortunately.

The term cardboard is an umbrella term for many different products. These can vary widely in thickness, size, color, and purpose. Really, all these different materials have in common is that they’re thicker and tougher than paper, despite usually being made with the same material, cellulose.

The manufacturer Whirlpool (whose product line includes several microwaves) warns against adhesives, waxes, and metal components that might be present in cardboard packaging on their website, cautiously advising their customers to avoid microwaving cardboard in their Whirlpool-branded machines altogether.

While it’s understandable that a company like Whirlpool would go with a simple ‘no’ answer here for liability purposes, the problem with their “better safe than sorry” approach is that it doesn’t account for the kinds of cardboard intentionally designed to be microwave-safe (so yes, they do exist).

To clear up any confusion, a few common cardboard products will be described below, along with definitive answers about whether it’s safe to put them in a microwave or not.

Can You Microwave Cardboard Pizza Boxes?

Depending on the box manufacturer, a takeout pizza box will sometimes have an aluminum coating to help keep the pizza steaming hot while it’s being transported. As we all should know, aluminum is definitely not microwavable!

Even when there’s no metal bonded to the surface, most corrugated cardboard (the cardboard most pizza boxes are made out of) has some kind of adhesive or glue binding its layers together. Beyond metal or glue, some pizza boxes are coated in PFAS to keep your pie from sticking to the box.

Unfortunately, all this adds up to a definite no when it comes to microwaving the box your cheesy dinner arrived in.

Can I Heat a Cardboard Pizza Box In a Conventional Oven?

For many of the same reasons you shouldn’t microwave a pizza box, you should also not heat it in the oven. Additionally, there is a much higher chance of the cardboard catching fire in a conventional oven. For that reason alone, this method is even more dangerous than microwaving your pizza in the box, which is the reason it’s been added here. If you were considering trying the oven since the microwave isn’t a good option, don’t do it!

Can You MIcrowave an Empty Cardboard Box?

Microwaving an empty cardboard box is incredibly dangerous, and you should never do it. In fact, it’s the second most dangerous item on this list, beat out only by tossing a pizza box in the oven.

Why? Just like with the oven, it goes back to the flammability of cardboard. The condensed version of the science on this one is that there’s a tiny bit of moisture in cardboard from the air, which will heat the cardboard up hot enough to evaporate itself. What’s left when that happens? Unbelievably dry tinder at a searing hot temperature that will inevitably turn your convenient microwave into a very inconvenient fireball.

Can You Microwave Disposable Coffee Cups?

Disposable coffee cups are usually made of thick paper. (or styrofoam, which is certainly not microwaveable). Because paper isn’t waterproof on its own, they’re almost universally coated in high-density polyethylene plastic (or HDPE) to prevent a soggy, leaky cup riding around in a customer’s cupholder.

A 2021 study in the Journal of Hazardous Materials found that exposing this type of cup to heated water both degraded the HDPE liner and caused it to leach toxic heavy metals, microplastics, and other nasty chemicals like sulfate, chloride, and nitrate.

Since microwaves heat unevenly, it stands to reason that microwaving a cup lined with HDPE could cause some spots to get even hotter than the cups in the study did. Without a doubt, this makes them unsuitable for microwave use as well.

Can You Microwave Fast-Food Sandwich Boxes?

The paperboard boxes McDonald’s serves many of its fast-food offerings in are marked with the PAP-21 recycling code, meaning it’s made of strict paperboard with no plastic or wax coating. This means it is probably at least a little bit safer to microwave their sandwiches in the boxes they came in compared to most other paper goods.

Other possible concerns in regards to microwaving a container made from PAP-21 material are dye, ink, and glue. These paperboard boxes are not made of corrugated cardboard so they usually don’t contain adhesive, although you would need to check the folded areas to make sure there’s nothing chemical holding things together.

Dyes and inks are perhaps the most concerning objection to microwaving your Big Mac in the box. The USDA specifically mentions the possibility of inks giving off toxic fumes if they are not intended for the level of heat you subject them to. This statement was specifically about cooking in brown paper bags (versus containers that are already intended to be in contact with hot food like sandwich boxes), but a 2018 study out of Istanbul University found excessive levels of mercury in many paper goods used for food packaging. Even more disturbingly, researchers collected data showing the packaging was consistently leaching cadmium, zinc, aluminum, and lead into the food it contained due to the heavy metals being present in the inks and dyes the packaging used.

All things considered, while PAP-21 burger and sandwich boxes seem like the least dangerous paper product to microwave that isn’t specifically produced for that purpose, it still appears to be a bad idea if it can be avoided.

Can You Microwave Cardboard Takeout Containers?

The actual name for those cute cardboard takeout boxes you get from Chinese restaurants is the oyster pail. While oyster pails are instantly recognizable (and might even trigger a craving for lo mein on sight), they are quite often coated in wax or plastic.

They also occasionally contain small pieces of metal on the sides, either as part of a handle or for the handle to loop through. There is also red ink used in most of them, which was the likeliest culprit for leaching lead and cadmium in the Instanbul University study mentioned above.

Unless the packaging specifically says it’s microwave-safe, the safest choice is to toss your leftover takeout onto a known microwave-safe plate before rewarming. On the bright side, this also serves as a good excuse to finish a whole box of noodles by yourself, right?

Can You Microwave Paper Plates?

Some paper plates are perfectly safe to microwave, while others range from questionable to outright dangerous.

Some varieties are waxed, coated in plastic, full of inks and dyes, or possibly even all of the above. The safest paper plates to microwave are uncoated plain white plates that are specifically marked as microwave-safe.

Of course, it’s also important not to make the mistake of microwaving styrofoam plates, which are usually sold alongside paper ones. They are usually cheaper, but they should never be microwaved or exposed to extremely high temperatures due to long-known leaching issues with styrofoam.

What Kinds of Cardboard Can Go In The Microwave, Then?

Now that we’ve covered some of the cardboard and paperboard products that range from ‘completely unsafe for the microwave’ to ‘it depends on the kind,’ it’s finally time to discuss the products that are not just safe to use in the microwave, but specifically meant for it!

Can You Microwave Frozen Meal or ‘TV Dinner’ Trays in Their Cardboard?

Yes! As long as there are microwave directions that specifically tell you to cook the food in the tray it comes in, frozen entrees are considered safe to cook in your microwave. The USDA does recommend only using the packaging once rather than reheating it later, though. This is because the packaging is specifically designed to be used once, and has not been tested to see if it will release anything harmful when used repeatedly due to the material degrading.

Can You Buy Microwave-Safe Cardboard Containers?

Absolutely, and there’s an enormous selection. As sustainable packaging has become more and more popular, cardboard plates, containers, boxes, bowls, and just about any other food packaging solution you can imagine has been designed to be specifically microwave-safe, disposable, and far more sustainable than the plastic and paper alternatives that once dominated.

For example, there are now 100% Compostable Clamshell Takeout Boxes, which could be brought along from home to the restaurant or used to serve food at a large gathering. These specific ones are made out of sugarcane fiber and have no coating whatsoever, providing the convenience of a takeout box you can microwave without the harmful chemicals so often present in traditional cardboard containers.

There are endless possible reasons why someone might seek out microwavable cardboard products, but the most obvious one would be buying a more environmentally friendly and sustainable product that is still disposable at the end of the day, ensuring that convenience isn’t sacrificed in the pursuit of ‘living green.’

Of course, these kinds of products are also just a good way to avoid the dangers you could potentially be exposed to with traditional cardboard and paperboard packaging. These Compostable Coffee Cups for example are made from corn starch. Considering that many specialty coffee shops allow you to bring your own to-go cup, these could be a fantastic option for someone looking to avoid exposure to the polyethylene that’s almost universally present in the cups the coffee shop would provide you themselves. Rather than having to bring a washable cup away from the home in the morning and back home at night to be washed, there’s a plastic-free alternative that can still be tossed out and replaced with a new one the next morning.

So basically, can you put cardboard in the microwave? The answer has firmly been established as yes, but only if it’s the right kind (and not empty).

Are There Alternatives to MIcrowaving Cardboard?

Again, the answer is a firm yes. If you’re not interested in buying safer cardboard meant for your microwave, there’s an array of microwave-specific cookware available that will not only let you avoid the dangers of microwaving conventional cardboard but will also yield a better result when reheating food.

Microwave cookware isn’t all plastic either. If you’re someone who prefers to avoid heating even microwave-safe plastics, products like this Ceramic Stoneware Cooker will yield a far better texture heating up a single serving of leftover takeout than drying it out (and potentially poisoning yourself) using the box you brought it home in.

There are also very practical options that pack a one-two punch. Silicon cookware is often not only microwave safe but oven safe as well. This Small Pot and Steamer Combo is one such example.

What’s the Bottom Line?

The first key takeaway: you can absolutely microwave cardboard, but you shouldn’t risk microwaving any cardboard or paper products (or really any materials in general) that aren’t specifically designed to be microwave-safe. There are too many variables involved with food packaging and safety to take chances, especially when you’re betting your long-term health on whether it’s going to work out or not.

The second key takeaway: if you don’t feel comfortable microwaving cardboard at all after learning about some of the risks today, you don’t have to! The culinary applications of ‘microwave radiation’ were first discovered in the 1940s, and we have had plenty of time to invent a wide variety of microwave-safe materials since then.

Whether you like the convenience of using cardboard disposables in the microwave or you’re looking for a reusable solution to replace it, you have options. Please throw the pizza box out now!

Sources

• Live Science
• McDonald’s
• Microwave Master Chef
• NC State University
• ScienceDirect
• The Guardian
• USDA
• USDA
• Whirlpool
• WIRED

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