There are quite a few things you should never put in the microwave, such as styrofoam, water bottle, plastic bags, aluminum foil, or stainless steel. If you do so, three things can happen depending on the material of the object you put in your microwave:
- The object will be damaged.
- The content of the object will be damaged.
- The microwave oven itself will be damaged.
Of course, any combination of the previous scenarios is possible, sometimes with some serious consequences. So it’s definitely worth checking what you are about to put in the microwave.
The decline of disposable plastic has made cardboard take-out containers more popular than ever and many people are unsure if it’s safe to put them in the microwave.
Can you microwave cardboard? Here’s a quick answer:
You can microwave pure cardboard containers with food or drink under supervision. However, it is not safe to microwave cardboard containing plastic, wax, ink (unless the label says otherwise), glue, or metal. Do not put cardboard in the microwave without any food or drink and avoid overheating.
Previously I always got my takeaway food from my favorite restaurant in a styrofoam container. I never put that one in the microwave because I knew that it could melt and chemicals might leach into the food (1).
FYI – Wondering if all types of styrofoam containers are hazardous when heated in the microwave? Find the answer in this article based on scientific research.
However, on the last few occasions instead of the usual styrofoam containers, I got cardboard ones. I knew that many other restaurants switched to cardboard containers previously and I wondered if it was safe to use the microwave to reheat my food (or drink) in a cardboard box.
So I’ve done some research on the topic to get a reliable, science-based answer (after all, it’s about our food). In this article, I’ll share all my findings with you so that you have an exact answer to your question.
Let’s see if you can microwave cardboard safely.
Do you like stuff that’s both useful and interesting? Then check out our other exciting articles that give you an answer based on science:
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Click on the titles and learn something new and useful about home safety!
How Does Microwave Warm Up Cardboard?
If we want to know how microwaves affect cardboard, first we have to understand what exactly happens inside the device. A microwave oven works by generating microwaves. Sure. But what are microwaves?
Microwaves are electromagnetic waves with a wavelength between infrared radiation and radio waves. Every microwave oven has a built-in vacuum tube called magnetron that’s responsible for generating microwaves at a frequency of 2450 Mhz (2, 3).
How do microwaves heat your food? They are absorbed by water, fat, and sugar molecules and as a result, these molecules start to rotate and interact with other molecules as well. This molecular dance generates heat and warms up your food, drink, and… cardboard.
Microwaves can also be absorbed by certain types of plastic, that’s why you should only put certified plastic containers in your microwave oven.
How Does Cardboard Interact with Microwaves?
Cardboard is used in many industries and an extremely wide variety of products are made from it: pizza boxes, coffee cups, shipping boxes, take-out food containers, pen cases, and laptop stands, just to mention a few.
There are many existing types of cardboard, such as corrugated or triple wall cardboard. However, they have one thing in common: they’re all made from wood fiber (cellulose), like paper (4).
Cardboard typically has a moisture content of 9-12% (5). If you put it in the microwave, the moisture content will start to decrease and can drop to 1% depending on the time the cardboard spends in the microwave oven and its surface area.
This process sometimes can occur pretty fast, and if you leave the microwave on, chances are the cardboard gets completely dry and catches fire. Note that this scenario applies only to those cases when you put cardboard in the microwave on its own with no other stuff (such as food or drink).
Therefore, do not microwave cardboard on its own because, under certain conditions, it may lead to a fire. Anything made of cardboard is potentially flammable.
I know you can find some comments and articles that claim that it’s okay to microwave cardboard even on its own (for drying purposes, for instance), however, I want to call your attention that if it’s not done with great care, it can be really hazardous. While it’s true that much depends on the type of cardboard you use, I do not recommend microwaving cardboard on its own.
What About Cardboard Boxes and Containers with Food or Drink?
Now that we’ve come to the conclusion that it’s not a good idea to microwave cardboard on its own, let’s examine those cases when you just want to warm your food or drink up in a cardboard container.
As I’ve already mentioned, there are many types of cardboard containers. Generally speaking, cardboard boxes that contain wood fiber without any additional material are usually safe to microwave until there’s food or liquid in them.
When you reheat food or drink in a container made of cardboard, the microwaves are primarily absorbed by the water, sugar, and oil molecules of the food, so the moisture content of the cardboard won’t decrease to a dangerous level.
Therefore, it’s usually safe to warm up food and drink in a container made of pure cardboard under supervision (unless the label says otherwise). Of course, you shouldn’t cook or boil in these containers, but reheating food is okay in most cases. Microwaving your food in a cardboard box for too long can, indeed, damage both the container and your food.
Cases When You Shouldn’t Microwave Cardboard
Keep in mind that some cardboard food containers are coated with plastic or wax or contain these materials. They are not safe to microwave unless labeled otherwise.
Wax used to coat paper and cardboard containers is, in most cases, no other than polyethylene (PE) plastic (6). As you may know, plastic can melt or warp at higher temperatures and may leach into your food (7). It’s needless to say that it’s anything but healthy. It’s usually easy to recognize wax-coated food containers: they’re shinier and smoother than pure cardboard.
Ink on the container can also be a concern. While it’s true that not all cardboard containers with ink on them are hazardous, I suggest you only put in the microwave oven those that are labeled microwave safe.
Glue is another substance that’s pretty common in packages. Make sure you don’t put any cardboard box in the microwave that contains glue: it can emit toxic fumes and may melt and migrate into the food.
Cardboard take-out containers sometimes also contain some metal (such as some Chinese take-out boxes). As you might know, you should never put anything that’s made of metal in your microwave: it can seriously damage the device and even lead to a fire (8).
There might be cases when you can’t decide if you can microwave an item made from cardboard. If you’re unsure, always use containers that are labeled for microwave use.
Note: It’s not recommended to heat food in brown paper bags in the microwave. Heat may cause them to ignite and the glue and ink can emit toxic fumes (9).
Don’t forget that reheated food and liquid sometimes can be extremely hot as well as cardboard containers. So always touch and consume them with precaution.
Putting It All Together
There could be many reasons why you would want to microwave cardboard, however warming up food or drink is probably the most common one. As you can see, microwaves significantly decrease the moisture content of cardboard if you put it in the microwave oven on its own. This may lead to hazardous situations because cardboard can ignite in the device. Therefore, you should never put an empty cardboard container in the microwave on its own.
On the other hand, pure cardboard boxes with food or drink can be reheated (but not cooked) in the microwave. However, always make sure the container does not contain any material that’s unsafe to use in the microwave, such as metal or plastic. When in doubt, use containers that are labeled microwave safe.