Can You Microwave Cardboard? Can They Set On Fire?!

Sometimes you may need to reheat some food quickly and want to avoid using dishes. So you may be wondering can you microwave cardboard?

Is it safe to put a pizza box in the microwave to warm up pizza? How about microwaving a leftover cardboard takeout container?

Also when is it ok to put cardboard in the microwave and when isn't it? Are there any health concerns we should be aware of?

I'm often reheating last night's pizza in the microwave on the cardboard box it came in and I wanted to do some research into whether or not this was ok. Here's what I found:

It is generally safe to microwave cardboard or reheat food in the microwave in cardboard boxes, bowls or containers. However, scrunched up cardboard can set on fire in the microwave. Some cardboards with plastic film, wax, ink, glue or metal are not safe to microwave due to health concerns or a higher likelihood of fires.

I wanted to get to the bottom of whether or not you can microwave cardboard so I decided to do some of my own experiments with cardboard in the microwave and see under what conditions it was dangerous.

What Happens When You Microwave Cardboard?

I decided to do some experiments myself to see exactly what happens to cardboard when you put it in the microwave.

You can see my video below:

When you put cardboard in the microwave it gets extremely hot (over 100ºC or 212ºF). If laid out flat it is highly unlikely to set on fire, but when scrunched up it can become hot enough to combust and burst into flames.

So what exactly happened when I microwaved cardboard?

To begin with I put in some smaller flat pieces of cardboard by themselves for 2 minutes.

When I pulled them out they were extremely hot – around 248ºF (120ºC) but they didn't catch on fire.

This is most likely what will happen if you are placing food on top of flat cardboard in the microwave.

Whether it's a pizza box or a cardboard takeout container it's unlikely to catch on fire while it's lying flat. Especially if there is food in the microwave to absorb a lot of the microwave's energy.

Next I put in a flat piece of cardboard that was from a chocolate wrapper.

The cardboard had a wax exterior but it also had some golden ink. I wanted to see if this metallic ink would set on fire.

Turns out it did, the ink obviously got hot enough to cause the cardboard to combust and catch on fire.

After that I put the test to the extreme and placed in some scrunched up cardboard and put the microwave on for a total of 5 minutes.

It only lasted about 2.5 minutes until the scrunched up cardboard burst into flames inside the microwave.

I've tested this idea of scrunched vs flat with cardboard, paper towels and paper and in all cases found that when flat the cardboard/paper gets hot but doesn't set on fire.

However, when it is scrunched or folded up given enough time it will always catch on fire.

Can You Microwave Cardboard Takeout Containers?

Generally speaking it is fine to microwave cardboard takeout containers. Given their generally flat shape and the fact that you have food in them they are unlikely to catch on fire.

However, I did discover in my article on microwaving cardboard takeout boxes that a bunch of cardboard these days has what's called PFAS, which is a class of toxic chemicals linked to cancer and other health issues.

So while you won't set your microwave on fire it's probably not the best idea health-wise to be reheating food inside cardboard takeout containers in the microwave.

The extreme temperatures the cardboard can get up to in the microwave make it more likely to leech chemicals into your food.

So use a glass or ceramic plate, bowl or cup where possible, even it if means having a little bit more to clean up.

Can You Microwave Cardboard Pizza Boxes?


The same deal goes for microwaving cardboard pizza boxes.

Generally you'll be microwaving them for less than a couple of minutes in order to warm up your pizza and they are highly unlikely to set on fire or even smoke in this short period of time.

However, prolonged periods in the microwave could cause the corners or a pizza box to catch on fire.

Plus there is the same health concerns with pizza boxes that we have with cardboard takeout containers. They may contain harmful PFAS that you want to avoid wherever possible.

Can Cardboard Set On Fire In The Microwave?

Cardboard can set on fire in the microwave if you aren't careful and there are a few common ways this can happen. Most commonly is having the cardboard scrunched up and not flat or having metal pieces or metallic paint on the cardboard.

These two scenarios create areas of concentrated areas of heat and the cardboard can combust and set on fire.

Some types of food you should be careful with are things like hot pockets and pizza that contain the foil-like crispers in them.

These are a super thin piece of foil that is designed to get extremely hot extremely quickly in the microwave. This helps to brown the food and give them some crisp, but they can also get so hot they can cause paper and cardboard to burn. So be careful.

You also need to be careful if you've got scrunched up paper or cardboard. Something like a McDonalds bag scrunched up to fit in the microwave is a big no-no.

The points where the cardboard is bend and compacted tends to get so hot it can all catch on fire. And the fire will happen really quickly.

In my experiment there was no fire at all and then all of a sudden BAM, the entire piece of cardboard was on fire.

So always watch your microwave if you're heating up something on cardboard.

Does Cardboard Leech Chemicals Into Your Food When Microwaved?

You would think cardboard is perfectly fine and safe to be heated up in the microwave. It's not going to leach harmful chemicals into my food like plastic does? Right?


Well potentially wrong.

While cardboard doesn't have any BPA like plastic does there are a range of other dangerous chemicals present in cardboard called PFAS.

According to Business Insider Australia a watchdog for evidence of PFAS, , a class of toxic chemicals linked to cancer, in paper to-go boxes and one sandwich wrapper product at Whole Foods Market.

A bit more research into PFAS and you find out that CNN have reported:

A growing body of science has found that there are potential adverse health impacts associated with PFAS exposure, including liver damage, thyroid disease, decreased fertility, high cholesterol, obesity, hormone suppression and cancer.

There is also this article from the United States Environmental Protection Agency warning us against the health dangers associated with PFAS.

But how can you know if you pizza box or cardboard take out containers contains PFAS or is safe to use?

You don't know

For a normal person like me and you there is absolutely no way of telling if the cardboard we are putting in the microwave is safe to reheat food in or not.

Would I Personally Put Cardboard In The Microwave?

Would I, after having done the research and experiments, put cardboard in the microwave?

Personally, I would avoid putting cardboard in the microwave and use a ceramic plate/bowl wherever possible. But for quick reheating of pizza, or coffee or something small I would feel comfortable putting cardboard in the microwave sometimes.

I wouldn't put cardboard in the microwave for a long period of time. Both because I know it can catch on fire as well as not being entirely sure whether or not it's going to leach harmful chemicals into my food.

If I can avoid that health risk but the tiny bit of extra effort it takes to use a plate then I'll go ahead and do that.

What about you? Would you put cardboard in the microwave knowing what you know now?