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Is It Safe to Microwave Cardboard?

Do you fancy takeout food or a delivery pizza? It’ll always come packed in cardboard or paper when you order from your local restaurant. Luckily, they give twice as much food as you require for a meal, meaning you’ll always have leftovers.

The microwave is the fastest and most convenient way to reheat leftover food. But if you don’t have microwave-safe dishes, you may be wondering, ‘is it safe to microwave cardboard?

Yes, you can microwave food inside cardboard. However, it can be dangerous because the materials can catch fire or leach harmful chemicals that contaminate your food.

Can you microwave food? Here’s everything you need to know about microwaving food in cardboard. So keep reading for more insight.

Related post: Can You Microwave Cardboard?

Can You Put Cardboard in the Microwave?

The science behind the safety of microwaving cardboard is quite confusing. That’s because most experts consider it a good and bad idea.

It’s generally safe to reheat paper plates in the microwave. You can also microwave cardboard safely without any problem. The thing is that the two consist of paper materials.

But it’s a bad idea to microwave cardboard boxes and paper. Why is that? These materials often provide a fast and easy way to start fires. However, considering the microwave’s operational concept, microwaving cardboard isn’t a big problem.

Food and drinks often contain water molecules. When microwaved, the water molecules move faster and at high frequencies. The process generates friction, causing food to heat up.

Let’s assume you use the microwave the right way. Cardboard may absorb some moisture from food while in the microwave. But it’s still impossible to reach higher temperatures that can cause combustion.

That doesn’t mean all cardboards are safe in the microwave or can’t spike fire. Before putting cardboard in the microwave, make sure it has a microwave-safe label.

Besides, ensure you microwave cardboard for a short period. And never microwave cardboard more than once.

How Do Microwaves Heat-Up Cardboard?

Microwave ovens often operate by producing microwaves. These are electromagnetic waves whose wavelength ranges between that radio waves and infrared radiation.

Each oven has a magnetron (an inbuilt vacuum tube) that generates microwaves at 2450 MHz. When you place your food in the microwave, the electromagnetic waves absorb water, sugar, and fat molecules from food or beverage.

These molecules will start rotating and interacting with other molecules. This molecular interaction generates heat to warm up your drink, food, and cardboard.

Can You Microwave Food in a Cardboard Box?

The answer is yes. You’d, however, do so carefully and in moderation. In other words, you must reheat cardboard and its contents for a shorter time.

Cardboard is quite a fast conductor of fire. So prolonged heating could make it catch fire easily. In most cases, corrugated board or cardboard is combustible at high temperatures of 4270C. It will start a fire when exposed to open flame.

Apart from that, most microwaves work at lower power levels than cardboard’s ignition point. Therefore, getting cardboard burnt while microwaving your leftovers won’t be a significant threat.

Cardboard boxes also contain wood fiber and pure pulp materials, making them ideal for microwaving food. Plus, they may have some metal compounds to keep the pieces intact to avoid water leaks. Microwaving cardboard can be dangerous in such cases

You’d avoid microwaving your food in cardboard for a long time. Moisture accounts for 9 to 12% of the cardboard’s structure. The moisture level will start falling to about 1% or even lower when heated. And once the fiber or pulp becomes very dry, it becomes easier to catch fire.

The process slows if the box contains food or drinks. Besides, heating cardboard for shorter periods allows your food to reheat faster. That’s because the oil, sugar, and water molecules in food or drinks absorb microwave energy.

Is it Safe to Microwave Cardboard?

Yes and no. It is safe to microwave cardboard with pure pulp and fiber wood materials. The process won’t damage your food, the box, or the microwave.

Not every cardboard is safe for microwaving, though. Cardboard boxes that contain glue, wax, metal, ink, or plastic on the external coating or layered between the corrugated metal are dangerous. Meaning they’re unsafe to go in the microwave.

It’s crucial to always read your container’s label to determine if it’s safe to put cardboard in the microwave to reheat food. Check for the microwave-safe label. If it has, the manufacturer has tested cardboard and found it safe to go in the microwave.

Ensure you read the manufacturer’s instructions carefully. And avoid cardboard containers with a disclaimer. They’re not safe.

Don’t also overheat cardboard boxes because they contain toxic chemicals which can leak into your food, making it poisonous to eat. The chemicals may affect your health and destroy your food’s delicacy.

Overheating cardboard at temperatures of 4270C may result in a fire as the material is combustible. Though that can be impossible if it happens, the fire may hurt you, damage your oven or even cause a fire disaster. It’s best to reheat your foods in the microwave for shorter periods.

How Long Should You Microwave Cardboard?

As stated above, cardboard shouldn’t stay in the microwave for extended periods. It’s advisable to microwave it for shorter periods ranging from 60 to 120 seconds.

If it’s a must to microwave cardboard for a longer period, consider pausing the timer at intervals of 30 seconds. Don’t carry out the heating process continuously for more than 120 seconds.

Foods can easily and quickly heat up in cardboard, but tea or coffee may not heat up much faster. That explains why cardboard beverage packs often need a longer time to heat up.

Can You Microwave Cardboard Pizza Box?

Yes. A cardboard pizza box has a unique design for delivery purposes. These boxes also boast incredible characteristics that make them ideal and convenient for reheating in the microwave. Such traits include:

  • Long-lasting for stacking
  • Warmth retention
  • Resistant to heat up to 800C
  • Ability to prevent pizza crust from getting dry

Despite these unique characteristics, not all cardboard pizza boxes are fit to be put in the microwave. The thing is, these boxes have additional inbuilt functionality. Their interior lining consists of aluminum foil, strengthened wax, or leach-proof plastic, making it dangerous for microwaving.

You must read the manufacturer’s instructions before reheating your cardboard pizza box in the microwave. Check if the package can withstand the heat microwaves generate to warm up food.

Tips for Microwaving Cardboard Safely

You can reheat cardboard in the microwave without any risks if you do it correctly. But not all types of cardboard are safe for microwaving. The following are the tips/things you should know to microwave cardboard safely.

1. Check for the “Microwave-Safe” Label

It’s crucial that you check the label on the cardboard box before putting it in the microwave. Most manufacturers will always provide instructions and label the packaging as microwave-safe. It means they have tested the material, and you can use it in the oven without any risks.

If cardboard doesn’t have this label, read the instructions on heating your food in the microwave. And if the package doesn’t have either the label or instructions, don’t use it in the microwave.

2. Check the Cardboard Material

Not all cardboards are ideal for putting in the microwave. Some have little metal contents, which are dangerous for microwaving. Ensure the packaging doesn’t have aluminum foil.

Besides, cardboard shouldn’t have plastic or wax coating as it can release toxic chemicals when heated in the microwave. Remove any metal clips or plastic lids that fasten the box before microwaving it.

3. Set Your Microwave to Ideal Heating Levels

I recommend that you use the lowest heating level on your microwave when reheating your food. In addition, limit the heating duration to between 60 and 90 seconds most of the time. If you’re warming beverages, you can extend the time to 120 seconds. Consider also pausing the heating process at intervals of 30 seconds.

4. Consider the Type of Food

Don’t heat meals with high moisture content. If it’s a must, make sure the box is open. A closed box causes the heat generated to radiate slowly, and your food will take much time to warm up.

Besides, the closed box traps a lot of pressure which may try to escape, resulting in an explosion. The heated liquid may also compromise cardboard material’s reliability, making it soggy and even break down.


From our discussion, you can safely reheat your foods and beverages in a cardboard box as long as it is microwave-safe.

The box should not contain any pieces of metal, plastic, glue, or wax as these elements can melt and leach toxic chemicals into your food. They may also cause damage to your microwave oven.

If you’re using microwave-safe cardboard, limit the heating time to about 60 to 120 seconds. You should avoid overheating cardboard as it may catch fire at temperatures of 4270C.

Always read the manufacturer’s instructions before using the microwave to reheat your food in the cardboard box.

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