Heating your favorite snacks and drinks while they’re still in their cardboard containers might seem like a simple way to cut down on dish clean-up. But putting cardboard in the microwave isn’t always safe, so it’s crucial to exercise caution when microwaving cardboard.
You can put cardboard in the microwave as long as it doesn’t contain wax, plastic, and metal. However, you’ll want to keep a close eye on cardboard containers while they’re being microwaved, as cardboard can catch flame when it dries out and reaches high temperatures.
So it’s possible to prepare a convenient meal (or defrost) in the microwave by heating your food in its cardboard container. But before you get started, you’ll need to understand how microwaves affect cardboard and what materials are dangerous to place inside a microwave.
How Microwaves Interact With Cardboard
Microwaves generate heat by exciting electrons. When an atom’s electrons begin to vibrate, the molecule containing the atoms begins to move rapidly, which then creates heat. But microwaves are designed to generate movement from components in food, like water and fat.
Cardboard typically doesn’t contain these elements, but rather, they are made with cellulose fiber.
When you place cardboard in a microwave, the microwaves first excite any water molecules in the cardboard, causing them to evaporate and produce steam. During this process, it’s unlikely that the cardboard will catch fire.
But if the cardboard is very dry, the cellulose fibers may vibrate rapidly, producing heat. When this heat isn’t allowed to escape the interior chamber by opening the microwave door, the cardboard can eventually catch fire.
Still, this is more likely to happen in an oven than in a microwave. But keeping a close eye on cardboard containers in the microwave is essential, as it can help prevent sudden fires and damaged microwaves.
The most dangerous aspect of microwaving cardboard is that some cardboards contain other materials like metal or single-use plastic.
Dangers of Putting Cardboard in the Microwave
Cardboard typically consists of a corrugated middle portion protected on either side by a thin paper-like covering. But cardboard may contain adhesives that keep the paper-like coverings attached or metal staples to make it more stable.
Additionally, many people mistake paperboard, which is a common food container material, for cardboard. But unlike standard cardboards, paperboard often has a wax or plastic lining to repel moisture. Unfortunately, this lining can melt or catch fire when exposed to microwaves.
A lack of moisture can also affect the safety of microwaving cardboard.
For example, microwaving a tasty bowl of soup in a cardboard bowl isn’t likely to pose many risks, especially when the bowl in question is made of pure cardboard with no adhesives, metal staples, or plastic linings.
After all, wet cardboard is challenging to set on fire. But when the cardboard is very dry, it can quickly catch a flame. This is especially true of dry corrugated cardboards placed inside microwaves.
The lack of moisture content means that the cardboard doesn’t generate steam while microwaving, so airborne water molecules don’t immediately dampen tiny flames that erupt from the cellulose fibers.
If you plan to microwave a cardboard food container, it’s best to ensure that it’s somewhat moist. A few drops of water sprinkled onto the cardboard can significantly reduce fire risks.
Materials That Aren’t Safe for Microwaves
Cardboard can be safe to microwave, but some materials should never be microwaved due to their risks. Some of the most notable materials that aren’t safe for microwaves include:
- Paperboard Containers
Each of these materials can be hazardous when microwaved. Some generate electrical damage, some can catch fire, and a few can fill your home with toxic fumes.
Let’s discuss these materials in greater detail to discover why they’re not microwave-safe.
Metal Can Burn the Inside of Your Microwave
Metal is one of the most dangerous things you can put inside your microwave. So when microwaving cardboard, it’s vital to ensure that the cardboard is free of metal staples or pieces.
Common metals such as stainless steel and aluminum foil are conductive, meaning they can hold and transmit electric currents. When you microwave metals, the electrical current can arc, shorting out your microwave’s internal electrical components and causing a fire.
For a more in-depth guide on how metals react to microwaves, check out this educational video discussing the subject:
Paperboard Containers Can Catch Fire Inside the Microwave
Many paperboard food containers feature plastic-coated or wax-coated sides that help keep foods from spoiling. Some paperboard containers are also sealed shut with glue. While these components don’t pose a severe risk to foods inside the container, especially when refrigerated or frozen, they can pose a risk when microwaved.
For example, paperboard with glue or wax can melt when exposed to microwaves.
Paperboard with plastic coatings can also melt in a microwave, and the plastic can release harmful fumes that linger in your home. These single-use plastics can also leach chemicals into your food with dangerous chemicals, potentially increasing your risk of developing some types of cancer.
Unless a paperboard food container states that it’s microwaveable, it’s best to remove the food from the container and microwave it in a microwave-safe container.
Styrofoam Can Leach Toxic Chemicals When Microwaved
Most styrofoam containers aren’t designed for microwave use. Styrofoam takeout food containers without a microwave-safe stamp can degrade or catch fire in the microwave.
When the styrofoam degrades, it can fall and melt onto your food or damage your microwave. But fire damage and a ruined meal aren’t the most worrisome dangers of microwaving styrofoam.
One of the main chemicals in styrofoam is styrene. When microwaved, this chemical can become a gas. So when you open your microwave to retrieve a styrofoam container, you may breathe in a dangerous amount of styrene gas.
If you inhale styrene, you can experience several symptoms, including:
- Concentration problems
Prolonged exposure to styrene gas may cause significant damage to the nervous system and liver, so it’s best to avoid microwaving styrofoam.
Microwaving cardboard containers is generally safe, especially when the cardboard is damp. But cardboard that contains plastics or metals can catch fire when microwaved or cause significant damage to microwaves.
If you attempt to microwave a cardboard container with food inside, it’s crucial to follow the manufacturer’s instructions and keep a close eye on the cardboard while it’s microwaving. Stop the microwave at the first sign of flame or sparking to prevent household fires or damage to your microwave.