Can I Put Cardboard in The Microwave? Answer May Surprise You

We’ve all been there – you have leftover pizza or cookies in a cardboard box and want to quickly heat them. It may seem harmless to pop cardboard in for just 30 seconds, but is it microwave-safe?

The short answer is no, you should avoid microwaving any type of cardboard. While brief exposure won’t immediately cause fires, the risks still exist. Cardboard and paper contain flammable coatings and fibers that can easily overheat and ignite.

However, with care and precautions, certain types of cardboard can be suitable for microwave use. In this guide, we’ll cover everything you need to know about cardboard microwave safety, fire hazards, microwave-safe options, and best practices for warming up cardboard-packed foods.

Cardboard Microwave Risks: Why You Should Generally Avoid It

Most cardboard boxes, containers, and plates contain cellulose fibers and paper fibers that are highly flammable when exposed to microwave radiation. The adhesives and clay coatings used in cardboard can also contain moisture, metals, and other materials prone to creating sparks and heat when microwaved.

When microwaved, the fibers and coatings rapidly reach temperatures above 475°F – hot enough to scorch paper, ignite grease, and release volatile fumes. This builds up intense heat that cannot escape the closed microwave, allowing fires to spark easily.

According to the U.S. Fire Administration, over 19,000 household fires per year start due to flammable items like paper products being ignited in microwaves.

Tip: Microwaves work by exciting water molecules and creating molecular friction that heats up moisture-containing foods. Cardboard has little moisture, so instead the microwave energy is absorbed by flammable fibers that can’t handle the heat.

For this reason, any cardboard containing boxes, containers, plates, etc. should never be microwaved for more than 30 seconds to simply warm – not cook or reheat – food. Even short 30-second bursts can be dangerous, as sparks or small embers may not be immediately visible.

Signs Your Cardboard Box Overheated:

If you do microwave cardboard briefly and notice any of the following signs, it likely overheated and risks were present:

  • Scorching, blackening, browning, or dark spots on the cardboard
  • Smoke coming out when you open the microwave
  • Cardboard feels overly warm or hot to the touch
  • A strong burning smell coming from the microwave
  • Visible sparks or flickers inside the microwave
  • Bubbling or warping of the cardboard container

If you notice any of these warning signs, stop microwaving cardboard immediately. Avoid reusing any overheated cardboard as well, even if no fire occurred. The compromised integrity means it could more easily ignite next time.

Are Special Microwave-Safe Cardboards Available?

The good news is that not all cardboard is created equal when it comes to microwave safety. There are specially manufactured cardboards designed to be microwaved:

  • Microwave-safe cardboard plates – These contain flame-retardant coatings to prevent overheating and fire. Only microwave as instructed.
  • Microwave-safe cardboard takeout boxes – Also treated with fire retardants. Still, avoid over 1 minute.
  • Microwave-safe cardboard trays – Found in the frozen meals section. Safe for brief reheating only.

These products will be clearly labeled “Microwave-Safe” and have directions on maximum microwave time, usually 1 minute or less. The flame retardants allow the microwave energy to pass through safely without igniting the substrate.

However, microwave-safe does not mean unlimited microwaving is safe. The surface can still get dangerously hot when microwaved for too long. Avoid exceeding the manufacturer’s recommended microwave time.

Can You Microwave a Pizza Box?

While it may seem harmless to quickly zap that leftover pizza slice in its box, the risks are elevated due to the perfect storm of cardboard fibers and oil/grease absorption. When the microwave’s high energy waves hit the box, the grease works as an accelerant that can cause scorching and ignition.

According to research by the NFPA, over 1,000 house fires per year originate from grease-stained cardboard-like pizza boxes being ignited in the microwave. The steam created by heating the oil and cheese toppings permeates the cardboard even more, worsening its integrity. Food scientists warn that even boxes that only appear slightly grease-stained can contain absorbed oils deep in the cardboard which makes it dangerous for microwave use. For true safety, transferring that slice onto a paper towel or microwave-safe dish is worth the extra seconds of effort. With pizza boxes, assumption and convenience can come at a fiery cost.

Can You Microwave Other Foods in Cardboard?

Pizza isn’t the only culprit when it comes to cardboard container microwave risks. Many other takeout foods and leftovers get packaged in flimsy cardboard boxes and containers that seem convenient to reheat as-is. However, the dangers of igniting the cardboard exist whether it’s pasta, burgers, fried chicken, or other greasy foods.

The steam released when cooking saucy foods can further compromise the cardboard’s integrity. One study showed that microwaving oil-soaked cardboard boxes led to ignition in under 60 seconds. Foods with high moisture like soups and casseroles boiled over easily too. For the shortest microwave times possible, transfer any cooked leftovers onto microwave-safe dishware first. The few extra seconds of prep beats a lifetime of replacing your microwave or kitchen due to avoidable cardboard fires.

Microwaving Alternatives That Are Safer Than Cardboard

To avoid the hazards of microwaving cardboard, use microwave-safe dishware whenever possible:

  • Ceramic plates, bowls, and mugs – Excellent microwave-safe option as ceramics handle heat well with no ignition risks. Can fully reheat foods.
  • Glass containers and jars – Another safe choice that won’t overheat. The glass allows microwave energy to pass through.
  • Microwave-safe plastic containers – Containers labeled microwave-safe won’t melt from hot foods. Allows thorough cooking.
  • Paper plates, towels, and napkins – Microwave-safe paper won’t ignite easily like cardboard and makes an ideal disposable option.

By transferring foods into proper microwave-safe containers, you can reheat and cook without fear of sparks, smoke, or accidental fires in your microwave. Using ceramic and glassware also gives more even heating compared to flimsy cardboard.

Microwave Fire Safety Tips

Microwaves provide fast, convenient cooking, but also require extra caution. With the rapid heat they generate, a small mistake can spark dangerous flames. If you do opt to microwave cardboard temporarily, be fire-aware.

First, never leave any microwaving unattended, especially cardboard. Stay by the microwave and watch closely for signs of smoke or sparks. Have a microwave-safe fire extinguisher mounted and easily accessible nearby, and test it monthly to ensure it’s functional. Also, ensure your kitchen smoke detector is working properly to alarm at the first hint of smoke.

If you see flames inside the microwave, do not open the door – the rush of oxygen can worsen it. Instead, immediately unplug the microwave, use the fire extinguisher aimed at the base of the flames, and call emergency services if it seems to be spreading. Trying to remove a burning container risks getting burned. After any microwave fire, thoroughly inspect the unit and wiring before using it again to ensure no lingering damage. Avoid reusing any cardboard that sparked – its integrity is likely compromised. Taking key precautions and knowing how to respond safely can prevent a minor microwave mishap from turning into a kitchen catastrophe.

The Bottom Line on Microwaving Cardboard

Microwaving cardboard boxes, containers, trays, plates, and other paper products comes with genuine fire hazards that consumers should take seriously. While the convenience is tempting, the risks outweigh the benefits when standard cardboard lacking flame retardants is used.

To stay safe, restrict any microwave use of cardboard to just 20-30 seconds to slightly warm up pizza, pastries, or foods. Transferring food out of cardboard into proper microwave-safe dishware is always recommended when possible. With caution and common sense, you can briefly leverage the convenience of microwaving cardboard while minimizing safety risks.

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