In this guide, we’ll cover everything you need to know about microwaving cardboard safely and effectively.
We’ll examine what makes plain cardboard generally microwave-safe while highlighting coatings, components, and materials that should be avoided at all costs.
You’ll learn how to identify microwave-safe labeling, inspect for hazardous materials, and even perform quick tests to check a container’s safety.
Can You Put Cardboard in Microwaves?
You may have wondered if it is okay to reheat food in cardboard containers in the microwave. The simple answer is yes, you can put plain cardboard in the microwave safely if you are careful.
Plain cardboard made from wood pulp fibers is usually safe to microwave. But, the cardboard cannot have any special coatings or materials that are dangerous to heat up.
Cardboard containers made for the microwave will say so on the packaging.
Things to Watch Out For
Temperature: Plain cardboard is safe in the microwave up to around 425°F. Higher temperatures can cause the cardboard to catch fire or chemicals to leak out.
Coatings: Avoid cardboard with plastic, wax, or other coatings that can melt into your food when heated.
Inks/Dyes: Some inks, dyes, or glues have metals that can get into your food when microwaved.
Metal Parts: Never microwave cardboard with metal handles, foil lining, staples or clips as this can damage the microwave.
Moisture: Cardboard is safer when heating moist foods versus an empty, dry container which can overheat.
So in simple terms, you can microwave plain cardboard containers safely as long as there are no coatings, metal pieces, or other unsafe materials that could cause problems when heated.
Is Cardboard Safe in the Microwave?
Yes, but there are some conditions attached to it. While it’s possible to microwave plain or pure cardboard for short periods of time, there are a few important things you should keep in mind:
The Dos and Don’ts of Microwaving Cardboard
First, let’s look at what makes cardboard generally microwave-safe and when it’s okay to microwave it:
DO Microwave:
- Plain microwave-safe cardboard food containers made of pure cellulose/wood fiber
- Containers clearly labeled as “microwave-safe”
- Containers holding food or liquids (not empty)
- For 60-120 seconds max at a low power setting
The presence of food or water molecules is key, as it helps absorb the electromagnetic waves so the cardboard itself doesn’t overheat and dry out to dangerous levels. Do well to also follow labeled instructions.
DON’T Microwave:
- Cardboard coated in plastic, wax, or polished surfaces
- Containers with metal handles, clips, foil linings, etc.
- Boxes containing adhesives, dyes, or unknown chemicals
- Empty containers with no moisture present
These types of cardboard containers can potentially melt, release toxic fumes, spark, or even catch fire when overheated in the microwave due to their components or lack of moisture.
Can I Microwave Takeout Containers?
There’s nothing quite like bringing home leftovers from your favorite restaurant to enjoy later. But when it’s time to reheat that takeout meal, can you simply pop the cardboard container right into the microwave? The answer is: sometimes (yes), but you need to exercise caution.
While microwaving cardboard takeout boxes and containers can be convenient, there are some important safety considerations to keep in mind.
The Dos and Don’ts of Microwaving Takeout Containers
DO Microwave:
- Containers with the Microwave-Safe label
- Plain cardboard/paper containers with no coatings
- For short periods only (under 2 minutes)
- Make sure there are no metal handles or attachments
DON’T Microwave:
- Containers with plastic, polished, or wax coatings that could melt
- Any containers with metal components
- For prolonged periods over 2 minutes continuously
- If there are any adhesives, dyes, or unknown materials present
The Safest Solution
For peace of mind, the safest option is to transfer any takeout or delivered foods to a dedicated microwave-safe dish or container before reheating.
Glass, ceramic, and specific plastics labeled for microwave use will eliminate any lingering doubts about chemical leaching or container safety.
How Do I Know if my Box is Microwavable?
While it’s possible to safely microwave plain cardboard boxes, there are certain materials and components you’ll want to avoid due to potential safety hazards. So how can you tell if that box is truly microwavable?
Start With the Labeling
The easiest way to check microwaveability is to look for clear labeling or symbols on the box itself.
Microwave-safe containers will typically have:
- The words “Microwave Safe” or “Microwaveable” printed on it
- A microwave icon (usually squiggly lines or an image of a microwave)
- Instructions for microwave use and any heating limitations
If the packaging has none of these indicators, that’s your first sign you may not want to risk microwaving it without further investigation.
Identify Risky Materials
Even if not labeled, you can inspect the container for any concerning materials or components that should be avoided in the microwave.
Examples include:
- Inks/Dyes
- Adhesives
- Coatings
- Metal Pieces
If you spot any of those red flags, transfer your food to a dedicated microwave-safe dish to avoid potential hazards like fires, warping, or chemical leaching.
Check for Safe Material Symbols
For unlabeled containers, see if you can identify microwave-safe materials used by finding recycling symbols and numbers, such as:
- #5 Polypropylene (PP) – This plastic is approved as microwave-safe.
- #1 PETE or #2 HDPE Plastics – These are generally microwave-safe as well.
- Glass/Ceramic – Look for these microwave-friendly materials.
Perform a Simple Safety Test
If you’re still unsure about a container after inspecting, try this simple heat test:
- Place the empty container in the microwave along with a microwave-safe cup of water.
- Microwave together at full power for 1 minute.
The safe outcome is:
- The container is cool/slightly warm
- Water is hot
If the opposite occurs (hot box, cold water), the box materials are absorbing too much microwave energy and it may be an unsafe choice.
What Containers Are Not Safe to Use in the Microwave?
Not all materials or containers can be safely used in the microwave without posing fire risks, chemical leaching, and other safety concerns.
The question now becomes: how can you tell if the container you are about to put in the microwave is safe or not?
To answer the question, let’s take an in-depth look at the containers you should never use in the microwave.
Materials That Absolutely Cannot Go in the Microwave
1. Metal Items
This is probably the most well-known microwave no-no. Any metal items like aluminum foil, utensils, cookware, or containers with metal components are an absolute fire hazard if microwaved. Metals are conductive and can arc or spark when subjected to microwave energy.
2. Styrofoam Containers
While convenient for takeout, Styrofoam containers are not designed to withstand the heat of a microwave. The material can warp, melt, or potentially release toxic styrene gas into your food when overheated.
3. Plastic Containers (Unless Labeled Microwave-Safe)
Most plastics are simply not meant for the high heat of microwave cooking. They can easily melt and become misshapen, plus have the potential to leach chemicals like BPA, phthalates, and others into food. Only use plastics clearly labeled as microwave-safe.
4. Paper Bags and Cardboard Boxes
Plain brown paper bags and cardboard boxes may seem harmless, but they actually pose fire and toxic fume risks in the microwave. The glues, inks, and materials in them are not meant for that level of heat exposure.
5. Anything With Metal Decorations or Trim
Even plates, mugs, or bowls decorated with metallic paint or trim are off-limits for microwave use, as the metal can create arcing issues similar to utensils.
6. Travel Mugs and Insulated Bottles
Most travel mugs and insulated bottles contain stainless steel or plastic components on the interior that make them unsafe for microwave heating.
7. Aluminum Foil
While you should never microwave a whole sheet of aluminum foil, even small amounts should be used with caution. Foil can still create arcing damage if it forms sharp edges or folds.
What to Use Instead
The safest materials for microwaving food and beverages are glass, ceramics with no metallic decorations, and plastics clearly labeled microwave-safe by the manufacturer.
Examples of microwave-safe containers:
- Glass plates, bowls, mugs
- Ceramic or stoneware without metallic trim
- BPA-free plastic containers marked “microwave-safe”
- Microwave-safe paper plates/towels (not bowls)
- Parchment paper or wax paper
Conclusion
Microwaving cardboard containers can be convenient but requires caution. Plain, uncoated cardboard is generally safe for short microwave periods when following guidelines.
However, certain materials like metal, plastic coatings, adhesives, and unknown chemicals can make cardboard hazardous to microwave due to fire risks and potential chemical leaching into food.
Always inspect containers closely for microwave-safe labeling and any concerning components before use.
When in doubt, err on the side of safety by transferring contents to glass, ceramic, or microwave-safe plastic dishes instead of risking damage, contamination, or fires.
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David Kessler is a CSLB licensed contractor with over 25 years of experience. He is specialized in home building, construction, and electrical works; he also has a keen eye for interior design and, over the years, has collaborated with various architects.
