Microwaves have to be one of the most common cooking appliances in the average American household. It makes sense why. You can melt, cook, boil, reheat, defrost — there’s not much that a microwave can’t do if you’re looking for a quick meal. That being said, there are some types containers that you shouldn’t put in the microwave, for a number of safety reasons.

It’s not simply a matter of heating materials may ruin your meal. If you use the wrong material enough, it can make you sick or worse. That’s why knowing what can and can’t be microwaved is paramount to safe cooking. We’ll look at a few of the big microwave safety hazards, but first let’s briefly discuss how microwaves cook.

How Do Microwaves Cook?

Understanding how a microwave works goes a long way toward explaining what materials work in the microwave and why they don’t work. Microwave cookers (we need to specify) are pretty simple to explain, even if they sound a little science-fictiony. The cooker is outfitted with a magnetron — a vacuum tube that creates microwaves. Microwaves are electromagnetic waves similar to radio or infrared waves. These waves are sent out through the machine, penetrating the surface of the food. Once the waves have made their way into the food, they excite the molecules and cause them to heat.

Microwaves are sent out through the machine, penetrating the surface of the food.

Since the microwaves are fairly short in length, the thicker the food is, the less likely the waves will penetrate through to the center. This explains the famous “cooked on the outside, ice on the inside” outcome that you sometimes get with the microwave. With that in mind, there are certain items that don’t cooperate with microwaves!

Metal

A quick search of YouTube or Google will give you many examples of when putting metal in the microwave goes very, very badly. But, not all metals lead to an explosive reaction. In fact, the outer window of the microwave is lined with metal to prevent the microwaves from escaping the inside of the cooker since metal actually deflects microwaves. Some microwavable pizzas also come with a small crisper tray that’s lined with metal. So, why are these metals okay while others aren’t?

If the electrical buildup becomes too great, it can cause a spark or a fire.

It all comes down to the shape and smoothness of the metal. The reason a flat plate like the crisper tray is okay in the microwave is because it reflects the microwaves back into the pizza. If there are curves or wrinkles in the metal, like with a fork or tin foil, it can trap the microwaves (an electromagnetic wave), causing them to build up. If the buildup becomes too great, it can cause a spark. If these sparks hit anything flammable, a fire can start. These sparks can even cause damage to the magnetron. So, while there are some metal items that can be microwaved, it’s better to be safe and avoid putting metal in the microwave, unless it’s specifically labeled as microwaveable.

Polystyrene and Foam

Polystyrene, colloquially known as “Styrofoam,” is a useful material for containers since these containers are cheap to make, hold their shape well, and are great for insulation. In other words, they keep hot foods hot and cold foods cold. So, if polystyrene can handle heat and the cold, why isn’t it good for the microwave?

At high enough amounts, these chemical compounds are carcinogenic and can be hazardous.

While polystyrene can handle some hot temperatures, it doesn’t handle high heat well. After a bit, it actually begins to melt pretty aggressively. This releases chemicals inside the foam, both into the air and into your food. At small doses, it may not immediately cause health concerns, but at high enough amounts, these compounds are carcinogenic and can be hazardous. There are some types of microwave-safe polystyrene, but in this case, unless you’re certain, it’s better to be safe than sorry. Just transfer your food to paper plate or microwave-safe plastic bowl before heating it.

Plastic

Speaking of plastic, this material is one of the iffier ones when it comes to whether or not you can use it in the microwave. There are many plastic containers that are microwavable, which complicates the whole situation. Many plastics contain chemicals similar to polystyrene and can be very bad for you if used improperly. When heated, these chemicals leach from the container into the food you’re cooking. It’s gotten to the point where some believe there’s a growing body of evidence that supports plastic being unhealthy for us in general. All plastic containers leak some chemicals to the food, but to be considered a microwave-safe plastic container, the FDA tests the container to ensure it’s within a safe, allowable amount.

When heated, these chemicals leach from the container into the food you’re cooking.

So, what type of plastics generally aren’t microwave safe? Thinner plastic (like sandwich or grocery bags) should not be microwaved, as they’ll easily melt in the high heat. The same goes for cold-storage containers, like margarine tubs and milk jugs. A commonly microwaved container is the to-go plastic container. While these look like many of your microwave-safe Tupperware containers, they aren’t made to be reheated. If your microwave-safe plastic container has gotten discolored and scratched, it’s also more likely to bleed over those chemicals, so it may be time to replace it.

Brown Paper Bags

Brown paper bags can’t withstand the high temperature of the microwave and are a fire hazard.

This last entry will be short, but it’s no less important. Despite what you see online, brown paper bags are not a good way to DIY popcorn at home. In fact, you should never, ever use brown paper bags in the microwave, according to the USDA. Most types of paper are okay, specifically white paper or paper plates. This isn’t the case with brown paper bags, which are usually made from recycled materials. They can’t withstand the high temperature of the microwave and are a fire hazard. For that reason, find alternative options to DIY popcorn.

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There may be times where you just aren’t certain if something is microwave safe. If you ever are in this questionable situation, and can’t search online for help, there is a way that you can test the container or utensil to see if it will be safe for the microwave. Place a measuring cup with about one cup of water in the microwave alongside of, but not touching, the container in question. Microwave on high for a minute and feel the container. If the container is hot, it’s not made of a microwave-safe material.

The microwave is a useful and versatile appliance in the kitchen, but like any kitchen appliance, it needs to be used properly to be safe. If you follow these suggestions, and always keep an eye on what’s microwave safe and what’s not, this handy appliance shouldn’t pose many risks!