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Microwave Safety Guide: What Not to Put Inside

Check in advance what could cause a spark or an explosion if you put it in without thinking.

Never use metal containers or aluminum foil

Metal reflects the microwaves inside a microwave oven, which can cause sparking (arcing) and, in severe cases, lead to a fire or damage the appliance. Avoid any dishware with gold or silver trim, stainless steel containers, and food wrapped in aluminum foil. Delivery food containers with metal handles should always be transferred to another dish before reheating. Some ovens allow small, smooth pieces of foil as shielding, but only if the manual says so.

Fully sealed containers risk exploding

A tightly sealed container, or food wrapped completely airtight in plastic wrap, can build up internal pressure rapidly and burst. Even with microwave-safe containers, you should leave the lid slightly open or poke a small hole in the plastic wrap to create a path for steam to escape. Highly airtight containers like canning jars should not be put in the microwave at all.

Heating a whole egg, shelled or hard-boiled, is dangerous

Putting a whole egg in the microwave, whether raw or already hard-boiled, can cause it to suddenly burst due to trapped internal steam pressure. The egg may even look intact and then burst when you cut or bite into it. To cook eggs in the microwave, you must crack the shell and pierce the yolk with something like a chopstick, then heat it briefly in several short intervals.

Always check for a "microwave-safe" label on plastic containers

Not all plastic is safe for microwave use, and most disposable delivery containers or standard airtight containers are generally not recommended. If the bottom of a container has no microwave-safe mark, it may warp from the heat or leach harmful substances, so it's safer to transfer food into a glass or ceramic container instead. Plastic wrap should also be draped loosely so it doesn't touch the food directly.

The "bumping" that happens when you heat plain water too long

If you heat only water in a smooth-surfaced cup for too long, it can become superheated, passing its boiling point without visibly boiling, and then suddenly erupt the moment it's disturbed, a phenomenon called bumping (superheating). This requires caution, as it can cause burns from hot water splashing onto your face or hands. Placing a wooden chopstick or teaspoon in the cup can help prevent bumping by giving bubbles a surface to form on.

Watch out for styrofoam containers and metal-rimmed paper cups too

Styrofoam containers without a microwave-safe label can warp or leach chemicals at high temperatures, and paper cups with a metallic rim can cause sparking. Paper with a thin metallic coating, like some takeout coffee cups, or paper with staples in it, is also unsuitable for the microwave. Thermal-coated paper, like receipts, should not be put in either.

Handling dishes and plastic wrap right after heating

Even if the dish itself doesn't look hot, heat from the food inside can transfer to it and cause burns. Liquids in particular (soups, milk, and the like) are often much hotter inside than they appear, so it's safer to let them cool briefly rather than drinking or touching them immediately after removing them. When removing plastic wrap, hot steam can also shoot out toward your face, so it's best to peel it back slowly starting from the side farthest from your body.

Tips for removing odors and cleaning your microwave

Mixing a few drops of vinegar or lemon juice into water, placing it in a heat-safe container, and heating it for 2-3 minutes lets the steam loosen baked-on grime on the interior walls, making it easier to wipe away. Don't wipe the hot interior directly with a dry cloth, as it can cause burns; open the door slightly to let the steam escape first, then wipe with a dry or damp cloth. Doing this regularly can help reduce lingering food odors.

Uneven heating and cold spots

Microwaves heat unevenly, leaving hot and cold spots, which matters for food safety as well as taste. Stir or rotate food partway through, arrange it in a ring rather than a mound, cover it loosely to hold in steam, and let it stand for a minute or two afterward so heat spreads. The USDA advises reheating leftovers to an internal temperature of 165 degrees Fahrenheit (74 degrees Celsius), which is easier to confirm with a food thermometer in several spots.

Baby bottles and infant food

Pediatric guidance generally advises against warming baby bottles or breast milk in the microwave, because the liquid can have scalding hot spots while the bottle feels only warm. Warm a bottle by standing it in a bowl of warm water instead, swirl it, and test a drop on your wrist. Baby food jars are similar: stir thoroughly and let the food cool before serving.

Other things to keep out

Do not run the microwave empty, since the energy has nothing to absorb and can damage the oven. Pierce foods with a skin or membrane, such as potatoes, sausages, and tomatoes, so steam can escape. Halved grapes joined by a thin bridge of skin are known to spark dramatically, and dry sponges or damp cloths used to sanitize can ignite, so avoid both. Foods with very little water, such as plain bread, can also overheat and burn.

Check your product manual as well

Safety standards can vary somewhat depending on your microwave model and the container manufacturer. This page summarizes widely known general precautions; if your specific microwave or container comes with separate instructions in its manual, it's safest to follow those first.

Why microwaves cause these specific problems

A microwave oven heats food by sending microwaves that make water molecules vibrate, and that vibration produces heat. This explains most of the rules above. Metal reflects the waves and can arc, trapped water turns to steam and builds pressure in eggs and sealed containers, and smooth plain water can heat without bubbling because there are no spots for bubbles to start. Once you picture heat coming from water inside the food, the rules become easier to remember than a list.

What to do if something goes wrong

If you see sparks or flames, stop the oven immediately, but keep the door closed so flames starve of oxygen, and unplug the oven if it is safe to do so. Do not use water on an electrical fire. If a fire does not die out quickly, leave and call emergency services. After any incident, check the door seal and interior for damage before using the oven again, and do not use it if the door does not close properly or the cavity is burned or arcing.

Curious about kitchen safety in general?

If you're curious about how to safely use cleaning products at home, see our guide to types of cleaning supplies and dangerous combinations as well.

Frequently Asked Questions

Should I never use a dish that has no microwave-safe label?

As a rule, yes. Without a label, there's no reliable way to confirm its safety. In particular, it's hard to judge by eye whether colored-glaze ceramics or older plastic containers are safe, so it's recommended to switch to containers with a clear microwave-safe label.

What should I do if my microwave smells really bad?

A common method is to mix vinegar or lemon juice into water, heat it, let the steam escape, then wipe the interior with a dry cloth. If the smell persists, it could indicate an issue with internal components, so it's a good idea to contact the manufacturer's service center for an inspection.

Is it safe to stand in front of the microwave while it runs?

Microwave ovens are built with shielding and door interlocks that keep the waves inside, and ordinary use with an undamaged oven is considered safe. Avoid using an oven with a damaged door, hinge, or seal, and stop if it runs with the door open.

Can I microwave water for tea in a mug?

Yes, but heat it for a shorter time than you think, avoid very long cycles, and put a wooden stick or microwave-safe spoon in the mug. Let it stand briefly after heating, and be careful when adding a tea bag or sugar, which can trigger sudden boiling.

Why does my food explode or splatter?

Steam builds up quickly inside foods with skin or tight covers. Cover with a loose, vented lid, pierce skins, and heat in shorter bursts at lower power.