Why Microwaves Can Catch Fire—And How to Use Yours Safely

Just about any appliance that cooks food can spew smoke or catch fire, and hardly anyone blames their cooktops or ovens or toasters for these kitchen mishaps. But when a microwave sparks or smolders, it seems to inspire a deeper sense of dread and apprehension.

Actually, microwaves are among the safest cooking appliances you can own. If they do start to burn, it’s usually for the same reasons as any other electric oven. (Well, a microwave is the only appliance where you can accidentally make something like ball lightning, but that’s not such a big deal, as it turns out.) Here’s what you need to know.

Overheated food or packaging is the main culprit

According to Bob Schiffmann, a 50-year veteran of the microwave industry and president of the International Microwave Power Institute,¹ microwave fires usually start for the same reason that oven or stovetop or grill fires do: “It’s simply due to [the food] overheating.”

Food-based fires in a microwave almost always happen because someone overestimated or mis-entered the cook time. An extra three minutes can be the difference between a nicely baked potato and a charred, smoky mess. Blasting a Lean Cuisine that you think is frozen but is actually closer to room temp could cause problems, too. Another culprit can be packaging that gets stuck near the wave emitter on the side of the machine—like a big bag of popcorn that gets wedged against the walls of a small oven as the kernels expand.

Schiffmann said that food-based fires like these “create a lot of smoke, but they tend not to be all that dangerous because the components can’t really catch fire.” If you spot smoke or fire in the oven cavity, turn off or unplug the microwave, but leave the door closed. UL-certified microwaves (which is almost all of them) are designed to contain small fires started by common foods (like popcorn or potatoes), so it should be safe to leave a microwave alone while the flames suffocate.

Other types of fires aren’t as common

Schiffmann said that he knows of three other ways that dangerous microwave fires can begin: 1) The power supply can short out and burn the components inside the housing (it’s a risk with any electrical equipment); 2) A power surge can trick the microwave into turning on, and it may run continuously until somebody notices and turns it off (there are reports of this happening with several brands); 3) Some part of the oven could be prone to catching fire when overheated (like these Whirlpool models).

Microwaves have built-in safety features to reduce the chances and severity of these types of fires. They don’t seem to be nearly as common as food-based fires, either: A representative from the National Fire Protection Association told us that 17 percent of microwave fires begin in the appliance housing (which is consistent with non-food fires). But there’s always a risk, and it’s important to follow the safety advice in the manual to try to prevent any of these from happening.

Unfortunately, you won’t know whether your microwave is particularly susceptible to these types of fires until you see it happen or there’s a class-action lawsuit and recall.

Sparks can happen, and you don’t really need to worry about them

Zaps and sparks inside the oven cavity aren’t a sign of a microwave malfunction, nor are they especially dangerous, according to Aaron Slepkov, a physics professor at Trent University in Ontario.

Sparks happen because the waves of energy that should get absorbed by your food instead bounce around between objects. The energy gets concentrated in a small space, so it’s prone to react with something in the air or food, creating a spark.

Metal with rough or pointy edges, like a fork or ball of aluminum foil, will almost always spark. Paper towels made from recycled materials can also (unintentionally) contain flecks of metal that might spark, according to the National Institutes of Health. It’s not a stretch to imagine that some food packaging might also contain hidden metal. Non-microwave-safe bowls and plates might crackle or spark too.

And in the right conditions, food can also create sparks. Grapes are the best-known culprit (video), and Slepkov co-authored a paper that got to the bottom of this phenomenon (his lab nicknamed the project “Grape Balls of Fire”). Grapes have the ideal size and moisture content for sparking, but it can happen with peas, blueberries, and other foods as well.

If you do see sparks inside the oven cavity, it’s nothing to be too concerned about—they’re like fireworks for nerds. Microwave sparks are a plasma, the fourth state of matter, that’s much rarer on earth than gas, liquids, or solids. Plasma feeds off of microwave energy, according to Slepkov, and if you glance up at the top of the oven cavity after you see the initial sparks, you might see plasma dancing around for the duration of the cook time. “Literally, it’s ball lightning,” Slepkov said (though some experts contend it’s not quite the same thing).

Given enough time and sparking, the plasma could char the top of your oven, and reflected microwaves could damage the magnetron (the oven’s core component, which generates the waves that heat your food). But the risk of an inferno is small, according to Slepkov. The plasma won’t burn your food, because it floats to the top of the cavity. “It’s not going to explode your microwave or anything. And if it does damage your microwave, it will only damage your microwave in a way that will immediately stop feeding this plasma.”

Microwaves are quite safe, as far as cooking appliances go

One fire is too many, but the numbers suggest that microwaves are quite safe. Estimates can vary, but at least 90 percent of US households have a microwave. According to the National Fire Protection Association, microwaves cause 4 percent of cooking-related house fires per year (PDF) and 1 percent of associated deaths. A similar percentage of homes have a range or cooktop, yet those appliances account for 62 percent of cooking fires and a whopping 89 percent of the deaths.

It’s still wise to stay near your microwave while it’s running, so you can try to stop small fires before they turn into big blazes. But of all the safety hazards in your kitchen, microwaves aren’t near the top of the list.

For more, check out our guide to the best microwaves.

Footnotes

1. Bob Schiffmann holds patents in microwave technology and does consulting work for microwave-adjacent companies, but he does not represent any manufacturers directly, nor is he paid by them. The International Microwave Power Institute is a membership industry group.
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