Is boiling water in a microwave dangerous?


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Dear Straight Dope: I received the following by E-mail and almost brushed it off as another “net myth,” but then the part at the bottom about the owner’s manual gave it a semblance of reality (at least to me). Can you check it out? I searched your archives and didn’t find a reference. I’m usually wary about stuff forwarded to me by mom since she tends to be a bit naive about all things Internet. If this is “fairly common” as it states, how come in all the years I’ve used a microwave it’s never happened to me or anyone I know and I’ve never heard of it happening? WARNING: WATER IN MICROWAVE I guess I didn’t know this. I heat water in a mug often in the microwave. I never thought of it as being too dangerous. I feel that the following is information that anyone who uses a microwave oven to heat water should be made aware of. About five days ago my twenty-six-year old son decided to have a cup of instant coffee. He took a cup of water and put it in the microwave to heat it up (something that he had done numerous times before). I am not sure how long he set the timer for but he told me he wanted to bring the water to a boil. When the timer shut the oven off, he removed the cup from the oven. As he looked into the cup he noted that the water was not boiling, but instantly the water in the cup “blew up” into his face. The cup remained intact until he threw it out of his hand but all the water had flown out into his face due to the build up of energy. His whole face is blistered and he has first and second degree burns to his face that may leave scarring. He also may have lost partial sight in his left eye. While at the hospital, the doctor who was attending to him stated that this is a fairly common occurrence and water (alone) should never be heated in a microwave oven. If water is heated in this manner, something should be placed in the cup to diffuse the energy such as a wooden stirrer stick / spoon, tea bag, etc. It is however a much safer choice to boil the water in a kettle. NOTE: Subsequently on reviewing the above I have consulted a set of microwave oven operating instructions and under the heading “Liquids” it states: Liquids that have been heated by Microwave can suddenly erupt. This is due to layers heated to higher temperatures being trapped under the surface. To avoid this happening to any liquid, e.g. coffee, custard, gravy etc.: 1) Stir the liquid thoroughly before heating in the microwave. 2) Stir the liquid at least twice during the heating time. 3) Stir the liquid again at the end. NEVER OVERHEAT LIQUIDS. 4) Always use a suitable sized container, at least one third larger, than the volume of liquid to be heated. If in doubt use conventional methods, kettle, etc. Julie Charity

David replies:

This one is one part net-myth, as you said, mixed in with one part truth. It has been discussed and debunked in countless newsgroup and message board discussions, web pages, news stories, etc. Here I’ll take a look at some of these.

Todd Campbell, at, wrote a response that directly addressed the e-mail you mention. (Cecil discusses the issue in more general terms at Campbell explains that microwaving water can indeed be dangerous under certain circumstances, but not to the extent mentioned in the e-mail legend. When you think of boiling water, you think of bubbles coming to the top. Where do the bubbles come from? They form at a nucleation site. Usually this consists of a small flaw in the container or a seam–say, where the sides meet the bottom–but Campbell notes that it could also be turbulence in the water due to convection. Such a flaw or turbulence is common in teapots or saucepans used to boil water on the stove. However, when you are boiling water in a microwave, you are probably using a ceramic mug, which is much smoother and may not have a good starting point for nucleation. On top of that, a microwave oven heats more or less uniformly, so the amount of convection and turbulence in the water is greatly reduced. Without nucleation, the water just gets hotter and eventually becomes superheated; that is, it exceeds the normal boiling point for water without actually boiling.

This is when things can get touchy. If you move the cup around a bit, or drop a teabag into it, or put your spoon in to stir it, or whatever, you can provide the nucleation site the water has been looking for. So it turns from a superheated liquid to a boiling one–quickly. Usually, this will just mean it boils over the top of the cup, in which case you’ll only be burned if you happen to be holding it (such as if you took it out of the microwave and jostled it enough to start it going). It could also splatter a bit, just as a pot of boiling water occasionally throws up some scalding drops. But it’s not going to "blow up" and throw all the water into your face, as described in the e-mail message. At least, it won’t unless you’re sitting there with your face an inch away from the water, staring at it (if that’s the case, perhaps getting burned isn’t your biggest problem).

Still, just because the e-mail contains an element of truth doesn’t mean it’s necessarily a true story. There’s no real way to find out if the story told here really did happen. The fact that it”s been passed around so much like other urban legends (remember poor dying Craig Shergold who needs postcards to break the world record?) makes me rather suspicious.

Whether the e-mail is true or not, the phenomenon has been noticed by more than just a few people who got horrific e-mail warnings. According to the Cleveland Plain Dealer (11/5/2000), "All microwave manufacturers are now including warnings to stir water before heating it in the microwave because that will stimulate the release of the vapor." Specifically, one Amana microwave warning quoted at the AFU & Urban Legends Archive ( says: "Liquids heated in certain shaped containers (especially cylindrical-shaped containers) may become overheated. The liquid may splash out with a loud noise during or after heating or when adding ingredients (instant coffee, etc.), resulting in harm to the oven and possible injury. In all containers, for best results, stir the liquid several times before heating. Always stir liquid several times between reheatings."

Even the FDA has gotten involved, saying they have received reports of burns due to this very issue ( But there’s no way to know if they got real reports or just multiple copies of this e-mail and got all alarmed like your mom.

But not you, Julie. You’re one of the few, the proud, the folks who check with us first. Thanks for doing your part to help us fight ignorance. Oh, and feel free to direct your mom to our archive in case there is anything else we can help her with.


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