Safety warning posted online by the Gurnee, Illinois Fire Department:
Liquids heated in a microwave oven may not turn into steam, even though they are very hot. Moving these containers of hot liquid, or putting a utensil or other object into them creates a "steam bubble" and the hot liquid may splash out, causing a scald burn. Liquids heated in a microwave are very hot, even if the container they are heated in is cool.
Scientists agree that this can happen, though they offer a somewhat more complicated explanation. As Richard Barton writes in New Scientist magazine, "A portion of the water in the cup is becoming superheated the liquid temperature is actually slightly above the boiling point, where it would normally form a gas. In this case, the boiling is hindered by a lack of nucleation sites needed to form the bubbles."
When water is heated on a conventional stove, the porous surface of the kettle and the convection caused by the hotter liquid rising from the bottom enable the water to convert to steam. It boils. But a stationary cup of water in a microwave oven can heat past the boiling point without actually boiling. If that happens, placing an object (like a teabag) in the water or jarring the cup could cause the sudden and explosive conversion of part of the water to steam.
"I imagine," adds Barton, "that by keeping the cup still and microwaving for a long time, one could blow the entire contents of the cup into the interior of the microwave as soon as you introduced any nucleation sites. It is this sometimes explosive rate of steam production that means you should take great care when using a microwave oven."
So, we know it's possible. But is it common? Does it happen frequently? No, says Louis Bloomfield, professor of physics at the University of Virginia. "Fortunately, serious microwave superheating accidents are unusual this is the first injury I've ever heard about." Which contradicts the email's claim that such injuries are a "fairly common occurrence."
A survey of the medical literature on microwave injuries also undermines that claim. There are references to trauma caused by everything from overheated pizza to exploding eggs, but no mentions I could find of serious injury due to surges or explosions of boiling water.
Lastly, is the warning useful, or just an alarmist rumor? It falls somewhere in between, it seems to me. Manufacturers do recommend that foods or liquids heated in a microwave oven be allowed to stand for a time before they're touched or consumed. Sounds like good advice to me.
FDA: Risk of Burns from Eruptions of Hot Water Overheated in Microwave Ovens
U.S. Food & Drug Administration, 4 April 2009
Sources and further reading:
Chapman, Murray, et al. "Microwave Madness." New Scientist. 19 Jan. 2000
Bloomfield, Louis A. How Things Work: Microwave Ovens. University of Virginia. 19 Jan. 2000
Microwave Oven Safety. Gurney, Illinois Fire Dept. 18 Jan. 2000
Unwise Microwave Oven Experiments. Science Hobbyist. 18 Jan. 2000
Microwaving Water. AFU & Urban Legends Archive: "Let's be careful out there..."
Last updated 11/01/10