Answer Geek: Dangerous Microwaves?

-- Q U E S T I O N: Is it true that it is dangerous to heat plain water in a Microwave oven? I have seen a lot of my co-workers do this. If there is some truth behind the stories that it is dangerous, I will at least let them know about this.

— Nikhil R.

Q U E S T I O N: When my European in-laws visit, they drink a lot of tea. They insist, however, that I don’t boil the water in the microwave because of the “dangerous radiation.” They also claim that water boiled in the microwave doesn’t taste as good and that it gets cold faster. I am playing the good host and using the stove, but I’d like to verify that these notions are absurd! — Ryan W.

A N S W E R: There’s an e-mail message on this very subject circulating around the Internet that looks suspiciously like one of the many hoaxes and urban legends that worm their way through cyberspace, causing otherwise rational people to believe, for example, Ericsson is giving away free cell phones and people have been killed by licking ATM envelopes coated with cyanide.

The microwave missive starts out like this: “I feel that the following is information that any one who uses a microwave oven to heat water should be made aware of.” It then goes on to tell the story of a “26-year-old son” who was severely burned when the cup of water he heated up in a microwave “blew up” in his face.

That sad tale about the maimed 26-year-old son has led many people to worry about the safety of using the microwave to boil water. But is it just another wacky piece of digital disinformation? Or is there something to it? Does boiling water in a microwave pose some danger?

Before we get to the answer, I have a question for Ryan. What’s up with you and your in-laws? I’m sensing quite a bit of hostility in your e-mail. I don’t have any specific advice for you — I’m the Answer Geek, not the Therapy Geek — but I’m wondering if maybe you and your wife could use a little bit of help sorting out this issue.

Understanding ‘Nucleation Sites’

So, back to microwaves and boiling water. Believe it or not, it turns out that boiling water in a microwave can be dangerous — although I tend to think the e-mail message is more of an urban legend than a true story.

To understand why there’s a potential problem, let’s look first at what happens when you heat water by more conventional means. Let’s say we fill a pot with water, set it on the stove and turn the burner on high. At the bottom of the pot, where it touches the stove, there are hot spots far warmer than the 212 degrees Fahrenheit needed to bring water to its boiling point. As the liquid heats up, the water begins to circulate, with hot water rising to the pot’s top and cooler water sinking to the bottom. When the water passing by the hot zones reaches the proper temperature, some of the water molecules separate from the others, changing from a liquid gas state into bubbles of steam that rise to the surface.

Eventually, you get a nice, roiling boil, with plenty of bubbles. That process — bubbles forming and circulating to the top, cooler liquid falling to the bottom — means that the water maintains a fairly constant temperature right around 212 degrees.

In order for those first bubbles to form, however, there needs to be some sort of trigger. Scientists call that something a “nucleation site.” Think of a nucleation site as the catalyst that gets the process going. In our pot of water, it could be a flaw in the metal, or the hot spots at the bottom of the pot, or the turbulence caused by the circulation of warm and cold liquid, or even an impurity in the water.

Bubble Trouble

Unfortunately, that coffee mug that you just filled with water and placed in the microwave is probably so smooth that it doesn’t have any nucleation sites. And because of the way a microwave works, the cup itself doesn’t get hot, nor does the water circulate. No nucleation sites, no bubbles. No bubbles, no roiling boil. No roiling boil, and the water just keeps getting hotter and hotter — hotter, in fact, than the normal boiling temperature for water. So what you have is a mug of superheated liquid.

Here’s why that’s dangerous. You take the cup out and bang it a bit as you set it down on the table. The bump disturbs the water. The disturbance serves as the nucleation catalyst, and there are a lot of pent-up water molecules in the cup ready and anxious to change into steam. Suddenly — sometimes violently — the water bursts into a boil. The same thing might happen if you stir instant coffee into the water or drop in a tea bag. If the water is at a high enough temperature, it could indeed splash out of the cup and cause a burn.

So, yes, accidents can happen, although the number of actual injuries is thought to be quite low. But if you are going to boil water in a microwave, it’s probably worth taking some precautions. You could let the cup sit undisturbed for a couple of minutes in the microwave before you take it out. Or you could put something in the water prior to placing it in the microwave that would provide nucleation sites. Sugar works. A wooden spoon would do the trick. Or a rock.

But Ryan, just between you and me, the claim by your European relatives that water boiled in a microwave doesn’t taste as good is pretty nutty — but that doesn’t let you off the hook for dealing with the unresolved feelings you have toward your in-laws. But, wait, that’s a subject for the Therapy Geek.

Todd Campbell is a writer and Internet consultant living in Seattle. The Answer Geek appears weekly, usually on Thursdays.