Steam comes off a microwavable steamer tray after a cut up sweet potato and eggplant. (Matt McClain/FOR THE WASHINGTON POST)

I use my microwave nearly every day, mostly for heating leftovers, rewarming my morning coffee and “steaming” vegetables. Because, according to the U.S. Bureau of Labor Statistics, more than 90 percent of homes in this country have the appliance, I figured it was universally regarded as safe; so I was taken by surprise when a recipe I posted online generated some passionate anti-microwave comments.

A quick Web search of “microwave dangers” brought up some startling claims and concerns. There are questions about radiation leakage, arguments that microwave cooking “violently” rips apart food molecules rendering them void of nutrients, and earnest warnings that ice crystals from microwaved water form a similar shape to those from water that was repeatedly exposed to the word “Satan.” (You just can’t make this stuff up.)

Although the devil-water concept is a bit, um, farfetched, some of the other claims cited seemingly sound sources, so I decided to investigate.

How do microwaves work?

The microwave contains an electron tube called a magnetron, which produces electromagnetic radiation with a wavelength a little shorter (“micro”) than that of a normal radio wave. These waves cause the water molecules in food to vibrate, which ultimately produces the heat that cooks the food. That’s why ingredients with a high water content, such as vegetables, cook very quickly in a microwave.

It’s a common myth that microwaves cook from the inside out. In fact their waves don’t actually penetrate foods very well, which is why you get cold spots in thicker foods and need to let them rest after microwaving to allow the heat to disperse for even cooking.

Do they emit radiation?

Microwaves do leak some radiation, but before you panic, read on. To put things in perspective, your laptop, cordless phone and cellphone all leak electromagnetic radiation, too.

The Food and Drug Administration has strict limits on the amount that can leak from a microwave oven throughout its lifetime, and it is far below the amount known to harm people. Part of the requirement is that the appliances are built with double “interlock” systems that stop the production of microwaves as soon as the latch is released.

The level of normal exposure also drops dramatically as you move away from the oven — so, to be extra cautious, simply stand away from it while it’s on. Microwaves turn on and off like a light bulb: when they are off, no waves are emitted, and microwave energy cannot linger in the oven or in food.

Although there is no clear evidence of harm, many people are concerned that low levels of electromagnetic radiation may impact human health over a long time. It is certainly something that needs to be studied. The thing is, when you consider the persistent and cumulative exposure from all your WiFi-enabled devices and the multitude of other radiation sources we have around us, the occasional microwaving of some leftovers seems the least of the problem, if there is one at all.

How does microwaving affect nutrition?

Microwaving is one of the most healthful ways to cook vegetables because it cooks them quickly and without a lot of water, much as steaming does. Multiple studies have supported this, including a 2010 study published in the Journal of Agriculture and Food Chemistry that identified microwaving as the best cooking method for maintaining the color and cancer-fighting phytochemicals in Brussels sprouts.

One study that came up in many Internet anti-microwave arguments was done in 2003, when Spanish researchers concluded that microwaving broccoli destroyed nearly all its flavonoids (antioxidant-like compounds). The problem is that the researchers microwaved the broccoli for a long time in a bowl of water. That made it more akin to boiling, which leads to similar nutrient loss.

To “steam” a vegetable in your microwave (one of my favorite techniques), just wash it and pat it dry, place it in a microwave-safe bowl, cover tightly and microwave it for the same amount of time you’d steam it — four minutes for a cut-up head of broccoli, for example. The result will be bright green, crisp and nutrient-rich.

Is it safe to heat in plastic containers?

Takeout containers, foam trays or any plastic storage or grocery bags not specifically designed for microwave use can melt and leach into food. So use plastic containers only if they are explicitly labeled microwave-safe. It’s best to look for a label on paper plates, too.

Thick, nondecorative glass, wax paper, parchment paper and white (undyed) paper towels are all fine in the microwave.

The bottom line

Microwaving can be a healthful, convenient way to cook. Just be sure your appliance is in good repair and use a microwave-safe container. The biggest health risk involved is probably the foods you choose to microwave. Rather than use it for heating hyper-processed salty meals and snacks, try it out with more healthful items, such as fresh vegetables.

Krieger is a registered dietitian, nutritionist and author. She blogs and offers a biweekly newsletter at She also writes weekly Nourish recipes in The Washington Post’s Food section.