If you’re concerned about carbon dioxide emissions, you might think twice about reheating your coffee five times in your microwave after reading a new study from researchers at the University of Manchester in England.

The study found that the energy required to build and operate 130 million microwave ovens in the European Union over a year created 7.7 million tons of carbon dioxide emissions—equal to the yearly tailpipe emissions from 6.8 million cars.

Microwave ovens don’t actually emit carbon dioxide. But the researchers tracked the energy needed and resulting emissions to extract raw materials and to bring the ovens to market, plus the emissions from creating the electricity needed to operate the appliances.

The researchers chose to look at microwaves, but other appliances could be studied in the same way. 

“We were not trying to demonize microwaves with this study,” says lead author Alejandro Gallego Schmid, a researcher at the school of chemical engineering and analytical science at the University of Manchester. “It’s just that consumers are not aware that these devices have an impact. That’s why we compared them to car usage: It’s easier to see how a car produces combustion and how that’s related to global warming.”

There are no studies tracking the U.S. figure, but 96 percent of U.S. households have at least one microwave, according to a study by the Lawrence Berkeley National Laboratory in California.

The Environmental Breakdown

Many scientists believe that carbon dioxide emissions contribute to global warming.

Schmid’s team found that regular use of a microwave accounts for 54 percent of the appliance’s environmental impact. Because electricity coming into your home is made up of a mix of energy sources, burning the fossil fuels to power your microwave is wreaking havoc indirectly. Extraction of raw materials (23 percent) and the production process (15 percent) are less harmful but still significant. The remaining 8 percent is chalked up to transportation and other costs.

So what’s an environmentally conscious consumer to do? Schmid offers some simple microwaving guidance: “Try to not overheat the food that you’re cooking.”

He also suggests resisting the urge to toss out your microwave if it breaks or if it doesn’t fit the aesthetic of the rest of your kitchen. “With all electronic devices, there’s a trend to throw them away before the technical life span is up,” he says.

Consumer Reports surveyed more than 35,000 subscribers on
brand reliability for over-the-range microwaves.

As a part of the study, Schmid looked at 100 microwaves destined for the landfill. “Around 30 percent were still working, and another 30 percent were really easy to fix,” he says.

Of course, fixing a microwave may be easier said than done. Schmid acknowledges that even for the 30 percent of microwaves that could be repaired, it’s not necessarily cost-effective to do so.

More on Kitchen Appliances

“We took a microwave to be fixed to an expert, and it took more than 2 hours to dismantle it—from a design standpoint, that’s really poor design,” Schmid says. “At some point, you might have to break parts, which means you can’t fix the microwave yourself.”

In terms of design, manufacturers could use more recycled materials and make it easier for microwaves to be recycled at the end of their lifespan. 

That’s not to say that using a microwave is environmentally irresponsible. “Microwave ovens are the most efficient form of cooking from an energy-use perspective,” says Pat Remick, a spokesperson at the Natural Resources Defense Council.

In the end, your microwave isn’t going to harm the planet by itself. But reheating your food the minimum amount of time can do some good, especially if your neighbors follow suit.