Q: In public toilets, I'm always torn about the most environmentally friendly way to dry my hands, when there is a choice.

Asked by Helen Kist, Edinburgh, U.K.

Do you reach for the paper towels or push the button on the blow dryer after washing your hands? Or do you simply stand there, wringing your hands over the best option? If you wring them long enough, evaporation might take care of things, but if you don't fancy that option (or want to use your jeans as a hand towel), let's examine the other choices available in public restrooms: paper towels or blow dryers.

The main impacts of paper towels happen outside of the restroom—whether and how much of the towels are made with recycled paper, and how they're disposed of after sopping up your rinse water. For blow dryers, most of which are electric, the source of electricity—whether it's from fossil fuels or a renewable source—matters a little, and the design of the blower matters a lot. Super efficient, high-speed dryers, which blast cooler air at higher speeds, can be as much as 80 percent more efficient than the older, hot-air models.

Here's the problem: most public and workplace restrooms don't have high-efficiency blowers or high-recycled content paper towels. Their virgin-paper towels are destined for the landfill, where they'll give off plenty of climate-warming methane as they decompose in the oxygen-starved environment; and their old-school blowers could double as marshmallow roasters.

The Climate Conservancy, a greenhouse-impact auditing outfit started by Stanford graduates, took a thorough look at the issue. "If you walk into an average bathroom and use an average hand dryer in an average way," they found, "you'll be causing somewhere between 9 and 40 grams of carbon dioxide emissions" every time you dry your hands. On the other hand, "If you use two towels, data suggests you will be responsible for roughly 56 grams of carbon dioxide emissions." Given than many of us use more than two towels at a go, using the hot-air blower instead will do more to help keep the climate cool.

By the way, don't let anyone tell you not to obsess over what might seem like a trivial issue. True, automatic faucets and unspool-proof paper towel dispensers have saved uncounted gallons of water and reams of pristine paper in the decades since they were introduced to public restrooms. But according to one recent survey by a trade group, 50 percent of us wash our hands more than 10 times a day—and with coughs, colds and other infectious diseases popping up all the time, the rest of us should probably follow suit. Shouldn't our drying choices be as clean as our hands?

Micki Ream received her master's in Earth systems in 2010.