Researchers coated subjects' hands with Lactobacillus, a harmless bacteria that you don't typically come across in a public bathroom. The idea was to mimic hands that hadn't been washed properly. After drying hands, researchers went in and conducted 120 air-sampling tests. They found that Lactobacillus counts in the air were 4½ times higher near high-powered jet dryers than around warm-air dryers. And bacteria counts were 27 times higher near warm-air dryers than when subjects used paper towels.
"Next time you dry your hands in a public toilet using an electric hand dryer, you may be spreading bacteria without knowing it," University of Leeds School of Medicine professor and study lead Mark Wilcox said in a statement. "You may also be splattered with bugs from other people's hands."
Another set of subjects coated their hands in paint to mimic how bacteria could be spreading.
The European Tissue Symposium, a group of tissue-product makers, funded the research. A spokesperson for Dyson, a company that makes hand dryers, told the Telegraph that the research was flawed. "They have tested glove-covered hands, which have been contaminated with unrealistically high levels of bacteria, and not washed," she said.
Previous research has looked more closely at the amount of bacteria left on the hands of people using hand dryers versus paper towels, and found that one of the biggest keys to reducing bacteria was getting hands as dry as possible. A 2011 study published in the Journal of Applied Microbiology found that paper towels consistently out-performed hand dryers. For that study, subjects handled chicken and then washed their hands, in order to replicate a common scenario.
What you do with your hands under a dryer makes a big difference: The 2011 study found that people who kept their hands still under dryers reduced almost the same amount of bacteria as those using paper towels. But rubbing hands together under a dryer actually spreads more bacteria around.
The 2011 study also suggested that ultra-rapid hand dryers could be superior to the warm-air dryers, given that the shorter drying time means likely greater compliance among users.
(There is also the separate and ongoing debate over the environmental impact of using paper products versus hand dryers.)
There isn't widespread agreement on which kind of hand dryer is better, as noted in a 2012 review of 12 previous studies, which was published in the peer-reviewed journal Mayo Clinic Proceedings. But, the authors write, from a hygiene standpoint, drying with paper towels is superior to using air dryers. "Paper towels should be recommended in locations where hygiene is paramount, such as hospitals and clinics," they say.