Handwashing is one of the most effective ways to remove germs, stave off illness, and prevent the spread of bad bacteria to others.

But you may be undoing all your handwashing work the moment you press the button on a hot-air dryer.

A new study from the University of Connecticut and Quinnipiac University shows that hot-air dryers may be acting like bacterial bombs, shooting loads of spores from bathroom air directly onto your hands.

Several years ago, in an effort to be more environmentally friendly, many businesses, universities, and large corporations shunned the stacks and rolls of paper towels for these hot-air and jet dryers.

They produced less waste and ultimately could help the company’s bottom line, the rationale went.

However, this new research, which was published in the April issue of Applied and Environmental Microbiology, suggests that the environmentally friendly option may be bathing your hands in hefty doses of bacteria, even some that are usually found in feces.

What the study churned up

Researchers placed germ-collecting plates in 36 men’s and women’s bathrooms across the University of Connecticut’s School of Medicine facilities.

Some plates sat for two minutes in still air, without hot-air dryers blowing. Others were blasted with dryer air for 30 seconds while they were held 12 inches from the dryer’s nozzle.

Any bacteria that landed on the plates would start growing, forming tiny, domed bacterial colonies. The researchers counted the number of colonies that appeared on each plate.

They said the results were staggering — if not also sickening.

In the still bathrooms, the plates collected an average of zero to one bacterial landings in the two-minute window. When left open for 18 hours, the number of colonies leapt to six per plate.

Plates that were exposed to hot-air dryer air for 30 seconds showed an average of 18 to 60 colonies per plate, with a range as high as 254 colonies.

The researchers also tested the theory of bacterial splatter from air dryers by leaving two plates open for 20 minutes in bathrooms in which air was circulated by small fans. In those 20 minutes, 12 and 15 colonies appeared on the two plates.

The key to the swirling bacterial cyclone, the researchers decided, was the movement of air, disturbance of bacteria, and the forceful direction of air into and out of hand dryers.

In other words, any air movement is a bacterial hazard.

“One reason hand dryers disperse so many bacteria is the large amount of air that passes through hand dryers, 19,000 linear feet per minute at the nozzle,” the researchers wrote in their study. “The convection generated by high airflow below the hand dryer nozzles could also draw in room air.”

The addition of high-efficiency particulate air (HEPA) filters cut the number of bacteria that landed on the plates by fourfold. However, it did not eliminate the bacteria entirely.

This new research squares with several recent studies that show the handy hot-air and jet dryers may be one of the highest sources of bacterial contamination in a public restroom.

A study in the Journal of Hospital Infection compared jet dryers, warm-air dryers, and paper towels in a simulation of poorly washed and contaminated hands.

People who used jet dryers had 4.5 times more bacteria on their hands than people who used a warm-air dryer, and 27 times more bacteria than people who used paper towels.

Another study in the Saudi Journal of Biological Sciences found large quantities of bacteria flowing through hand-dryer air in a 15-day study. This study also found that the bacteria were present even when the dryer was not being used to dry hands.

To confirm the source of the bacteria was the bathroom air and not the dryers themselves, the researchers took samples of the surfaces inside the dryers. The results showed “minimal bacterial levels.”

“These results indicate that many kinds of bacteria, including potential pathogens and spores, can be deposited on hands exposed to bathroom hand dryers, and that spores could be dispersed throughout buildings and deposited on hands by hand dryers,” the study’s researchers wrote.

The best way to dry your hands

Before this news sends you into a full state of hygienic hysteria, Dr. Thomas S. Murry, an author and researcher in the study as well as a professor and infectious diseases expert, cautions that they did not find evidence of dangerous or deadly bacteria.

“Importantly, we did not prove that the bacteria deposited by hand dryers are responsible for disease,” he told Healthline. “In fact, for people with a healthy immune system, it is unlikely to be a problem as the hand dryer is concentrating environmental bacteria from the air probably found in most places where people congregate.”

However, Murray points out that the results of the finding show “it does to some extent defeat the purpose of hand washing to remove bacteria when you just blow them back onto your hands.”

For their part, the findings have already led to one big change at the University of Connecticut.

“Paper towel dispensers have recently been added to all 35 bathrooms in basic science research areas in the UConn School of Medicine surveyed in the current study,” the authors said.

Charles Gerba, PhD, a microbiologist and professor at the University of Arizona, has studied germs for more than 30 years. He stands by the tried and true handwashing and paper towel-drying techniques.

“Hand washing remains the gold standard of getting rid of germs on hands. However, if it is not done the right way, for at least 15 to 20 seconds including between the fingers and on top of the hands, then we can recontaminate our hands again the moment we touch the next surface at home or in public,” he told Healthline.

Gerba recommends you dry your hands when you’re in public with a disposable towel. Hand sanitizer is a great tool in follow-up to handwashing, he suggests. He prefers one that provides residual, long-lasting protection, such as Zoono GermFree24, which he says offers 24 hours of protection against germy hand hitchhikers.

At home, Dr. Katy Burris, a dermatologist at ColumbiaDoctors and an assistant professor of dermatology, suggests you have little to be worried about.

“As long as you are cleaning your bathroom and washing your towels frequently, you should not be overly concerned,” Burris told Healthline. “It is the volume of people that use a restroom that increases the overall bacterial presence. A few people in a home versus hundreds to thousands in a day makes a big difference.”

Eve Early, the corporate director of infection prevention and control at Orlando Health, says there is one time when you might want to treat your home like a busy hospital or airport bathroom and swap out cloth hand towels for paper towels.

“Certainly, a used towel can be a source of germs, but this is more of a concern in the home setting if there is someone sick in the home,” she told Healthline. “In that case, you might want to switch to paper towels in the bathroom until they are past the infectious stage.”

For routine precautions, Early says, just make sure you change your towels on a regular basis.