The bacterial horror of hot-air hand dryers

John Ross, MD, FIDSA

Contributing Editor

Follow me on Twitter @JohnRossMD

If you’re the kind of person who avoids public bathrooms at all costs, you may feel validated, as well as disturbed, by a new study from researchers at the University of Connecticut and Quinnipiac University. They suspected that hot-air hand dryers in public restrooms might be sucking up bacteria from the air, and dumping them on the newly washed hands of unsuspecting patrons.

To test this theory, scientists exposed petri dishes to bathroom air under different conditions and took them back to the microbiology laboratory to look for bacterial growth. Petri dishes exposed to bathroom air for two minutes with the hand dryers off only grew one colony of bacteria, or none at all. However, petri dishes exposed to hot air from a bathroom hand dryer for 30 seconds grew up to 254 colonies of bacteria (though most had from 18 to 60 colonies of bacteria).

Were the bacteria multiplying inside the hand dryers, or were they being pulled into the hand dryers from the air inside the bathroom? To answer this question, the researchers attached high-efficiency particulate air (HEPA) filters to the dryers, which would eliminate most of the bacteria from the air passing through the dryer. When they exposed petri dishes to air from the hand dryers again, the quantity of bacteria in the dishes had fallen by 75%. As well, the researchers found minimal amounts of bacteria on the nozzles of the hand dryers. They concluded that most of the bacterial splatter from the hand dryers had come from the washroom air.

How did the bacteria get into the air in the first place? Unfortunately, every time a lidless toilet is flushed, it aerosolizes a fine mist of microbes. This fecal cloud may disperse over an area as large as six square meters (65 square feet). Aerosols from flushed toilets may be especially harmful in the hospital setting as a means of spreading Clostridium difficile.

Is there any good news from this study? Well, the vast majority of the microbes that were detected do not cause disease in healthy people, with the exception of Staphylococcus aureus. Some of the bathroom bacteria, such as Acinetobacter, only cause infections in people in the hospital, or in those with weak immune systems. The others that were found are relatively harmless. In addition, air from real-world bathrooms may contain fewer bacteria than the bathrooms in the study. The sampled restrooms were located in a university health sciences building, and at least some of the bacteria came from experiments going on in laboratories within the building.

So what’s a person to do to avoid picking up bacteria in a bathroom? You should still dry your hands, as not drying them after washing them helps bacteria to survive on them. Paper towels are the most hygienic way to dry your hands. For this reason, use of paper towels is already routine in health care settings. You may also wish to avoid jet air dryers, which have also been associated with the spread of germs in bathrooms. And remember that your chances of picking up a serious pathogen in a restroom are small. Direct contact with other people is much more likely as a means of acquiring infection.


  1. Ray

    From this study it seems that the first order of business is to make sure all public toilets have lids, and educate the public about shutting before flushing. Automatic lowering of the lid would be ideal, but until then take a piece of toilet paper in your hand, and use that to lower the lid BEFORE flushing (done this all my life to lower the seat after a man, who was not taught by his parents to do so, leaves the seat up). Second order of business is to, obviously, remove the air dryers. They slow down the traffic in a busy public bathroom (keeping us in there longer than we want), and now we know of their aiding bacteria dispersal. Paper towel dispensers come in many flavors. Automatic “eye” ones are the best (especially if they roll out one LONG sheet that’s enough to dry your hands), the old fashioned “grab and pull down the next towel” come in second, and the worst are the ones requiring you to manually forward the towel, exchanging bacteria with the last user and the next user all the while water is dripping down your arm into your sleeve. I don’t like public bathrooms, but where else are you going to go?

  2. KW

    John Ross,
    Why are you concerned if it’s harmless and found widely throughout the environment? I just read up on it a bit and it’s actually used in food preparation of fermented foods (natto) and as an alternative treatment for digestive issues. We should probably be glad if it’s in us; it seems our digestive microbiome probably needs it.

  3. Connie White, RN, CIC

    Maybe we need to invent an automatic cover attached to the toilet that was activated by the actual flush, i.e. a cover which came down over the bowl when the handle was pressed, and when it completed the task, then and only then would the flush occur. Next, you would have to determine how long before the cover would recede to contain the aerosolized material, preventing the next person from being able to use said toilet.
    Actually teaching people to physically use the cover would result in massive hand contamination, which would put us back into the same boat as the original handwashing fight. Better not to take that chance, mechanized cover would be safer.

  4. vinu arumugham

    And any electric dryer, motor, fan emits high levels of Extremely Low Frequency (ELF) magnetic fields.

    International Agency for Research on Cancer
    “Extremely low-frequency magnetic fields are possibly carcinogenic to humans (Group 2B).”

  5. Lisa

    Any thoughts on the level of risk to the individual from air dryers versus the level of risk to the planet from the combination of creating, delivering, and disposing of the paper towels? Instead of the conclusion being to use paper towels, is there any thinking of how to decrease harmful bacteria in the air? Would encouraging public toilets to have lids (and educating people to use them) be worthwhile, especially in healthcare facilities? Should people at home be encouraged to lower their toilet lids each time they flush? It’s so hard these days to figure out exactly what is and isn’t worth being afraid of, and to not avoid one risk at the expense of running right into a different more significant risk.

  6. John Ross

    From the paper: “One of the organisms recovered from multiple bathrooms was B. subtilis,” of which a particular type, “strain PS533 [was] used to prepare large amounts of spores in a research laboratory on the 2nd floor of research building 1.” About 2.5-5% of the total bacteria isolated was confirmed to be of this “non-pathogenic” B. subtilis strain, which is found “widely throughout the environment.” (If I worked in the building, I probably wouldn’t be too thrilled about this.)

  7. Tootles LeBeau

    “The sampled restrooms were located in a university health sciences building, and at least some of the bacteria came from experiments going on in laboratories within the building.”


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