You hands get more dirtier after washing them with air dryer.

Bacteria are like biology's glitter; they get absolutely everywhere.

Scientists still don’t understand how bacteria can make their way into seemingly every nook and cranny on Earth. Most recently, researchers at the University of Connecticut discovered (pdf) that bacterial spores they had been growing in labs somehow wound up in the air of 36 bathrooms, some in the same building wing as the lab and some in two other wings connected to it. Subsequently they found that air dryers in these bathrooms played a large part in circulating these bacteria—meaning the machines are probably coating the newly-washed hands of anyone using them with bacteria.

To be clear, this work doesn't necessarily mean air dryers pose a health risk for hand-washers. It does, however, suggest in some cases paper towels may be superior to air dryers. In areas where there are pathogens known to cause people harm (like certain labs), or where a large portion of potential air-dryer users have compromised immune systems (like a retirement home or perhaps certain wings of a hospital), it's best not to spread around microbes any more than necessary.

Peter Setlow, a microbiologist and one of the lead authors of the paper, frequently studies spores of the benign bacterial species Bacillus subtilis, and so needs to produce huge quantities of them for his work. He and his team were curious to see if these spores, which should be unique to his lab, were capable of making their way into other parts of the building he worked in—namely the bathrooms, which are communal space accessed at some point by everyone in the facility.

They took samples from inside bathroom air dryers, from air blown by both dryers, and from air blown slower fans they brought in specifically for the study. Nothing much grew in the petri dishes hosting samples from the air dryer itself, but the petri dishes with samples taken from the air blown on them grew up to 60 colonies of live bacteria in addition to B. subtilis spores. Researchers also left petri dishes out in the ambient air and found just a few colonies growing in them. This suggests that there are always bacteria floating around bathroom air, including some from different parts of the building, and that although air dryers don’t necessarily cultivate these microbes, they do circulate them.

Setlow also doesn't consider these findings particularly worrisome. Microbes, after all, are everywhere, and hand dryers aren’t making anyone sick as far as science can tell. And this work supports conclusions from researcher at the Mayo Clinic from 2012: After reviewing 12 studies, they found that paper towels are probably better from a hygiene perspective.

But they may not be better overall—they’re more expensive, cause more waste, and, if used insufficiently, don’t actually get hands all the way dry—which is a key step to preventing bacterial spread from person to person.

Thomas Murray, a pediatrician at Connecticut Children's Medical Center and co-author of the paper, says that in general, the findings don't present enough of a concern for everyone to switch to using paper towels. Although there may be more bacteria floating around the air, most of these potential pathogens are no match for a healthy human immune system, he says.