Q: Considering news coverage of how hot-air hand dryers do little more than blow bacteria around, am I better off just wiping my hands on my pants when using a public restroom?
A: First, let's consider why this would have seemed a ridiculous question only two decades ago — before bathroom hand dryers had proliferated throughout the developed world. The use of paper towels in bathrooms had created concerns about environmental impact, both in connection to the trees used to make the paper towels and in the amount of trash produced. Bathroom hand dryers, although they used energy, decreased the use of, and trash from, paper towels — not to mention they were (and remain) — very cost-effective. The operation of bathroom hand dryers is only one-tenth to one-twentieth the cost of a year's worth of paper towels.
But, yes, now there are concerns about bathroom hand dryers and hygiene. In short, air hand dryers can aerosolize bacteria. A recent study at the University of Connecticut School of Medicine evaluated whether hand dryers contaminated the area around them with bacteria. The authors looked at three buildings within the school and placed bacterial growth plates (supplemented with sugar) in the bathrooms underneath the hand dryers. Each plate was placed 12 inches from the outlet of the hand dryer, left open to air for 2 minutes and then closed.
After 48 hours, the growth of bacteria was then measured. The plates near non-operating hand dryers grew 0 to 1 colony of bacteria. Those plates exposed to 30 seconds of air from operating hand dryers grew 18 to 60 colonies of bacteria, depending on the bathroom. That's a very big difference.
Multiple strains of bacteria, from 21 different species, were seen on the growth plates. The bacteria appeared not to come from the hand dryer itself, but from bacteria in the bathroom that was sucked into the air coming out of the fan. The reason: A high volume of air passes through these driers (19,000 linear feet per minute at the nozzle).
Two interesting notes: First, researchers saw no difference between women's and men's restrooms. Second, placing a HEPA filter within the hand dryer led to a fourfold reduction in the number of bacteria seen on the plates.
But because the hand dryers disperse bacteria does not necessarily mean they will increase the risk of infections, at least for most people. Those who have compromised immune systems, disruptions of the skin or intravenous lines, however, could potentially develop invasive infections. Furthermore, if you're washing your hands to prevent passing an infection to others, using a hand dryer may make the action moot. When you shake someone else's hand, you may transfer bacteria from the dryer air to them.
It may defeat the purpose of washing your hands in the first place, right? Not quite. The logic behind that conclusion is still hypothetical. A better study would be comparing the bacteria upon the hands of people who used either hand dryers or paper towels.
Until then, we're still not willing to suggest you dry your hands on your pants. It's simply better to use paper towels. Perhaps, sometime in the future, hand dryers with HEPA filters will become widely available.