Our Microbial Overlords are always trying to outwit us. And usually succeed without our realising. Their ability to propagate themselves can be frightening – especially if you culture what’s lurking in the average hot air hand dryer. But first a note of caution. There are a many studies out there that have examined this subject, some good, some bad, many indifferent. But most people only get to see these through the lens of whatever spin the newspapers put on them. What you really need to do is evaluate the source material. How the study was designed. How the data was collected. Are the conclusions cromulent with the data? And who funded it – do they have a dog in the fight? (You can also get ‘meta analysis’ or ‘systematic review’ of a bunch of studies. The theory is that if you amalgamate the data from lots of small studies you can draw better conclusions because the resultant data set is bigger. In practice these often just combine a collection of dog turds and then claim the result has become the pinnacle of accuracy and significance rather than just an mammoth pile of pooch poop.)
Anyway, fortunately you’ve got me to do all that for you, so what’s the real hand dryer hot poop? Hand washing is really important in food prep and healthcare infection control. But effective hand drying is just as vital because it’s far easier to spread germs via wet hands than dry ones. But I don’t really trust studies that just look at dryness. Yes, it’s important but the real test of hygiene performance is not the percentage dryness of the hands but the number of bacteria removed from, or remaining on, the hands after use. The study quoted in the Daily Fail above finds similar results to most other data out there: a standard hand dryer takes 30 seconds longer than a towel and “…rubbing hands while under hot-air dryers leads to greater bacterial numbers and airborne dissemination… it might be that rubbing hands causes bacteria to migrate from hair follicles to the skin surface.” I’m not sure I entirely agree with Dr Huang’s migration hypothesis principally because other studies have found that:
- Hot air dryers produced significant increases in all bacteria (by more than 260%);
- Gut bacterial counts rose by 438%, the largest increases being in Enterobacteriaceae and coliforms, including Escherichia coli (395%) – or (in English) the hand dryer is blowing someone else’s poo on to your hands;
- Sampling nine types of location (including hospitals, eating places, railway stations, public houses, colleges, shops and sports clubs) at least 6 species of gut bacteria were isolated from the air flows of 63% of dryers indicating faecal contamination.
Ick. So what about ‘jet’ type dryers, then? There are claims that a certain brand is the ‘most hygienic hand dryer’ on the market. This is true but only if ‘hand dryer’ refers to electric devices only – and if you go back to the source of that statement it compares the ‘jet’ dryer with the traditional warm air / poo mix ones. Other studies show the jet’s performance in terms of the numbers of all types of bacteria remaining on the hands of users compared to paper towels was significantly worse. So, compare traditional dryer to jet, jet = better. Introduce paper into the comparison and you get a very different result.
“…although the jet air dryer showed a similar drying efficiency to paper towels… its hygiene performance, although better than the warm air dryer, was significantly worse than the two types of paper towel tested in this study.”
You can imagine the war of words this sort of stuff provokes between the paper industry and manufacturers of hand dryers, but if we apply basic biological plausibility to the data across many studies it is most likely paper towels work better because they dry more quickly and will help remove any residual dirt, grease or transient bacteria not removed by washing from the hands with friction – whereas the jet or warm air dryers will not, as well as blowing additional bacteria on to hands being dried.
- In one study paper towels reduced the mean numbers of all types of bacteria by between -44.6% and -91.5% for fingerpads and between -32.8 to -85.2% for palms.
- The warm air dryer increased the mean numbers of all types of bacteria by between +114.1% to +414% for fingerpads and +230.4% to +478.8% for palms.
- The jet air dryer increased the mean numbers of bacteria by between +28.0% and +193.3% for fingerpads and +9.1% to +82.2% for palms.
- Comparing cloth towels with hot air dryers showed 10 seconds’ drying with a cloth towel was about equivalent to 45 seconds’ use of a hot air dryer.
- It is true that 10 seconds’ drying with either paper towels or jet air dryers was equally efficient at removing water, but traditional hot air dryers took about 40 seconds to achieve the same level of dryness.
Let’s put that into English. Dryers coat your hand with other folks’ poo bacteria (Ick!) and because most people don’t dry their hands properly (due to impatience) they are more likely to spread them onto door handles and other touch surfaces. Or shake hands with you. Ick. But that’s not the only cross-contamination risk. Traditional dryers spread the bacteria on to surfaces up to three feet from the unit, jet dryers by twice that – and also lead to a large number of airborne bacteria near the dryer. This is not a good look in food prep or healthcare environments. No such spread is seen with either paper or cloth towels.
Warm Air Dryer
Total number of bacteria increased by 194% on the finger pads and by 254% on the palms; bugs spread up to 3 feet;
Jet Air Dryer
Total number of bacteria increased by 42% on the finger pads and by 15% on the palms, 400 mph wind spreads bugs up to 6 feet;
Total number of bacteria reduced by 76% on the finger pads and by 77% on the palms, paper towels showed no significant spread of microorganisms. And if you think that’s bad, don’t read the norovirus poster…