Friday November 21 2014
Jet air dryers spread the most germs
"Hand dryers 'splatter' users with bacteria," The Daily Telegraph reports.
The headline is prompted by an experimental study that compared the potential transfer of germs to the surrounding environment, users and bystanders when using three methods of hand drying:
- paper towels
- warm air dryers – the sort you see in most public toilets
- modern "high-tech" jet air dryers, such as the Dyson Airblade model
Testers wore gloves coated in a solution of bacteria. Air samples taken after drying with the hand dryers showed significantly higher bacterial counts than when drying with paper towels, and were highest for the jet air dryers.
They then assessed the potential for spread to users and bystanders, this time using the proxy of gloves coated in black paint and a white body suit.
They found there was no contamination of the body after towel drying, but paint spots were on the body after the use of air dryers, which again was higher with jet dryers than standard warm air dryers.
One important limitation of this study is it essentially replicates the scenario of someone going to the toilet and then proceeding straight to the hand dryer without washing their hands first.
A more suitable test may have been to coat the gloves with the marker, wash them with soap and water as recommended, and then proceed to the hand dryers.
But the overall message of this study is consistent with current hand washing recommendations, including the use of disposable paper towels in healthcare settings.
Where did the story come from?
The study was carried out by researchers from the University of Leeds and the microbiology department at Leeds General Infirmary.
It was funded by the European Tissue Symposium (ETS), from whom one author reports having received honoraria.
The ETS produces paper tissue, including toilet paper, household towels and paper napkins, which may be seen as a potential conflict of interest.
The study was published in the peer-reviewed Journal of Hospital Infection.
The Daily Telegraph and the Mail Online's reporting was accurate, but neither appeared to have considered some of the limitations of this research.
What kind of research was this?
This was an experimental study that aimed to compare the tendency for three common hand drying methods – jet air, warm air hand dryers, and paper towels – to spread germs and contaminate the environment, users and bystanders.
Like thorough hand washing, thorough hand drying is just as important to prevent the transfer of germs from person to person or the surrounding environment.
According to hand washing protocols, the optimal way to dry hands is to use a disposable paper towel, which is then used to turn off the tap to avoid re-contaminating hands.
The main concern with using hand dryers is that people may not dry their hands as completely as they would with paper towels, and may go away while they are still damp. If hand dryers are used, it is advised that the hands are rubbed together under the dryer until they are totally dry.
However, another unclear and often speculated issue when using hand dryers is the possible transfer of aerosolised germs to the surrounding environment and people, possibly increasing the spread of infection.
This study aimed to compare the different hand drying methods, looking at whether they can contaminate the surrounding environment, users and bystanders.
What did the research involve?
The researchers carried out a series of hand drying tests in a single room with standard ventilation (not air conditioned). They first tested the possible contamination of the environment, and then people.
Gloved hands were immersed in a solution of lactobacilli bacteria (cultured from Actimel Danone yoghurt) before being dried with either:
- a warm air dryer – hands were rubbed together for 30 to 40 seconds until dry
- a jet air dryer – hands were placed into the unit and slowly drawn up and down for 15 seconds until dry
- paper towels – four paper towels were taken from the dispenser and were rubbed over hands for 15 seconds until dry
The tests were conducted over six weeks. A total of 120 air samples were taken – 60 made after drying contaminated hands (20 collections after each drying method: 10 in close proximity, 10 one metre away) and 60 control air samples taken before hand drying. Air samplers were left running for 15 minutes after each drying process.
They then repeated the tests, this time looking at the possible contamination of people standing nearby. This time, gloved hands were coated in black water-based paint rather than bacteria, and the user wore a disposable white hooded suit.
Another bystander in a similar suit stood diagonally adjacent to the dryer user one metre away to replicate the scenario of another user waiting to dry their hands. There was a total of 30 drying tests in this manner, 10 for each drying method.
What were the basic results?
The researchers found the lactobacillus count in air samples taken in close proximity to the dryers were 4.5-fold higher for the jet dryer (70.7 colony forming units, or cfu) compared with the warm air dryer (15.7cfu), and 27-fold higher compared with paper towels (2.6cfu).
Counts for the warm air dryer were also significantly higher than with paper towels.
A similar pattern was seen for the air collection one metre away, where counts were 89.5cfu with the jet dryer, 18.7cfu with the warm air dryer, and 2.2cfu with paper towels.
"Settle plates" underneath each hand dryer had the highest bacterial count for the warm air dryer (190cfu) compared with the jet air dryer (68.3cfu) and the paper towel drying (11.9cfu). Respective figures at plates one metre away were 7.8cfu, 2cfu and 0.7cfu.
As would be expected, the control air samples taken before drying found no lactobacilli.
On the person-contamination experiments, no paint spots were seen on paper towel users. For both the jet air and warm air dryers, spots predominated in the upper body area, with the number of spots significantly higher with jet dryers (144.1) compared with warm air dryers (65.8).
The number of paint spots was higher for all body areas with jet dryers, with the exception of both arms. With both hand dryers, however, there were relatively few paint spots remaining on the hands.
The number of paint spots detectable on the bystander was generally low for both air dryers and was not significantly different between the two (average count 1.6 spots for jet dryers and 1.5 for warm air dryers).
How did the researchers interpret the results?
The researchers concluded that, "Jet air and warm air dryers result in increased bacterial aerosolisation when drying hands.
"These results suggest that air dryers may be unsuitable for use in healthcare settings, as they may facilitate microbial cross-contamination via airborne [spread] to the environment or bathroom visitors."
Overall, this experimental study found the airborne spread of lactobacilli bacteria from contaminated hands was significantly higher with air dryers than with paper towels. Of the two, jet dryers caused higher air bacterial counts than standard warm air dryers.
Similarly, when assessing spread on to the body of the user and bystander using the proxy measure of black paint dispersal, there was no contamination of the body with paper towels, but paint spots were on the body after use of air dryers, again higher with jet dryers than standard warm air dryers.
It is well known that thorough hand drying is as key to preventing spreading infection as thorough hand washing. One of the recognised problems with hand dryers is that people may not dry their hands as completely as they would with paper towels.
What is less clear, and is often speculated about, is the possible transfer of aerosolised germs to the surrounding environment and people, possibly increasing the spread of infection.
This study appears to demonstrate cause for this concern. However, there are some points worth consideration when interpreting this study:
- One important limitation of the study is it may not replicate the real-life condition of someone having just thoroughly washed their hands with soap and water, and then drying their hands. In this experimental situation, the users had gloved hands contaminated with either lactobacilli or black paint and then dried their hands. In effect, this may be seen more to replicate the scenario of someone going to the toilet and then proceeding straight to the hand dryer without washing their hands first. A more suitable test may have been to coat the gloves with either bacteria or black paint, wash them with soap and water as recommended, and then proceed to the hand dryers to see how many bacteria or paint were spread.
- The spread of heavier black paint may also not be equivalent to the spread of viruses and bacteria, though it may represent the spread of water.
- Aside from the assessment of the surrounding environment and bystanders, another important area of consideration would also be to compare how much bacteria remained on the surface of the users' hands after drying with each of the three methods. This is of equal importance in knowing how much bacteria remains on the users' hands that could be transferred to other surfaces. It would be valuable to know whether there was any difference. This study has not specifically examined this aspect, though in fact it did note few paint spots remained on the hands after drying with either of the hand dryers.
- It also would have been valuable to consider comparing the amount of bacteria or paint left on the towel dispenser or hand dryers after use, and how much of this would usually be transferred to the next person's hands during hand drying.
Despite these limitations, the overall message of this study is consistent with current handwashing recommendations, particularly when it comes to healthcare settings.
Of course, disposable paper towels are not available in all facilities. If only hand dryers are available, hands need to be rubbed together until they are completely dry.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.