Two previous studies
carried out by the University of Westminster in 1993 and 1994 showed that on
average warm air dryers substantially increase the number of bacteria on the
hands of users and that paper towels and continuous roller towels decrease
the number. Since these studies all other investigations by the University
have consistently shown that towels, both continuous roller towels and paper,
perform significantly better than warm air dryers in terms of speed, drying
efficiency, hand hygiene and bacterial contamination.
However, new types of electric hand dryer have been introduced in recent
years where users insert their hands into a slot whilst unheated air is forced
out through holes or slits at high speed producing jets or sheets of air which
remove water from the hands by scraping. This type of electric hand dryer had
not previously been tested by the University. The present study summarized
in this article aimed to compare the performance of one of the new type jet
air dryers with other hand drying methods. The study was performed in 4 parts
(A - D) and the following hand drying methods were tested:
Paper towel 1: (PT 1) 2-ply 100% recycled. Art. 217010 (Wepa).
Paper towel
2: (PT 2) 2-ply 100% virgin. Art. 403360 (Sofidel).
Paper towel 3: (PT 3) 2-ply
through-air dried (TAD). 50% virgin - 50% recycled. Art.6769 (Kimberly-Clark).
Paper towel 4: (PT 4) 1-ply 100% recycled. Art. 6019 (Kimberly-Clark).
Paper
towel 5: (PT 5) 1-ply 100% recycled. Art. 404638 (Sofidel).
Warm air dryer:
(WAD) Electric-Aire(tm), model LE48 (World Dryer Corporation).
Jet air dryer:
(JAD) Airblade(tm), model AB01 (Dyson).
Part A: The drying efficiency of different hand drying methods.
The amount of water remaining on the hands of volunteers after using three
different hand drying methods (paper towel, warm air dryer, jet air dryer)
for different times (0 - 60 seconds) were measured and the average percentage
dryness at different times was calculated for each hand drying method.
It was found that the jet air dryer had a drying efficiency equal to that
of the five types of paper towel (PT 1 - PT 5) tested and, like them, achieved
at least 90% dryness in approximately 10 seconds. The warm air dryer took an
average of 47 seconds (over four times as long) to achieve 90% dryness, i.e.
was slower than the jet air dryer or the paper towels at drying the hands.
Part B: Changes in the number of different types of bacteria on the hands
before and after drying using paper towel, warm air dryer and jet air dryer.
After visiting a washroom, the fingerpads and the palms of the hands of 20
volunteers were sampled before and after washing and drying using two types
of paper towel (PT 1 and PT 3), a warm air dryer, and a jet air dryer. Combined
results of the changes in bacterial numbers on three different bacterial growth
media are shown in Figure 1.
The two types of paper towel tested produced mean decreases in bacteria grown
on the three media after washing and drying of the hands. The decreases were
respectively -51% (from means of 195 to 95) and - 76% (from 170 to 41) for
the fingerpads and -48% (from means of 290 to 149) and -77% (from 278 to 65)
per square centimetre for the palms. The warm air dryer (WAD) produced a mean
increase in bacteria on the fingerpads by +194% (from 123 to 362) and the palms
by +254% (from 215 to 762) of the hands after use. Although the JAD performed
better than the WAD, it did not perform as well as the paper towels and did
not on average reduce the numbers of bacteria on the hands after use but increased
them on the fingerpads by +42% (from 183 to 260) and on the palms by +15% (from
411 to 472). Except for the JAD/palm one, changes in numbers of bacteria were
significant at p = 0.1 or lower.
Part C: Potential contamination of other users and the washroom environment
caused by paper towel, warm air dryer, and jet air dryer.
In this part of the study the hands of subjects were artificially contaminated
with yeast suspension. Subjects then dried their hands using two types of paper
towel, a warm air dryer or a jet air dryer for set times (10 seconds for paper
towel and JAD; 20 seconds for WAD). Yeast was detected at different distances
from each hand drying device by growth of colonies on agar plates.
The results showed that the jet air dryer dispersed the artificial contamination
on the hands for distances of up to 2 metres. The paper towels performed better
than the warm air dryer directly below the device but both hand drying methods
showed no significant dispersal beyond 0.25 metres, unlike the jet air dryer.
The capacity of a hand drying device to disperse artificial contamination demonstrates
its potential to disperse actual contamination from the hands to other users
and the washroom environment.
Part D: Bacterial sampling of jet air dryers in public washrooms.
The surfaces and air flows of 16 jet air dryers were sampled in the male
and female washrooms of a main line London railway station on different days
and at different times.
The mean bacterial numbers per square centimetre ranged between 85 and 171
on the inner surfaces and slits of the dryers and between 4745 and 7537 at
the bottom of the dryers. Counts of a 10-second air sample ranged between 10
and 20 colonies per agar plate.
Some of the bacteria isolated from jet air dryers in public washrooms are
potential pathogens (disease-causing) and/or indicators of faecal or other
types of contamination, e.g. 94% of dryer samples showed the presence of human
skin bacteria with 71% showing Staphylococcus aureus and 43% faecal/gut bacteria.
The bacterial contamination of jet air dryers paralleled the dirt often apparent
on their inner surfaces and could allow transmission of bacteria if the user's
hands touch the inner surfaces, the air outlet slits or the bottom of the hand
drying chamber. It is possible to use these dryers without touching their surfaces
but that will not be the case with all users, as was observed with some subjects
in Part B of this study.
Discussion
The issue of warm air dryer hygiene is controversial. Some studies have shown
that warm air dryers are hygienically inferior to towels and may actually increase
the number of bacteria on the hands after use. Other studies have shown that
there is little significant difference between towels and warm air dryers.
Only a few studies have shown warm air dryers to be generally hygienically
superior to paper towels.
The discrepancies between the results of different studies can usually be
explained by differences in the experimental protocols used, such as abnormally
long drying times and by the testing of new dryers in laboratories rather than
old ones in public washrooms. The experimental protocol used in this study
attempted to reproduce the public's normal hand washing and drying practices
as closely as possible.
The results of this study suggest that the use of electric hand dryers should
be carefully considered in locations where hygiene is paramount, such
as hospitals, clinics, schools, nurseries, care homes, kitchens and other
food preparation areas. TW
Note: This article is only a summary of the study which has not been peer
reviewed but the methods are provided in sufficient detail in the full report
to enable their repetition and confirmation of the results by those who wish.
Keith Redway, Senior Academic, Department of Biomedical Sciences,School of
Biosciences, University of Westminster, London. website: www.westminster.ac.uk/~redwayk
