European Tissue Symposium(ETS)
A comparative study of threedifferent hand drying methods:
paper towel, warm air dryer, jet air dryer.

Prepared by
Keith Redway & Shameem Fawdar,
School of Biosciences,
University of Westminster,London.

November 2008

European Tissue Symposium (ETS), Brussels
A comparative study of different hand drying methods: paper
towel, warm air dryer, jet air dryer.
Prepared by
Keith Redway & Shameem Fawdar,
School of Biosciences,
University of Westminster,
115 New Cavendish Street,
London W1W 6UW
Tel: +44 (0)20 7911 5000 Ext. 3686
E-mail: redwayk@westminster.ac.uk
Web: www.westminster.ac.uk/~redwayk
November 2008

Part A: The drying efficiency of different hand drying methods.
Part B: Changes in the number of different types of bacteria on thehands before and after drying using paper towel, warm air dryer andjet air dryer (and other observations).
Part C: Potential contamination of other users and the washroom
environment caused by paper towel, warm air dryer and jet air dryer.
Part D: Bacterial sampling of jet air dryers in public washrooms.

Keith Redway
University of Westminster
ETS 2008 Report1Introduction
Following Semmelweis’s observations on the effect of hand washing on theincidence of puerperal fever in a maternity ward in the 19th Century, good handhygiene has been recognized as an important factor in controlling the spread of
infectious disease and, more recently, antibiotic-resistant bacteria in hospitals
and in the community. Meticillin-resistant Staphylococcus aureus (MRSA),
Clostridium difficile, Enterococcus faecalis and other agents causing hospitalacquired infections can be transmitted to patients by the hands of medical staff.
Similarly, food-poisoning organisms can be transmitted to food by dirty hands
and subsequently cause illness to those eating it.
There have been many studies on the benefits of hand washing and on theefficacy of different hand washing agents but relatively few on the contribution ofhand drying to hand hygiene. However, there is increasing awareness of its
importance in the overall hand hygiene debate.
Disregarding the types of textile towel where users dry their hands on the same
area of material as previous users and which have been condemned on hygiene
grounds for many years, the three main hand drying methods available in publicwashrooms have until fairly recently been: paper towels, continuous roller towels(where a fresh area of towel is available for each user) and warm air dryers. In
recent years manufacturers such as Dyson, Mitsubishi and Veltia haveintroduced new types of electric hand dryer (jet air dryers) where users insert
their hands into a slot whilst unheated air is emitted at high speed and removes
water from the hands by scraping. In this study, only a jet air dryer with thehighest claimed velocity of air movement was tested.
Blackmore (1989) showed that in normal use warm air dryers increase thenumber of bacteria that can be isolated from the fingerpads after drying. She
also recorded decreases in the bacterial numbers on fingerpads when paper
towels and continuous roller towels were used for hand drying. Two previous
studies carried out by the University of Westminster (Knights et al., 1993;
Redway et al., 1994) showed similar results in that on average warm air dryers
substantially increase the number of bacteria on the hands of users. Comparedto the number present on subjects' hands before washing and drying, the firststudy found the mean percentage increase in the number of bacteria on thefingerpads after using a warm air dryer was 504%. The second study foundmean percentage increases in different types of bacteria on the fingerpads ofsubjects after using a warm air dryer ranging from 169% to 438%. Conversely,
both studies showed that paper towels and continuous roller towels decrease themean number of all types of bacteria on the fingerpads of users.
Since these studies all other investigations by the University of Westminster
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, until this present
study, the performance of a jet air dryer had not been investigated by the
University nor compared to other hand drying methods.
Keith Redway
University of Westminster
ETS 2008 Report2The main aims of this study were to:
·
·

·
·
·

Measure the drying efficiency of paper towel, warm air dryer and jet air
dryer.
Calculate any changes in the numbers of different types of bacteria on thefingerpads and palms of 20 subjects (10 male, 10 female) before and after
washing and drying their hands using three different hand drying
methods: paper towel, warm air dryer, jet air dryer.
Assess any potential contamination of users and the washroom
environment caused by the use of paper towel, warm air dryer and jet air
dryer.
Sample any bacterial contamination of jet air dryers in public washrooms.
Make any other observations, measurements and recordings of relevanceto the comparison of the three different hand drying methods used in thisstudy.

Part A: The drying efficiency of different hand drying methods.
Introduction
It is generally accepted that the transmission of bacteria and othermicroorganisms is more likely to occur from wet skin than from dry skin (Gould
1994). This happens partly because of the ease of water transfer from one
surface to another and partly because microorganisms prefer a damp
environment and, therefore, may be in a better physiological state to colonize
touched surfaces. The amount of residual water left on the hands of users after
drying is directly related to the number of bacteria that are transferred by contact- the greater the amount, the more bacteria (Patrick et al., 1997). Knights et al.
(1993) showed that warm air dryers in normal use do not dry the hands as
efficiently as either paper or continuous roller towels. Warm air dryers in normal
use achieved only 55% dryness of the hands of men and 68% of the hands ofwomen. In contrast, both types of towel in normal use achieved 93% or more
dryness of the hands of both sexes. It is highly likely that the significantly poorer
hygiene performance of warm air dryers shown in this and other studies waspartly due to the low drying efficiency of dryers and the consequent greater
amount of water remaining on the fingerpads and palms of the hand after theiruse.
In this present study a similar method to that used by Knights et al. (1993) was
employed to compare the drying efficiency of 5 different types of paper towelwith a warm air dryer and a jet air dryer.

Keith Redway
University of Westminster
ETS 2008 Report3Methods and materials
Hand drying methods used:
i)
ii)
iii)
iv)
v)
vi)
vii)

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™, model LE48 (World Dryer
Corporation).
Jet air dryer (JAD): Airblade™, model AB01 (Dyson).

1.

Sets of 5 paper towels (PT 1) were placed in sterile plastic bags andweighed prior to use.

2.

Two volunteers were asked to dip their hands up to the wrists in warm water(temperature = 40oC) for 10 seconds, shake them thrice, and then dry themfor 10 seconds using one of the 7 hand drying methods.

3.

All the water remaining on the surface of the hands was then carefully
removed by the investigator with one of the sets of 5 pre-weighed paper
towels using a standardized protocol for 40 seconds.

4.

The damp towels were returned to their plastic bag, re-weighed and theamount of water removed from the hands calculated.

5.

The operation was repeated using increasing drying times at 10-second
intervals: 20, 30, 40, 50 and 60 seconds.

6.

To estimate the total amount of water on undried hands immediately after
wetting and with no drying (time = 0 seconds), wet hands were driedthoroughly on 5 pre-weighed paper towel and the weight gain recorded. It
was then possible to calculate the percentage dryness of the hands as themean percentage of the total water load removed after the use of eachdrying method at each time as follows:
Percentage (%) dryness =weight of water on undried hands - weight of water on dried hands
weight of water on undried hands

7.

x 100

The order of drying times and the drying methods were randomised tominimize any possible effect of external factors such as variations in room
temperature, relative humidity or human behaviour.

Keith Redway
University of Westminster
ETS 2008 Report4Results
Table 1Showing the mean percentage dryness (%) of the hands of subjects
after using five different types of paper towel (PT), a warm air dryer(WAD), and a jet air dryer (JAD) for different drying times.
DRYING
TIME(seconds)
0
10
20
30
40
50
60

PT 1
00.0
90.6
95.5
97.3
99.0
98.3
97.9

PT 2

HAND DRYING METHOD
PT 3
PT 4
PT 5

WAD

JAD

00.0
93.0
96.6
97.9
97.5
97.5
97.9

00.0
93.0
96.6
97.9
97.5
97.5
97.9

00.0
33.7
55.4
70.9
84.1
92.1
96.8

00.0
89.3
95.5
97.0
97.3
97.7
98.4

00.0
87.3
96.0
97.4
97.7
98.0
98.2

00.0
91.0
94.9
96.0
96.9
97.4
97.4

(N = 2)
Figure 1
Graph of the mean percentage dryness of the hands of subjects
against drying time using five different types of paper towel (PT),a warm air dryer (WAD), and a jet air dryer JAD).
100

MEAN PERCENTAGE DRYNESS (%)

90
80
70

PT 1

60

PT 2
PT 3

50

PT 4
PT 5

40

WAD

30

JAD

20
10
0
0

10

20

30

40

DRYING TIME (seconds)(N = 2)
Keith Redway
University of Westminster
ETS 2008 Report

5

50

60

Table 2
Showing the mean times to achieve a minimum of 90% drynessof the hands using five different types of paper towel (PT),a warm air dryer (WAD), and a jet air dryer (JAD).
HAND DRYINGMETHODMEAN TIME TO ACHIEVEA MINIMUM OF 90%
DRYNESS (seconds)

PT 1
PT 2
PT 3
PT 4
PT 5
WAD
JAD

10
10
10
11
10
47
10

(N = 2)Conclusions and discussion
The results shown in Tables 1 and 2 show that the 5 types of paper towel and thejet air dryer were equally efficient at drying the hands of users, all of themachieving 90% dryness in approximately 10 seconds. Any slight differences
between these 6 hand drying methods were not considered significant. However,
the results show that the warm air dryer was considerably less efficient (i.e. slower)
than the 5 types of paper towel and also the jet air dryer and took over 4 times aslong to achieve 90% dryness of the hands. The results in Table 1 are represented
graphically in Figure 1.
Knights et al. (1993) also showed warm air dryers were much less efficient than
towels (paper or textile) at drying the hands. Their results were similar to those ofthis study with the difference that the jet air dryer was unavailable and not tested
at that time. They also found that it is impossible to achieve 100% dryness of thehands by any hand drying method within a 1-minute drying period.
Therefore, the instructions on a jet air dryer to use it for 10 seconds seemappropriate. Similarly, their claim that it is “the fastest hand dryer” seem to be borneout by the results of this part of the study which suggest that it represents a
considerable improvement over warm air dryers in terms of drying efficiency, i.e.
speed of drying.
However, although damp hands encourage the transmission and survival ofbacteria on the hands (Gould, 1994; Patrick et al., 1997), there are other factors
which can affect the hygiene performance of a hand drying method. These factors
include: the degree of frictional removal of dirt, grease, bacteria and skin squames
from the hands; the absorbance and softness of material used to dry the hands;
the emission of bacteria in the air flows of electric hand dryers; the contaminationof the surfaces of hand drying devices. The real test of hygiene performance is not
the percentage dryness of the hands alone but the number of bacteria removed
from, or remaining on, the hands of users after use. This, and other indicators ofhygiene performance, including the manufacturer’s claim that the tested JAD is“the most hygienic hand dryer”, were investigated in other parts of this study.
Keith Redway
University of Westminster
ETS 2008 Report

6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(and other observations).
Introduction
Previous studies (Blackmore,1989; Knights et al., 1993; Redway et al., 1994)
have used the 'contact plate' method to assess changes in the number ofbacteria present on the hands before and after washing and drying. The method
involves pressing the fingerpads onto nutritive agar plates, growing any
transferred bacteria at 37oC overnight and then counting the number of colonyforming units (cfu's) present. This method has been shown to be relatively quick
and sufficiently accurate for this type of study (Sanderson & Weissler, 1992). In
addition to contact plates, this present study also used swab sampling of an areaof the palm of the hand before and after the use of paper towel, warm air dryer or
jet air dryer.
The hand drying times used in this part of the study for the paper towels (10seconds) and the warm air dryer (20 seconds) were based on observations(Redway et al., 1997) in public washrooms of the average times used bymembers of the public. However, because it is relatively new, no suchobservations were available for the jet air dryer and the manufacturer’s
recommended time of 10 seconds was used.
Methods and materials
1.

20 subjects (10 male and 10 female) were recruited covering an age
range 18 to 60 years.

2.

Subjects were asked to visit a public washroom in a normal fashion andreturn to the laboratory without washing their hands. No instructions were
given by the investigator as to how they should use the washroom or what
they should do in it.

3.

Three different agar growth media were used to sample the dominant
hand of subjects before washing and drying (BD) and after washing anddrying (AD). The media used and in this order were:
Nutrient Agar [NA] (Oxoid)
NA is a non-selective, general purpose growth medium which would be
expected to grow most non-fastidious types of bacteria, including skin andgut bacteria.
Cystine-Lactose-Electrolyte-Deficient Medium [CLED] (Oxoid)
CLED medium supports the growth of potential pathogens from the gut
giving good colonial differentiation and clear diagnostic characteristics forEscherichia coli, Salmonella species, Enterococcus species, etc.
Escherichia coli produces large yellow colonies due to fermentation of theKeith Redway
University of Westminster
ETS 2008 Report7

lactose, Salmonella species produces flat blue colonies and Enterococcus
species produce small yellow colonies. Other types of bacteria produce
different colonial morphologies.
Mannitol-Salt Agar [MSA] (Oxoid)
MSA is a selective growth medium used for the isolation of staphylococci;
most other bacteria are inhibited by the high salt content. Presumptive
pathogenic, coagulase-positive Staphylococcus aureus colonies are
surrounded by yellow zones (due to acid production from the fermentationof mannitol) whilst non-coagulase-positive staphylococci produce colonies
with reddish purple zones.
4.

5.

Areas of hand sampled were: fingerpads (by direct contact with the agar
plate surface) and the palm (by swabbing and inoculation of agar plates).
a)

For sampling fingerpads, subjects were asked to firmly press thefingerpads of their ring, middle and index fingers onto the surfaceof 3 agar plates in turn (NA, CLED, MSA). A sterile swab
moistened with ¼ strength Ringers solution was then used to swab
the entire surface of each agar plate so as to spread and disperse
any potential colonies and enable them to be counted more easily.

b)

For sampling palms, a sterile metal former with a circular hole in it(diameter 4.2 cm) was placed on the palm of subjects and a sterileswab moistened with sterile ¼ strength Ringers solution was thenused to swab half the area. The cotton bud of the swab was thenaseptically removed to 3 ml of ¼ strength Ringers solution andvortexed for 10 seconds. 0.1 ml of this suspension was thendispensed onto the surface of 3 agar plates (NA, CLED, MSA) andspread using a sterile glass spreader.

Subjects were then asked to wash and rinse their hands for a total of 10
seconds using one squirt (0.83 ml) of a commonly available liquid soap
(Johnson Diversy “Soft Care” hand washing cream) from a dispenser
which was operated by the researcher and running tap water. Subjects
were then requested to dry them using one of the following 4 hand drying
methods and for the times indicated:i)Paper towel 1 (PT 1): 2-ply 100% recycled. Art. 217010 (Wepa).
10 seconds

ii)

Paper towel 3 (PT 3): 2-ply through-air dried (TAD). 50% virgin 50% recycled. Art. 6769 (Kimberly-Clark). 10 seconds.

iii)

Warm air dryer (WAD): Electric-Aire™, model LE48 (World
Dryer Corporation). 20 seconds.

iv)

Jet air dryer (JAD): Airblade™, model AB01 (Dyson).
10 seconds

Keith Redway
University of Westminster
ETS 2008 Report8Subjects were not given any instructions as to how to dry their hands andwere allowed to take as many paper towels as they wished (mean = 2.5)
but only within 10 seconds. Similarly, subjects were not instructed as to
how they should use the WAD or JAD devices but were stopped after 20
and 10 seconds respectively. However, subjects were given ademonstration of the JAD in case they had not encountered this type of
hand dryer previously.
6.

The sampling technique as in Stage 4 was repeated after washing anddrying (AD), viz. fingerpad and palm inoculation of the three different agar
growth media in turn. For palm sampling, the half of the circular area notswabbed previously for the BD sample was used.

7.

All agar plates were incubated at 37oC and examined after 1 and 2 days
for bacterial growth. The number of colonies on each plate was recorded
and, where appropriate, differentiation made between different types ofcolony, e.g. yellow zones around colonies on MSA indicating mannitol
fermentation and presumptive identification as Staphylococcus aureus.
Counts on plates which showed too many colonies to count were scoredas 200, which is considered the upper limit for accurate counting.

8.

All 20 subjects were re-tested exactly as in Stages 2 - 6 above but on adifferent days when they were required to a different hand drying method
each day.

9.

The order that subjects were required to use the four different hand drying
methods was randomised between subjects to minimize any external
effects such as variation in temperature or humidity on different days.

10.

Results were recorded, tabulated and statistically analysed. The
percentage (%) changes in bacterial numbers (as colony-forming units) on
the hands were calculated as follows:
number after drying – number before drying
number before drying

x 100

The paired t-test was used to establish if there were any significant
differences between the mean number of different types of bacteria on thehands of subjects before washing and drying their hands (BD) and after
washing and drying their hands (AD) using the four different hand drying
methods. The analysis was applied to all bacterial types that grow onnutrient agar, plus potential skin pathogens on MSA and gut bacteria on
CLED. The 4 different drying methods were also statistically compared byt-tests on the AD counts of subjects after using them.
11.

Controls: Plates of all 3 agar growth media were used at regular intervals
to test samples of the paper towels, the air flow of the warm air dryer, theair flow of the jet air dryer, the liquid soap and the tap water for thepresence of bacteria. For the paper towels bacterial contamination wastested by using the end of a sterile glass beaker to press a set area

Keith Redway
University of Westminster
ETS 2008 Report9(15.90 cm2) of towel onto an agar plate. Similarly, the liquid soap and thetap water were tested for the presence of bacteria by plating out 0.1 ml
aliquots onto agar plates and spreading with a sterile glass spreader. The
warm air dryer’s airflow was tested by holding agar plates beneath it at a
distance of 10 cm for 20 seconds. The jet air dryer’s airflow was tested byholding agar plates in the air flow emitted from the sides of the device for
10 seconds. Control plates were incubated at 37oC and examined after 1
and 2 days for the presence of bacterial colonies.
12.

Measurements were taken using an environmental meter (CEM DT-8820)
at regular intervals of the laboratory ambient temperature, tap watertemperature, air flow temperature from the two dryers, the relative
humidity in the laboratory and the noise levels when the dryers wererunning. The power consumption of the 2 electric dryers was also
recorded. Some of the same measurements were also made in the publicwashrooms used in Part D of this study.

Keith Redway
University of Westminster
ETS 2008 Report10Results
Table 1
Mean counts and percentage changes in bacterial numbers(CFUs) on fingerpads before and after washing and drying
hands using different hand drying methods.
HAND
DRYINGMETHODGROWTHMEDIUM

COLONY
TYPE

PT 1
PT 3
WAD
JAD
PT 1
PT 3
WAD
JAD
PT 1
PT 3
WAD
JAD
PT 1
PT 3
WAD
JAD
PT 1
PT 3
WAD
JAD
PT 1
PT 3
WAD
JAD

NA
NA
NA
NA
CLED
CLED
CLED
CLED
MSA
MSA
MSA
MSA
MSA
MSA
MSA
MSA
MSA
MSA
MSA
MSA
TOTAL
TOTAL
TOTAL
TOTAL

ALL
ALL
ALL
ALL
ALL
ALL
ALL
ALL
MAN +
MAN +
MAN +
MAN +
MAN MAN MAN MAN ALL
ALL
ALL
ALL
ALL
ALL
ALL
ALL

MEAN
BEFORE
DRY
COUNT
(BD)
73.9
64.8
38.2
63.2
53.4
39.0
40.4
64.6
25.4
26.0
11.4
14.9
42.7
40.4
33.1
40.4
68.1
66.4
44.5
55.2
195.4
170.1
123.0
183.0

MEAN
AFTER
DRY
COUNT
(AD)
41.0
15.0
109.3
96.6
24.9
11.5
123.0
82.7
10.6
2.2
58.6
43.6
18.6
12.8
70.8
37.0
29.1
15.0
129.4
80.5
95.0
41.5
361.6
259.8

MEAN
CHANGE(%)

T-TEST
(p)

-44.6
-76.9
+186.4
+52.8
-53.4
-70.5
+204.3
+28.0
-58.5
-91.5
+414.0+193.3-56.6-68.3+114.1
-8.4
-57.3
-77.4
+191.0
+45.8
-51.4
-75.6
+193.9+42.0

0.09800.00200.00020.03100.17000.01700.00020.28900.27600.07000.01000.01200.03600.03700.02000.82000.03200.02400.00010.07000.06600.00500.0001
0.0650(N = 20)
Key to Tables 1 – 4 and Figures 1 - 4:
PT = paper towel (1 or 3); WAD = warm air dryer; JAD = jet air dryer.
CFU = colony-forming unit; NA = nutrient agar; CLED = cystine-lactoseelectrolyte-deficient medium; MSA = mannitol salt agar; MAN + = acid frommannitol positive; MAN - = acid from mannitol negative; ALL = total number of
CFUs (all types of colony); TOTAL = total number of colonies on all three media(NA, CLED, MSA).
¯ = decrease in bacterial count after washing and drying;- = increase in bacterial count after washing and drying.
Change statistically significant at the limit of probability as follows:* p < 0.1; ** p < 0.05; *** p < 0.01; **** p < 0.001.
The result shown in Table 1 are summarized in Table 2 and represented
graphically in Figures 1 – 4.

Keith Redway
University of Westminster
ETS 2008 Report11Table 2
Summary of mean percentage changes in bacterial
numbers on fingerpads before and after washing and drying
hands using different hand drying methods.
GROWTHMEDIUM

COLONY
TYPE

NA
CLED
MSA
MSA
MSA
TOTAL

ALL
ALL
MAN +
MAN ALLALL

PAPER
TOWEL 1
(PT 1)-44.6 ¯ *-53.4 ¯
-58.5 ¯
-56.6 ¯ **-57.3 ¯ **-51.4 ¯ *PAPERTOWEL 3
(PT 3)-76.9 ¯ ***-70.5 ¯ **-91.5 ¯ *-68.3 ¯ **-77.4 ¯ **-75.6 ¯ ***WARM AIR
DRYER
(WAD)
+186.4 - ****
+204.3 - ****
+414.0 - ***+114.1 - **+191.0 - ****
+193.9 - ****JET AIR
DRYER(JAD)+52.8 - **+28.0 +193.3 - **-8.4 ¯+45.8 - *+42.0 - *Table 3
Mean counts and percentage changes in bacterial numbers(CFUs per cm2) on palms before and after washing and drying
hands using different hand drying methods.
HAND
DRYINGMETHODGROWTHMEDIUM

COLONY
TYPE

PT 1
PT 3
WAD
JAD
PT 1
PT 3
WAD
JAD
PT 1
PT 3
WAD
JAD
PT 1
PT 3
WAD
JAD
PT 1
PT 3
WAD
JAD
PT 1
PT 3
WAD
JAD

NA
NA
NA
NA
CLED
CLED
CLED
CLED
MSA
MSA
MSA
MSA
MSA
MSA
MSA
MSA
MSA
MSA
MSA
MSA
TOTAL
TOTAL
TOTAL
TOTAL

ALL
ALL
ALL
ALL
ALL
ALL
ALL
ALL
MAN +
MAN +
MAN +
MAN +
MAN MAN MAN MAN ALL
ALL
ALL
ALL
ALL
ALL
ALL
ALL

MEAN
BEFORE
DRY
COUNT
(BD)
129.7
105.0
79.0
155.0
76.6
86.8
77.9
126.2
13.4
16.2
14.3
82.1
69.9
70.4
43.7
47.4
83.4
86.6
58.0
129.5
289.7
278.4
215.0
410.7

MEAN
AFTER
DRY
COUNT
(AD)
50.0
23.2
261.1
169.1
45.5
24.7
267.2
143.5
6.9
6.3
82.7
72.7
47.0
10.4
151.1
86.4
53.9
16.7
233.8
159.1
149.4
64.5
762.1
471.8

MEAN
CHANGE(%)

T-TEST
(p)

-61.4
-77.9
+230.4
+9.1
-40.7
-71.6
+242.8
+13.7
-48.4
-61.3
+478.8
-11.3
-32.8
-85.2
+245.5
+82.2
-35.3
-80.8
+303.0
+22.9
-48.4
-76.8
+254.5
+14.9

0.046
0.063
0.005
0.773
0.352
0.132
0.021
0.698
0.193
0.065
0.126
0.830
0.146
0.170
0.027
0.052
0.125
0.110
0.006
0.545
0.083
0.093
0.004
0.664

(N = 20)
The result shown in Table 3 are summarized in Table 4 and represented
graphically in Figures 1 – 4.
Keith Redway
University of Westminster
ETS 2008 Report12Table 4
Summary of mean percentage changes in bacterial
numbers on palms before and after washing and drying
hands using different hand drying methods.
GROWTHMEDIUM

COLONY
TYPE

NA
CLED
MSA
MSA
MSA
TOTAL

ALL
ALL
MAN +
MAN ALLALL

PAPER
TOWEL 1
(PT 1)-61.4 ¯ **-40.7 ¯-48.4 ¯
-32.8 ¯
-35.3 ¯-48.4 ¯ *PAPERTOWEL 3
(PT 3)-77.9 ¯ *-71.6 ¯
-61.3 ¯ *-85.2 ¯-80.8 ¯
-76.8 ¯ *WARM AIR
DRYER
(WAD)
+230.4 - ***+242.8 - **+478.8 +245.5 - **+303.0 - ***+254.5 - ***JET AIR
DRYER(JAD)+9.1 +13.7 -11.3 ¯+82.2 - *+22.9 +14.9 -Table 5T-test results (p values) comparing the bacterial after dry (AD) counts on
subjects’ hands after using different hand drying methods.
DRYINGMETHOD
PT 3
WAD
JAD

PT 1
0.073 *
0.00003 ****
0.0005 ****FINGERPADS
PT 3
NA0.000001 ****
0.00001 ****

WAD
NA
NA
0.037 **

PT 1
0.090 *
0.005 ***
0.012 **

PALMS
PT 3
NA
0.003 ***
0.006 ***Key to Table 5:
PT = paper towel (1 or 3); WAD = warm air dryer; JAD = jet air dryer;
NA = not applicable (redundant comparison).
* p < 0.1; ** p < 0.05; *** p < 0.01; **** p < 0.001.

Keith Redway
University of Westminster
ETS 2008 Report

13

WAD
NA
NA
0.185

Figure 1
GRAPH OF MEAN PERCENTAGE CHANGE IN NUMBERS OFBACTERIAL COLONY-FORMING UNITS (CFUs) ON NUTRIENT AGAR
AFTER HAND DRYING USING 4 DIFFERENT METHODS350

MEAN % CHANGE IN CFUs

300
230

250
186

200
150
100

FINGERPADS

53

PALMS

50

9

0
-50

-45

-100

-61

-78

-77

-150
PT 1

PT 3

WAD

JAD

HAND DRYING METHOD(PT = PAPER TOWEL; WAD = WARM AIR DRYER; JAD = JET AIR DRYER)
Standard error bars shownN = 20Figure 2

350

GRAPH OF MEAN PERCENTAGE CHANGE IN NUMBERS OFBACTERIAL COLONY-FORMING UNITS (CFUs) ON CLED AGAR
AFTER HAND DRYING USING 4 DIFFERENT METHODSMEAN % CHANGE IN CFUs

300
243

250

204

200
150
100

FINGERPADS
28

50

14

0
-50
-53

-100

-41
-70

-72

-150
PT 1

PT 3

WAD

JAD

HAND DRYING METHOD(PT = PAPER TOWEL; WAD = WARM AIR DRYER; JAD = JET AIR DRYER)
Standard error bars shownN = 20Keith Redway
University of Westminster
ETS 2008 Report

14

PALMS

Figure 3

GRAPH OF MEAN PERCENTAGE CHANGE IN NUMBERS OFBACTERIAL COLONY-FORMING UNITS (CFUs) ON MSA MEDIUMAFTER HAND DRYING USING 4 DIFFERENT METHODS
450

MEAN % CHANGE IN CFUs

400
350

303

300
250

191

200
150

FINGERPADS

100

46

50

PALMS
23

0
-50
-57

-100

-35
-77 -81

-150
PT 1

PT 3

WAD

JAD

HAND DRYING METHOD(PT = PAPER TOWEL; WAD = WARM AIR DRYER; JAD = JET AIR DRYER)
Standard error bars shownN = 20Figure 4
GRAPH OF MEAN PERCENTAGE CHANGE IN NUMBERS OF BACTERIAL
COLONY-FORMING UNITS (CFUs) ON ALL 3 GROWTH MEDIA(NA, CLED, MSA) AFTER HAND DRYING USING 4 DIFFERENT METHODS400

MEAN % CHANGE IN CFUs

350
300

254

250
194

200
150

FINGERPADS

100

PALMS

42

50

15

0
-50
-51 -48

-100

-76

-77

-150
PT 1

PT 3

WAD

JAD

HAND DRYING METHOD(PT = PAPER TOWEL; WAD = WARM AIR DRYER; JAD = JET AIR DRYER)
Standard error bars shownN = 20Keith Redway
University of Westminster
ETS 2008 Report15

Control results
Table 6Means of bacterial colony counts for controls on
different growth media after incubation at 37oC for 2 days.
CONTROL ITEM
2
PT 1 (per cm )2
PT 3 (per cm )WAD (20 sec.)
JAD (10 sec.)
Sterile Ringer’s solution (per ml)
Tap water (per ml)
Liquid soap (per ml)

NA
0.13
0.00
1.40
1.00
0.00
30.00
0.00

CLED
0.06
0.00
0.00
0.00
0.00
30.00
0.00

MSA
0.06
0.00
0.40
0.20
0.00
40.00
0.00

(N = 5)
Key to Table 6:
PT = paper towel (1 or 3); WAD = warm air dryer; JAD = jet air dryer;
NA = nutrient agar; CLED = cystine-lactose-electrolyte-deficient medium;
MSA = mannitol salt agar.
Measurements
Table 7
Measurements in the laboratory and a public washroom.
MEASUREMENToLaboratory ambient temperature ( C)oWashroom ambient temperature ( C)oLaboratory tap water temperature ( C)o
WAD air flow temperature ( C) [20second run]o
JAD air flow temperature ( C) [10second run]Laboratory relative humidity (%)
Washroom relative humidity (%)
Background laboratory noise level (dB)
Noise level (dB) with laboratory JAD on
at 0.5 m distance
Noise level (dB) with laboratory JAD on
at 1.0 m distance
Noise level (dB) with laboratory JAD on
at 2.0 m distance
Background washroom noise level (dB)
Noise level (dB) with one washroom
JAD on at 2.0 m distance
Noise level (dB) with one washroom
JAD on at 10.0 m distance
Noise level (dB) with two washroom
JADs on at 2.0 m distancePower consumption of WAD (W)Power consumption of JAD (W)MINIMUM

MAXIMUM

22.9
21.8
21.3
50.5

26.7
26.9
24.9
59.1

MEAN
(N = 10)
24.3
23.2
22.7
55.6

36.6

40.7

39.2

33.6
36.6
51.2
94.7

51.6
49.2
52.7
93.7

47.0
44.6
51.8
94.1

85.1

89.3

87.4

85.4

87.6

86.3

55.5
ND

58.7
ND

57.8
83.6

ND

ND

77.9

ND

ND

92.0

ND
1
(standby)

1400-1600
1600

N/A
N/A

Key to Table 7:
WAD = warm air dryer; JAD = jet air dryer; dB = decibel: ND = no data;
W = watts; N/A = not applicable.
Keith Redway
University of Westminster
ETS 2008 Report

16Conclusions and discussion
The experimental protocol used in this study attempted to reproduce the public’s
usual hand washing and drying practices as closely as possible. The times used
for washing and drying the hands were based on those shown by a previous
study (Knights et al., 1993) to be the averages for men and women using paper
towels (10 seconds) and warm air dryers (20 seconds) in real public washrooms,
i.e. under 'normal', non-laboratory conditions. The average time that men use
warm air dryers was found to be 20 seconds whilst for women it was 25
seconds. By comparison, Patrick et al. (1997) found that the average time formen using warm air dryers was 17.0 seconds and 13.3 seconds for women.
However, the study by Knights et al. (1997) involving 292 subjects showed thatmen used warm air dryers for 15.4 seconds on average and women for 17.7
seconds. A survey on the “Country Doctor” website (2006) gives the averagetime for men using a warm air dryer as 20 seconds and for women as 16
seconds. Therefore, the drying time of 20 seconds for both sexes used in thepresent study is likely to be longer than the actual average time that the public
uses warm air dryers and would favour them compared to towels so that any
poor results from dryers cannot be explained by the drying time used in thisstudy being too low. The drying time of 10 seconds used for the jet air dryer was
not based on observations of the public but on the manufacturer’s
recommendation as given on the dryer itself.
Using the three different growth media it was hoped to count most of the bacteriapresent on the subjects' hands before and after washing and drying. In addition,
it was also hoped that information would be obtained about the incidence of thefollowing types of bacteria in particular:
·

·

·

·

Escherichia coli, a bacterium found in the human gut and a good indicatorof faecal contamination. Some strains are pathogenic and cause disease,sometimes severe, e.g. O157. This bacterium produces large yellow
colonies on Cystine-Lactose-Electrolyte-Deficient Medium (CLED).
Other coliforms also grow on CLED. Distinction between normal
commensals and pathogens would require further tests which were notdone in this part of the study but any presence of coliforms is indicative of
faecal contamination and poor hygiene.
Acid from mannitol negative staphylococci and micrococci. The former
can sometimes be pathogenic and cause disease. These bacteria growon mannitol salt agar and are normal commensal inhabitants of humanskin and nostrils.
Acid from mannitol positive staphylococci. These were differentiated on
mannitol salt agar by the production of yellow zones around colonies due
to acid production and presumptively identified as Staphylococcus aureus.
This organism can be found on the skin and in the nostrils of healthypeople but it is a common potential pathogen causing a toxigenic foodpoisoning, abscesses, boils and other problems. However, pathogenicity
and antibiotic resistance vary greatly between different strains, which
include meticillin-(methicillin-) resistant Staphylococcus aureus (MRSA), a
common hospital-acquired infection. The presence of any type of
Staphylococcus aureus on the hands of a worker in the food industry or

Keith Redway
University of Westminster
ETS 2008 Report17

medical field should be taken seriously as should any increase in its
numbers caused by particular hand drying methods.
The issue of warm air dryer hygiene is controversial. Some studies (Blackmore,
1989; Blackmore & Prisk, 1984; Gould, 1994; Knights et al., 1993; Knights et al.,
1997; Ngeow et al., 1989; Redway et al., 1994; Redway et al., 1995) haveshown that warm air dryers are hygienically inferior to towels and may actually
increase the number of bacteria on the hands after use. Other studies (Davis etal., 1969; Gustafson et al., 2000; Matthew & Newsom, 1987; Meers & Leong,
1989; Patrick et al., 1997; Taylor et al., 2000) have shown that there is little
significant difference between the three hand drying methods. Only a few studies(Ansari et al.,1991) have shown warm air dryers to be generally hygienicallysuperior to paper towels. Yamamoto et al. (2005) found warm air dryers reduced
bacterial numbers if subjects held their hands stationary in the airflow rather thanrubbing them which caused an increase but this method is likely to take longer todry the hands. They also found that paper towels reduced the bacterial numberson the fingertips more than warm air dryers; a result which agrees with thepresent study. However, their observation that paper towels did not reducebacterial numbers on the palms is not in agreement with the results of otherstudies or the present one but may be explained by a different sampling method.
The large discrepancy between the results of different studies can invariably be
explained by differences in the experimental protocols used, such as abnormallylong drying times of up to 1 minute (when the average time used by the public isless than 20 seconds) and by the use of new dryers in laboratories, rather thanregularly-used, and often contaminated, dryers in public washrooms. Used
dryers are commonly contaminated and emit bacteria in their air flow (Redway etal., 1994). It should be noted that a new warm air dryer and new jet air dryer
were used in a laboratory in this part of the study and that regular tests showedno significant numbers of bacteria in their air flows. Therefore, any increases inbacterial numbers after use of dryers in this part of the study must have been
due to factors other than bacterial contamination of the dryers themselves.
It is generally accepted that the transmission of bacteria and othermicroorganisms is more likely to occur from wet skin than from dry skin (Gould
1994). This happens partly because of the ease of water transfer from one
surface to another and partly because microorganisms prefer a damp
environment and, therefore, may be in a better physiological state to colonize
touched surfaces. The amount of residual water left on the hands of users after
drying is directly related to the number of bacteria that are transferred by contact,
the greater the amount, the more bacteria (Patrick et al., 1997). Knights et al.
(1993) showed that warm air dryers in normal use do not dry the hands as
thoroughly as either type of towel. Warm air dryers in normal use achieved only
55% dryness of the hands of men and 68% of the hands of women. In contrast,
both types of towel in normal use achieved 93% or more dryness of the hands ofboth sexes. It is highly likely that the significantly poorer hygiene performance ofwarm air dryers compared to towels shown in this study is mainly due to the low
drying efficiency of dryers and the consequent greater amount of waterremaining on the fingerpads and palms of the hand after their use. However,there must be other factors operating on the bacterial load on the hands of users
Keith Redway
University of Westminster
ETS 2008 Report18

because although the jet air dryer showed a similar drying efficiency to paper
towels (see Part A of this study), its hygiene performance, although better than
the warm air dryer, was significantly worse than the two types of paper towel
tested in this study. It is possible that paper towels work better because theyfrictionally remove dirt, grease, bacteria and skin squames from the hands
whereas the jet air dryer, like the warm air dryer, does not. The superior
performance of paper towels over the two types of electric dryer in reducing thenumbers of bacteria was shown with both the fingerpads and the palms ofsubjects.
The room temperature, the tap water temperature and the relative humidity
varied in the laboratory from day to day (see Table 7) but any effect that thesefactors may have had on the results were minimized by randomising the order ofhand drying method tested and subjects used.
In this study both types of paper towel (PT 1 and PT 3) tested reduced the mean
numbers of all types of bacteria tested on the fingerpads and the palms ofsubjects. The percentage mean reductions ranged from –44.6% to –91.5% for
fingerpads and from –32.8 to –85.2% for palms. Reductions were shown with all
types of bacteria on all 3 growth media. The majority of these reductions weresignificant suggesting that they were not due to chance alone but to the action of
the towels.
The warm air dryer increased the mean numbers of all types of bacteria testedon the fingerpads and the palms of subjects. The percentage mean increases
ranged from +114.1% to +414% for fingerpads and from +230.4% to +478.8%
for palms. Increases were shown with all types of bacteria on all 3 growth media.
The majority of these increases were significant, some highly so, suggesting thatthey were not due to chance alone but to the action of the warm air dryer.
The jet air dryer increased the mean numbers of most types of bacteria tested on
the fingerpads and the palms of subjects. The percentage mean increases
ranged from +28.0% to +193.3% for fingerpads and from +9.1% to +82.2% forpalms. Increases were shown with most types of bacteria on all 3 growth media,
the only exceptions being reductions on the fingerpads of mannitol-negative
bacteria and reductions on the palms of mannitol-positive bacteria. However,neither of these decreases was significant, whereas some of the increases were.
Comparisons of the after dry bacterial counts on the fingerpads of subjects using
the paper towels with the warm air dryer and with the jet air dryer showed that
there were highly significant differences between the towels and both types ofdryer, i.e. the superior performance of the towels in reducing bacterial numbers
was confirmed. Both types of dryer caused mean increases in the bacterial
counts on the fingerpads of subjects but the jet air dryer performed better than
the warm air dryer in that the increases were not as great. Differences between
the two types of dryer were less significant than for the towels compared toeither dryer.
Results for the palms were similar. Comparisons of the after dry bacterial counts
on the palms of subjects using the paper towels with the warm air dryer and with
Keith Redway
University of Westminster
ETS 2008 Report19the jet air dryer showed that there were significant differences (although not as
great as for the fingerpads) between the towels and both types of dryer. Again,
the superior performance of the towels in reducing bacterial numbers was
confirmed. As for the fingerpads, the jet air dryer performed better than the warm
air dryer in not increasing mean bacterial count on the palms as much but thisdifference was not significant.
Therefore, the manufacturer’s claim that the tested JAD is the “most hygienic
hand dryer” is confirmed, especially for fingerpads and assuming that the term
“hand dryer” refers to electric devices only because its performance in terms ofthe numbers of all types of bacteria remaining on the hands of users comparedto paper towels was significantly worse.
Other observations
The temperature of the air flow from the warm air dryer was higher than that fromthe jet air dryer which, unusually, uses unheated air (see Table 7). The latter’s
air flow temperature was higher than that of the room because when air is forced
through any orifice (in this case the slits of the dryer) its temperature rises.
Neither type of electric dryer would produce temperatures high enough, and forlong enough, to kill most bacteria.
The jet air dryer was particularly noisy compared to all other methods of handdrying tested, including a warm air dryer which was obviously noisier than thepaper towels. Several subjects commented on the noise level of the jet air dryer
and the decibel levels recorded in the laboratory and a public washroom confirm
it (see Table 7). The mean decibel level at 0.5 m is 94.1dB and in excess of thatof a passing heavy lorry 3 m away. The mean decibel levels at 1.0 and 2.0 m arein excess of a typical busy street at 87.4 and 86.3 dB respectively. In washrooms
with more than one dryer the noise level would be even higher and could
constitute a risk to those exposed to it for long periods, such as washroom
attendants. The two public washrooms tested for bacterial contamination in Part
D had 8 jet air dryers each. Table 7 shows the decibel level recorded when 2dryers were being used at the same time at a distance of 2 m as 92dB, thesecond highest recording taken and only being exceed by the noise level at 0.5
m.
However, assessment of the risk of noise is complex, involving average andpeak levels, and the decibel scale is sometimes misunderstood, e.g. an increaseof 3 dB represents an actual doubling of the noise level. The Health and Safety
Executive (2008) discuss using hearing protection devices for someenvironments with noise levels in excess of 85 decibels but length of exposureand the frequency of the sound must be considered in addition to the decibellevel alone.
Any further discussion of noise is beyond the scope of this present study as is
the claim that the jet air dryer consumes less power than a warm air dryer. In thisstudy the two types of dryer had similar power consumptions (see Table 7) but
any cost savings would presumably be due to the fact that the tested jet air dryer
dries the hands more quickly and is, therefore, not used for so long.
Keith Redway
University of Westminster
ETS 2008 Report

20Part C: Potential contamination of other users and the washroom
environment caused by paper towel, warm air dryer and jet air dryer.
Introduction
In this part of the study artificial contamination of the hands of subjects was usedin an attempt to demonstrate and compare the potential contamination of otherusers and the washroom environment caused by the different hand drying
methods tested: paper towel, warm air dryer and jet air dryer.
A previous study at the University (Redway et al., 1995) compared the antibacterial performance of textile towels and warm air dryers after artificial
contamination and washing of the hands. It showed that towels were superior towarm air dryers in producing significantly greater reductions in bacterial numberson the hands of subjects. This previous study used the bacterium Micrococcus
luteus as the artificial contaminating agent. In a more recent study (2008,
unpublished) the contaminating agent was changed to the yeast Saccharomyces
cerevisiae and similar results obtained. The yeast proved a good model andbetter than the bacterium in being safer, more acceptable to volunteers, easier todetect on appropriate growth media and easier to distinguish from normal hand
flora. Therefore, in this present study yeast was used to artificially contaminate
the hands of subjects prior to washing their hands and drying them using
different methods. Any potential microbial contamination originating from thehands of users that could contaminate other users or the washroom environment
was demonstrated by the isolation of yeast colonies on agar plates positioned atvarying distances from the different hand drying devices.
Methods and materials
1.

9 open Sabouraud dextrose agar [SDA] (Oxoid) plates were placedaround the hand drying device (a paper towel dispenser, a warm air dryer
or a jet air dryer) at the following distances (m) from the device: 0.00
(directly below), 0.25, 0.50, 0.75, 1.00, 1.25, 1.50, 1.75, 2.00.
With the paper towel dispenser and the warm air dryer, the test plates
were placed on a bench surface 31 cm and 42 cm respectively below thedevice. Due to the different design of the jet air dryer, the test plates wereplaced on a surface 40 cm below the bottom of the hand drying chamber.

2.

3 ml of ¼ st. sterile Ringer’s solution was added to each of 4 Sabouraud
dextrose agar (Oxoid) slope cultures of the yeast Saccharomyces
cerevisiae, which had been grown at 30oC for 2 days, and mixed using asterile Pasteur pipette to obtain an homogeneous cell suspension. The
volume of the suspension was made up to 500 ml with sterile distilledwater.

3.

50 ml of the suspension was used to wet the hands of subjects who wereasked to use it to mimic washing their hands in water, spreading it as
evenly as possible over both their hands using a wringing action followedby three shakes.

Keith Redway
University of Westminster
ETS 2008 Report

21

4.

Two subjects (one male, one female) were then asked to dry their hands
using one of the following four hand drying methods and for the times
indicated:i)Paper towel 1 (PT 1): 2-ply 100% recycled. Art 217010 (Wepa).
10 seconds.

ii)

Paper towel 3 (PT 3): 2-ply through-air dried (TAD). 50% virgin 50% recycled. Art. 6769 (Kimberly-Clark).10 seconds.

iii)

Warm air dryer (WAD): Electric-Air™, model LE48 (World Dryer
Corporation). 20 seconds.

iv)

Jet air dryer (JAD): Airblade™, model AB01 (Dyson).
10 seconds

Subjects were allowed to take as many paper towels as they wished but
only within 10 seconds and used the same drying behaviour each time.
Similarly, subjects used the same drying behaviour each time for the
WAD and JAD dryers but were stopped after 20 and 10 seconds
respectively.
5.

Plates were re-covered and incubated at 30oC for 2 days.

6.

The number of yeast colonies on each plate was counted. Other types ofcolony, if present, were ignored.

7.

The experiment was repeated 4 more times for each subject making 10
runs in total.

8.

The paired t-test was used to establish if there were any significant
differences between the different hand drying devices in terms of themean number of yeast colonies isolated at varying distances from them.

9.

Controls: Sabouraud dextrose agar (SDA) plates were left open for 1
minute and at the same distances from each hand drying device as in
Step 1 above with the electric hand dryers off. The warm air dryer’s
airflow was tested for the presence of yeast by holding SDA platesbeneath it at a distance of 10 cm for 20 seconds. The jet air dryer’s airflow
was tested by holding SDA plates in the air flow emitted from the sides ofthe device for 10 seconds.
The two subjects were asked to firmly press the fingerpads of their ring,
middle and index fingers onto the surface of SDA plates. A sterile swab
moistened with ¼ strength Ringers solution was then used to swab theentire surface of each agar plate so as to spread and disperse any
potential colonies and enable them to be counted more easily.
Samples (0.1 ml) of the sterile distilled water and the Ringer’s solution
were plated out on SDA and spread using a sterile glass spreader.

Keith Redway
University of Westminster
ETS 2008 Report22All control plates were incubated at 30oC for 2 days followed by their
examination to count any yeast colonies present. Other types of colony, if
present, were ignored.
10.

Measurements were made of the distances between pairs of jet air dryers
in the male and female washrooms of a main line London rail station.

Results
Table 1
Mean number of yeast colonies isolated on SDA plates placed atvarying distances from different hand drying devices usedby subjects with artificially contaminated hands.
HAND
DRYINGDEVICE
PT 1
PT 3
WAD
JAD

DISTANCE (m)
0.00
4.0
3.2
34.1
47.0

0.25
1.2
1.7
2.8
76.2

0.50
0.1
0.1
0.0
37.5

0.75
0.0
0.0
0.0
21.8

1.00
0.0
0.0
0.1
16.0

1.25
0.0
0.0
0.0
11.9

1.50
0.0
0.0
0.0
8.1

1.75
0.0
0.0
0.0
4.8

2.00
0.0
0.0
0.0
1.2

(N = 10)
Key to table:
SDA = Sabouraud dextrose agar; PT = paper towel (1 or 3);
WAD = warm air dryer; JAD = jet air dryer.
The results shown in Table 1 are represented graphically in Figure 1.
T-test results
Significant differences (p < 0.001) in the number of yeast colonies were foundbetween both types of paper towel and the jet air dryer at all 9 distances used.
Differences between the paper towels and the warm air dryer were only
significant at 0.00 m (directly below the device). The warm air dryer was
significantly different from the jet air dryer for all distances except 0.00 m.
Control results
Growth of yeast colonies was not observed on any of the control plates.
Measurements
The distances between 6 pairs of jet air dryers in the male and female
washrooms of a main line London rail station ranged from 0.36 metres to 0.45
metres with the mean being 0.39 metres.

Keith Redway
University of Westminster
ETS 2008 Report

23Figure 1

MEAN NUMBER OF COLONIES PER PLATE

SHOWING THE MEAN NUMBER OF YEAST COLONIES ISOLATED ON
SABOURAUD DEXTROSE AGAR (SDA) PLATES AT VARYING DISTANCESFROM DIFFERENT HAND-DRYING DEVICES USED BY PARTICIPANTS
WITH ARTIFICIALLY CONTAMINATED HANDS

80.0
70.0
60.0
PT1

50.0

PT3
40.0

WAD

30.0

JAD

20.0
10.0
0.0
0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

DISTANCE OF SDA PLATE FROM HAND-DRYING DEVICE (metres)(PT = PAPER-TOWEL DISPENSER; WAD = WARM-AIR DRYER;
JAD = JET AIR DRYER)
N = 10. Standard error bars shown.

Conclusions and discussion
The results suggest that paper towels are likely to cause considerably lesscontamination of other users and the washroom environment than the jet air
dryer which was shown in this study to disperse artificial hand contamination to a
distance of at least 2 metres. Paper towels were better than the warm air dryer at
0.00 metres (directly below the device) but there was no significant difference atgreater distances when their performances were similar and both significantly
better than the jet air dryer.
The mean distance between pairs of jet air dryers in the washrooms tested was
0.39 metres. This distance is less than some of the distances used in this study
and at which significant contamination from the hands of users of the dryer was
detected. The implications are that if the hands of a user of a jet air dryer were
contaminated with a potential pathogen, it could be dispersed and transmitted toother users in the washroom, and the air and surfaces in the washroom. Anysuch contamination could be blown over the person using the adjacent dryer andcould be inhaled by any persons present in the washroom.
The main reason for the large distances over which the jet air dryer spread theartificial contamination in this study is that, according to the manufacturer: “Air isforced through two continuous apertures the width of an eyelash – creating
sheets of air travelling at 400 mph.” Air movements of 400 miles per hour (640kilometres per hour) are sufficiently powerful to blow material in them overKeith Redway
24
University of Westminster
ETS 2008 Reportconsiderable distances as demonstrated in this study by the wide dispersal ofyeast cells. This, coupled with the fact that this type of dryer emits air sideways
rather than downwards (as with a warm air dryer) helps explain the results.
It is well known to microbiologists that air movements encourage the dispersal
and transmission of micro-organisms and increase the chances of thecontamination of materials or persons in any situation. This makes paper towels,where little air movement is generated, the most hygienic option tested in thisrespect followed by the warm air dryer and, lastly, the jet air dryer.

Keith Redway
University of Westminster
ETS 2008 Report25Part D: Bacterial sampling of jet air dryers in public washrooms.
Introduction
Previous studies (Blackmore, 1989; Knights et al., 1993; Redway et al., 1994)
have shown that warm air dryers in public washrooms can be contaminated withvarious bacteria. This contamination can occur on inner surfaces, including air
inlets and nozzles, and be detected in the air flow from warm air dryers. Suchcontamination can be transmitted to the users of warm air dryers by depositionon their hands and to all the users of a washroom through the air.
Jet air dryers are comparatively new and have not been widely tested. In thispart of the present study the bacterial contamination of jet air dryers in publicwashrooms was investigated.
Methods and materials
1.

Sterile swabs were moistened with 3 st. sterile Ringers solution and used
to sample the inner surfaces of 8 jet air dryers in the gents washroom and
8 in the ladies washroom of a main line London rail station. One swab
sample was taken from each dryer by swabbing horizontally once along
the back inner surface of the hand drying chamber, once along the front
inner surface, once along the front air slit and once along the back air slit.
Another swab sample was taken along the bottom of the hand drying
chamber.

2.

Swabs were returned to their sterile containers which were then removed
to the laboratory for analysis.

3.

Samples of the air emitted by the jet air dryers were taken using open
agar plates of three different growth media: Nutrient Agar (NA), CystineLactose-Electrolyte-Deficient Medium (CLED) and Mannitol-salt Agar
(MSA) held for 10 seconds at the side of the hand drying chamber in a
vertical orientation whilst the dryer was running. Plates were removed to
the laboratory for incubation and analysis.

4.

Using gloves, the ends of the swabs were transferred to 3 ml of 3 st.
sterile Ringers solution and vortexed for 10 seconds to release bacteria
adhering to the cotton wool of the swab.

5.

0.01 ml aliquots were then transferred to the surface of plates of threedifferent growth media: Nutrient Agar (NA), Cystine-Lactose-ElectrolyteDeficient Medium (CLED) and Mannitol-salt Agar (MSA) and spread using
a sterile glass spreader.

6.

All plates were incubated at 37oC and examined after 1 and 2 days forbacterial growth. The number of colonies on each plate was counted and,
where appropriate, distinction made between different types. Counts onplates which showed too many colonies to count were scored as 200,
which is considered the upper limit for accurate counting. Counts on theKeith Redway
University of Westminster
ETS 2008 Report26inner surfaces of the dryers were calculated as number of colony-forming
units per square centimetre. Counts on 10-second air flow samples were
calculated as number of number colony-forming units per agar plate.
7.

Any colonies on MSA showing yellow zones were presumptively identified
as Staphylococcus aureus and further tested using the coagulase test to
confirm their identification by a positive reaction.
Any confirmed colonies of Staphylococcus aureus were further tested fortheir sensitivity or resistance to meticillin (methicillin) by growth on
Diagnostic Sensitivity Test Agar (Oxoid) with the addition of a 5
microgram meticillin disk. This showed if any of the isolates were
meticillin-resistant Staphylococcus aureus (MRSA).

8.

Colonies of interest on CLED medium were further identified using theGram stain, the catalase test, the oxidase test and, when a member of thefamily Enterobacteriaceae was indicated, API20E (Biomerieux)
biochemical test identification strips.

9.

Control samples were taken of the inner surfaces and air flow of thelaboratory-based jet air dryer which had never been used in a publicwashroom. These controls were performed to help ascertain if any
bacterial contamination found in dryers in public washrooms was due
mainly to their location and usage rather than to any other factors.

10.

The inner surfaces of the 16 jet air dryers were sampled on two differentdays and at different times of the day making 32 samples in total. The air
samples were taken on three different days and at different times of the
day making 48 samples in total.

Results
Table 1
Mean bacterial colony counts on different growth media of samples fromjet air dryers in public washrooms and the laboratory.
SAMPLE TYPE
INNER SURFACES AND
2SLITS (S) PER CM
BOTTOM OF DRYING
2CHAMBER (B) PER CM
10-SECOND AIR SAMPLE (A)PER AGAR PLATE

W/L
W
L
W
L
W
L

NA
171.4
(7 – 1429)
0.0
7002.5(1137 – 7843)
0.0
14.1(2 - 35)
1.0

GROWTH MEDIUM
CLED
85.3
(0 – 1429)
0.0
7536.8(0 – 7843)
0.1
19.8(0 – 200)
0.0

MSA
127.2
(0 – 1429)
0.0
4745.1(0 – 7843)
0.0
9.7(0 – 25)
0.2(N = 32 S swab samples; N = 32 B swab samples; N = 48 air samples;
N = 5 laboratory controls)
Key to Table 1: W = public washroom samples; L = laboratory control samples;
NA = nutrient agar; CLED = cystine-lactose-electrolyte-deficient medium; MSA = mannitol salt
agar. Ranges are given in brackets. Upper limits of swab ranges are minimum values due toupper limit of sampling method used (<200 colonies per plate).
Keith Redway
University of Westminster
ETS 2008 Report27

Table 2
Identifications and incidences of some of the bacteria isolated
from samples of jet air dryers in public washrooms.
SOURCE OFBACTERIA

IDENTIFICATION

Staphylococcus aureus *Human skin, hair,nasal secretions
Other Staphylococcus species

Escherichia coli

Human gut, faeces

Klebsiella species

Serratia marcescens
Erwinia species
Hafnia alvei
Enterobacter species
Pseudomonas aeruginosa
Water, soil

Various

Bacillus species
Chryseobacterium meningosepticum
Chryseobacterium indologenes
Pasteurella pneumotropica

SAMPLE
TYPE
S
B
A
T
S
B
A
T
S
B
A
T
S
B
A
T
B
S, B
A
B, A
S
B
A
T
S, B
A
A
A

N

%

15
25
40
80
29
28
48
105
1
17
8
26
1
6
4
11
1
2
1
2
8
10
6
24
24
1
2
1

46.9
78.1
83.3
71.4
90.6
87.5
100.0
93.8
3.1
53.1
16.7
23.2
3.1
18.8
8.3
9.8
3.1
3.1
2.1
2.5
25.0
31.3
12.5
21.4
37.5
2.1
4.2
2.1

Key to Table 2: S = inner surfaces/slits; B = bottom surface; A = 10-second air
sample; T = total (S+B+A); N = number of isolates (out of 32 S samples, 32 B
samples, and 48 A samples); % = percentage of dryer samples that tested
positive.
* No meticillin-resistant Staphylococcus aureus (MRSA) strains were isolated.
Conclusions and discussion
Bacteria were isolated from the inner surfaces of the hand drying chamber, theair outlet slits, the bottoms of the hand drying chamber and the air flows all 16 jet
air dryers tested on different days. The highest mean bacterial counts wereobtained from the bottom of the hand drying chamber. This part of the dryer wasobserved to often be wet which would encourage bacterial colonization andsurvival and probably explains this part of the results. Also, swabs from thebottom were often visibly dirtier after sampling than swabs taken from the innersurfaces and slits. Bacterial counts on agar plates of a 10-second air flow
sample were lower than either swab sample, as would be expected from thenature of the sample.
Keith Redway
University of Westminster
ETS 2008 Report28

The mean numbers of bacteria isolated on agar plates from the air flows of thedryers in this study are similar to the results obtained for warm air dryers inprevious studies by the University of Westminster (Knights et al., 1993; Redway
et al., 1994) but lower than those found by Blackmore (1989). This may be
explained by differences in the frequency of use of dryers and on their location.
This study sampled jet air dryers in the washrooms of a very busy mainline
London railway station and it would be expected that more bacterial
contamination would occur than with dryers that are less frequently used.
Both the numbers and types of bacteria found to be contaminating the surfacesof the jet air dryers in this study are similar to those found in another study(Redway et al., 1994) associated with warm air dryers. The same types ofbacteria have also been found in washroom environments by other workers(Mendes & Lynch, 1976). The sources of these bacteria are mainly human skin(e.g. Staphylococcus aureus and other species), human gut and faeces (e.g.
Escherichia coli and other species of Enterobacteriaceae) and water and soil
(e.g. Pseudomonas aeruginosa and Bacillus species). Although carried on theskin of a proportion of the population, Staphylococcus aureus is an important
pathogen, especially if in a hospital environment and antibiotic resistant, e.g.
meticillin-resistant Staphylococcus aureus (MRSA). No MRSA strains were
found in this study but since its physical properties are identical to other strainsof Staphylococcus aureus, the results show that if MRSA was present in alocation it would be likely to contaminate jet air dryers in the same way and could
be transmitted by touch or dispersed in the air.
The bacterial counts of the bottoms of jet air dryers were particularly high,probably due to the water from users hands that collects in this area. Counts on
the other inner surfaces of the dryers (inner surfaces and slits) were lower,probably due to the fact that these surfaces are generally drier than the bottoms.
The bottom counts showed on average thousands of bacteria per squarecentimetre and the figure could be even higher due to the upper limit of thecounting method used. These bacterial counts are higher than those found by
Mendes & Lynch (1976) on the average toilet seat. Their study showed thatabout 80% of toilet seats in public washrooms had lower mean bacterial counts
than the bottoms of the dryers sampled here. However, although any surface ina washroom can become contaminated with bacteria, moist surfaces particularly
encourage bacterial colonization and survival and this probably explains the highcounts at the bottoms of the jet air dryer chambers which in a busy washroom
may be wet much of the time.
Escherichia coli and the other species of Enterobacteriaceae isolated from jet air
dryers in this study demonstrate the potential for this type of device to becomecontaminated with human faecal material. Escherichia coli is a well known
marker organism of such contamination. The detection of gut bacteria indicatesthe potential for this type of dryer to disseminate other more pathogenic types ofEnterobacteriaceae (e.g. Salmonella and Shigella species) since the propertiesof all members of the Enterobacteriaceae are very similar. As with
Staphylococcus aureus, these bacteria could be transmitted by touch ordispersed in the air emitted by contaminated dryers.
Keith Redway
University of Westminster
ETS 2008 Report29

It was not surprising that Pseudomonas aeruginosa was found associated withdryers in this study since it is commonly found in water but, like Staphylococcus
aureus, it can cause severe problems in hospital environments and with certain
types of patient. The species is generally resistant to many antibiotics but some
hospital strains can be especially resistant and infections caused by them very
difficult to treat. The results of this study suggest that the spread of such strains
in a hospital environment could be increased by the use of jet air hand dryers.
The Bacillus species isolated in this study from some jet air dryer surfaces
probably have less significance with respect to human health. Bacillus species
are commonly found in soil and the general environment. There are only two
pathogens in this group, one causing anthrax and the other a toxigenic foodpoisoning, but the former is very unlikely to have been isolated in this study
although the latter, often associated with rice-borne food poisoning, could havebeen. However, speciation of this group was not carried out in the present study.
The bacterial contamination of jet air dryers could allow transmission of bacteria
contaminating them 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 them but that will not be the case with all users. In Part B of thisstudy some subjects were observed touching the bottom, sides and slits of thehand drying chamber accidentally. Members of the public, especially those in ahurry, could touch the inner surfaces of this type of dryer and their hands becontaminated by any bacteria present. This type of dryer may actually be worsein this respect than warm air dryers because putting the hands into a narrow slot
is more likely to cause contact with the device than holding the hands beneathan air outlet nozzle.
However, even if the dryer is not touched, the results of this study show thatbacteria are transmitted in the air flow and could contaminate the hands, otherparts of the user, their clothes, other washroom users, or be inhaled.

Summary of main conclusions and discussion
Part A of the study shows that the drying efficiency of paper towels and the jet air
dryer were equal. Both methods achieved at least 90% dryness of the hands
within approximately 10 seconds and both were faster than the warm air dryer,
which took over 4-times as long to achieve the same level of dryness. Therefore,
the manufacturer’s claims for the tested JAD that it is “the fastest hand dryer”
and that it “dries hands twice as fast” seem confirmed with the one proviso that it
does not dry faster than towels but is certainly faster than other electric handdryers such as a warm air dryer.
Part B of the study shows that both types of paper towel tested reduced themean numbers of all types of bacteria tested on the fingerpads and the palms ofsubjects. As shown by other studies, the warm air dryer increased the mean
numbers of all types of bacteria tested on the fingerpads and the palms ofsubjects. The jet air dryer also increased the mean numbers of most types ofKeith Redway
University of Westminster
ETS 2008 Report30

bacteria tested on the fingerpads and the palms of subjects but the increases
were less than with the warm air dryer. Therefore, the manufacturer’s claim thatthe tested JAD is the “most hygienic hand dryer” is confirmed, assuming that theterm “hand dryer” refers to electric devices only because its hygiene
performance compared to paper towels was significantly worse in all respects.
Part C of the study shows that paper towels are likely to cause considerably lesscontamination of other users and the washroom environment than the jet air
dryer which was shown in this study to disperse artificial hand contamination to a
distance of at least 2 metres, well within the range of adjacent dryers observed ina real washroom. Paper towels were better than the warm air dryer forcontamination levels directly below the device but there were no significant
differences at greater distances when their performances were similar and bothwere significantly better than the jet air dryer. Therefore, the manufacturer’s
claim that the tested JAD is the “most hygienic hand dryer” is not confirmed inthis study with respect to its potential for dispersing bacteria.
In Part D of the study various bacteria were isolated from the surfaces and the
air flows of all jet air dryers tested on different days in public washrooms. The
main sources of these bacteria are human skin, human gut and faeces, waterand the general environment. Many of the bacterial types isolated are potential
human pathogens. The results show that jet air dryers can be colonized bybacteria and have the potential to transmit them to washroom users. Both thenumbers and types of bacteria found to be contaminating the jet air dryers in thisstudy are similar to those found associated with warm air dryers. Therefore, themanufacturer’s claim that the tested JAD is the “most hygienic hand dryer” wasnot confirmed in this study with respect to its potential for contamination andcolonization by bacteria.
The results of all parts of this study suggest that paper towels should be used inlocations where hygiene is paramount, such as hospitals, clinics, schools,
nurseries, care homes, kitchens and other food preparation areas. Warm air
dryers and jet air dryers should be carefully considered for these types oflocation because of their poorer hygiene performance and the increased
likelihood of transmission of bacteria, including potentially pathogenic types, via
the fingerpads and palms of the hand and their air flows. The performance ofboth the warm air dryer and the jet air dryer was inferior to paper towels in all
respects (drying efficiency, bacterial numbers on the hands, bacterial
contamination of the air flow and surfaces of the devices, and transmission ofbacteria in the washroom) with the one exception that the jet air dryer is equal in
drying efficiency. The jet air dryer was shown to be superior to the warm air dryer
in all respects except for similar bacterial contamination and greater transmission
potential. Although representing a considerable improvement over warm air
dryers in speed, the jet air dryer’s overall performance, with the exception ofdrying efficiency, was significantly poorer than that of paper towels in all otherrespects tested in this study.

Keith Redway
University of Westminster
ETS 2008 Report31References
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Note: this study has not been peer reviewed but it is intended that the test
methods described in this document are provided in sufficient detail to allow
replication by those who wish to confirm the results.

Keith Redway
University of Westminster
ETS 2008 Report34