Dental unit waterlines: source of contamination and cross ... - CCIH

Journal of Hospital Infection (2010) 74, 99e111
REVIEW
Dental unit waterlines: source of contamination
and cross-infection *
S. Kumar, D. Atray, D. Paiwal, G. Balasubramanyam, P. Duraiswamy,
S. Kulkarni*
Department of Preventive and Community Dentistry, Darshan Dental College and Hospital, Loyara,
Ranakpur road, Udaipur 313001, India
Available online 21 January 2010
KEYWORDS
Dental unit waterlines;
Dentist;
Disinfection;
Infection;
Microbial
contamination;
Patient
Introduction
Modern dental chair units (DCUs) are classified as
medical devices under the European Union Medical
* Proceeded without author corrections.
* Corresponding author. Address: Department of Preventive
and Community Dentistry, Darshan Dental College and Hospital,
Loyara, Ranakpur Road, Udaipur 313001, India. Tel.: þ91
9983681656.
E-mail address: sanrescommunity@yahoo.com
Available online at www.sciencedirect.com
www.elsevierhealth.com/journals/jhin
Summary Dental chair units (DCUs) are used in the treatment of many
patients throughout each day and microbial contamination of specific component
parts is an important potential source of cross-infection. The quality
of dental unit water is of considerable importance since patients and
dental staff are regularly exposed to water and aerosols generated from
the dental unit. This water hosts a diverse microflora of bacteria, yeasts,
fungi, viruses, protozoa, unicellular algae and nematodes which may be
contaminated with micro-organisms found in the biofilm formed due to
water stagnation in the narrow-bore dental unit waterline (DUWL) tubings.
The water thus contaminated, when used for various treatment procedures
through dental handpieces, air/water/three-in-one syringe, etc., produces
aerosols that can cause infection. The present review emphasises the risks
of infection from DUWL and various water treatment procedures available
to disinfect the DUWLs.
ª 2009 The Hospital Infection Society. Published by Elsevier Ltd. All rights
reserved.
Devices Directive. 1 As DCUs are used in the treatment
of many patients throughout each day, microbial
contamination of specific component
parts is an important potential source of crossinfection.
2
Furthermore, droplets and aerosols generated
by DCU handpieces may be inhaled by patients and
dental healthcare personnel. 3e6 DCUs consist of
several complex, integrated equipment systems
that are central to the practice of modern dentistry.
These systems are designed to provide the
0195-6701/$ - see front matter ª 2009 The Hospital Infection Society. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.jhin.2009.03.027

100 S. Kumar et al.
instruments and services necessary for a diverse
range of dental procedures.
The quality of dental unit water (DUW) is of
considerable importance since patients and dental
staff are regularly exposed to water and aerosols
generated from the dental unit.
This water hosts a diverse microflora of bacteria,
yeasts, fungi, viruses, protozoa, unicellular algae
and nematodes.
Water with

Dental unit waterlines 101
Handpiece Syringe Scaler Basin
Compressed
air
Shut-off valve
Biofilms in DUW
Bacteria in aquatic environments interact with
surfaces to form a biofilm, a strategy developed
to aid survival and to optimise available nutrients.
The physics of laminar flow of DUW passing through
the waterlines results in maximum flow at the
centre of the lumen and minimal flow at the
periphery, encouraging deposition of organisms
onto the surface of the tubing. 16 Intermittent use
patterns of dental lines leads to stagnation of the
entire water column within the waterlines for extended
periods during the day, thus promoting further
undisturbed bacterial proliferation. Bacteria
adhere more readily to hydrophobic polymeric
plastic tubing of the type used in dental equipment
(for example, polyvinyl chloride, polyurethane)
than to those composed of glass or steel. 17 Susceptibility
of medical equipment such as catheters to
biofilms has been reduced by coating with heavy
metals or incorporating biocides into the fabric
of the tubing that inhibit bacterial growth. 18 Similar
materials may be of future value in dental
units. Organisms in DUW biofilm are predominantly
derived from the incoming mains water. Once
a new DUW system is connected to mains water
supply, even when it is not used for patient treatment,
a biofilm will form (and be releasing high
numbers of planktonic organisms) within 8 h. 19
The biofilm will develop to reach a climax community
of microcolonies embedded in a protective
R
e
s
e
r
v
o
i
r
Silver
compound
added as
liquid, powder
or tablet
Figure 1 Dental chair unit water supply.
extracellar amorphous matrix by six days. 19 Bacteria
shed from the biofilm during use maintain
the bioburden of planktonic (suspended) organisms
detected in DUW. Characteristically biofilm bacteria
exhibit greater resistance to surfactants, biocides
and antibiotics than organisms floating
freely in fluids. 20 Various types of organisms isolated
from DUW are listed in Table I.
Bacterial indicators of risk
In potable water supplies, concentrations of >1coliform/100
mL suggest faecal/sewage contamination.
However, coliforms are not usually recovered from
DUWL, excepting unusual circumstances, e.g.
Table I Types of organisms isolated from dental
unit water
Bacteria Fungi Protozoa
Achromobacter Phoma spp. Acanthamoeba
xyloxidans Penicillium spp.
Acinetobacter spp. Cryptosporidium
spp.
Cladosporium spp.
Actinomyces spp. Microsporidium
spp.
Alternaria spp.
Alicaligenes
dentrificans
Bacillus spp.
Bacteroides spp.
Caulobacter spp.
Flavobacterium
spp.
Fusobacterium
spp.
Klebsiella
pneumoniae
Lactobacillus
spp.
Legionella spp.
Micrococcus spp.
Moraxella spp.
Mycobacterium
avium
Nocardia spp.
Pasteurella spp.
Proteus vulgaris
Pseudomonas
aeruginosa
Burkholderia
cepacia
Streptococcus
spp.
Staphylococcus
aureus
Mycobacterium
spp.
Xanthomonas spp.
spp. Giardia spp.
Scopulariopsis
spp.

102 S. Kumar et al.
following manual manipulation of independent reservoir
bottles by staff with poor hand hygiene. Reliance
solely on the aerobic count in DUWL as an indicator of
the associated health risks may underestimate the
risk, as aerobic counts will not highlight the presence
of respiratory and opportunistic pathogens. Isolation
of the latter organisms in water intended for irrigation
of a medical device such as a dental handpiece may
constitute both an infection control risk and health
hazard. 21 DUWL contaminated with 30% of opportunistic
pathogens such as Pseudomonas spp. or Legionella
spp. are likely to constitute a greater health risk
to staff and patients than DUWL with a high bacterial
count of millions of organisms per/mL in the absence
of known pathogens. An alternative approach for risk
assessment and management is to use epidemiological
and serological techniques to screen for evidence
of DUWL-associated infection in both the dental team
and dental patients.
What are the risks to patients?
There is no evidence of a widespread public health
problem from exposure to DUW. Nevertheless, the
goal of infection control is to minimise the risk
from exposure to potential pathogens and to
create a safe working environment in which to
treat patients. The ever-increasing number of
patients who are either immunocompromised or
immunosuppressed due to drug therapy, alcohol
abuse or systemic disease has produced a cohort of
patients susceptible to environmental waterborne
opportunistic pathogens such as those prevailing in
DUW. The organisms recovered from DUW vary
with geographic location. They include fungi, freeliving
amoebae, protozoa and nematodes as well
as the consistently reported recovery of saprophytic
and opportunistic Gram-negative pathogens
such as Pseudomonas spp., Klebsiella spp. and Flavobacterium
spp.. 22 Of particular concern are the
primary respiratory environmental pathogens
found in DUW that can cause pneumonia, milder
flu-like respiratory infection and, less commonly,
wound infections, e.g. Legionella pneumophila
and non-pneumophila Legionella spp. as well as
Mycobacterium spp. including Mycobacterium
avium-intracellulare. Mycobacterium avium-intracellulare
can cause disseminated infection in
HIV-seropositive patients following ingestion and
colonisation of the gut. 23,24 Numbers of non-tuberculous
mycobacterium (NTM) in DUW exceeded
that of drinking water by a factor of 400. 25 High
numbers of non-tuberculous mycobacteria may
be swallowed, inhaled or inoculated into oral
wounds during dental treatment with the potential
for colonisation, infection or immunisation. Priming
of the immune system by exposure to environmental
NTM helps to maintain the anti-tuberculin
immune response. 26 The true extent of the risk
posed by NTM in DUW to the immunocompromised
patient has yet to be fully elucidated. Similarly,
the primary pathogen acanthamoeba is recovered
from DUW and biofilm. 22 These organisms are
reputed to cause amoebic keratitis in contact
lens wearers who clean their lenses in tap water. 27
It is unknown whether they present a risk in the
dental setting, but routine use of protective eye
wear by both the dental team and patients should
shield the eyes from any possible exposure.
Pseudomonas aeruginosa
This can be highly resistant to biocides, including
antibiotics, and can grow in dilute disinfectants
such as chlorhexidine and iodophors. It is able to
thrive in low nutrient environments such as distilled
water, which is often used by dentists in
bottled-water systems.
The infective dose for colonisation in healthy
human volunteers is >1.5 10 6 cfu/mL. 28 Such high
concentrations are rarely encountered in DUWs. 29
Antibiotic treatment makes patients more susceptible
to opportunistic pathogens and markedly
lowers the required infectious dose. The estimated
risk of colonisation by daily exposure to water with
low levels of Pseudomonas aeruginosa is 1.7 10 8 .
Therefore, the risk of a healthy person becoming
colonised is vanishingly low. 29 The only proven evidence
was published in 1987. 30 Two patients with
solid tumours were unwittingly exposed to DUWs
contaminated with P. aeruginosa. Both patients
subsequently developed gingival abscesses which,
as later confirmed, were caused by the same strain
of P. aeruginosa as that isolated from the turbine
waterlines. In a prospective study, other non-compromised
patients treated in one of six P. aeruginosa-contaminated
dental units were transiently
colonised for three to five weeks with P. aeruginosa,
but no infection ensued. 30 (It should be noted
that transient colonisation commonly occurs e for
instance, after eating a salad e without any
adverse health consequences.)
Non-tuberculous Mycobacteria spp. (NTM)
These are opportunistic pathogens causing pneumonia,
cutaneous and disseminated disease. There
is little evidence for person-to-person transmission
and the organisms are transmitted from environmental
sources by ingestion, inoculation or inhalation.
Worldwide, there is an increasing

Dental unit waterlines 103
incidence of infection by NTM in immunocompetent
patients, which is thought to be acquired from
environmental sources such as drinking water. 31
Strains of Mycobacterium spp. have been isolated
from infected AIDS patients and their home coldwater
drinking water tap. 32 Fortunately, most
NTM infection is asymptomatic as studies suggest
that w12% of the population in the USA has been
colonised by the NTM Mycobacterium aviumintracellulare.
33
In hospital outbreaks of NTM infection, the
source of the organism has been tracked back to
contaminated taps and showerheads. NTM are
isolated in low numbers from municipal water
supplies, the prevalence rate varies from 1% up
to 50% depending upon the exact geographical
location. 29 It follows that their presence in DUWLs
fed by mains water also reflects local geographical
variations. 34,35 Only a small number of published
studies evaluate the prevalence or health risk
from NTM in DUWLs. 34,36e39 NTM are commonly
isolated from DUWLs, e.g. in England and parts of
Europe, and have been shown to proliferate in
the biofilm. 29,38,39 The numbers of non-tuberculous
mycobacterium in DUWLs exceeded that of
drinking water by a factor of 400. 38
The obvious concern is that large numbers of NTM
may be swallowed, inhaled or alternatively inoculated
into oral wounds during dental treatment with
the potential for colonisation and infection.
Gargling with water containing NTM resulted in
respiratory colonisation. Prosthetic heart valve
infection with M. gordonae and another two cases
of NTM cervical lymphadenitis following dental extractions
has been reported. 40,41 Low-level exposure
of dentists to DUWLs could have a positive
effect. Priming of the immune system by exposure
to environmental NTM is thought to be beneficial
as it helps to maintain the bacille CalmetteeGuérin
vaccine (anti-tuberculin) immune response.
Legionella spp.
Legionellae may enter the DUWL from the mains
drinking water. 42 Six to thirty percent of domestic
hot water systems harbour legionellae. 43 In order
to multiply in the DUWL, legionellae require other
micro-organisms, particularly amoebae, a supply
of nutrients and temperatures in the range of 20e
45 C. 44 Concentration of Legionella spp. in DUWL
is reported to be in the range of 102e105 cfu/
mL. 45e47 However, once established, legionella colonisation
may persist in waterlines for years. 48,49
Legionellae suspended in aerosols at 65% relative
humidity can survive in laboratory conditions for
2h. 50 Legionellosis can present either as an
atypical pneumonia or as a milder flu-like illness,
known as Pontiac fever. Although more than half
of the >46 species that comprise the family Legionellaceae
have been linked to disease, the vast majority
of reported cases of Legionnaires’ disease are
caused by L. pneumophila serogroup 1. 51 The prevalence
of legionella in DUWL varies widely from 0 to
68% depending, in part, on the isolation procedures.
46e48,52,53 Risk factors include male sex, age
>45 years, smokers, alcoholics, diabetics and people
with chronic respiratory or renal disease and
cancer. 54 Many serological surveys for legionellosis
among dental personnel have been conducted,
and we could find no study involving the dental
patients. An outbreak of Legionnaires’ disease
was reported from Stafford District General Hospital:
68 patients were found to be seropositive
among whom 22 died. The source of infection was
identified as the chiller unit of the air-conditioning
plants. 55
What is the risk to dental surgeons?
Considerable attention has focused on the plight of
the susceptible patient but the clinical members of
the dental team inhale aerosols generated by
dental equipment on a daily and long-term basis.
Abnormal nasal flora in dental personnel has been
linked to water system contamination. 56 The clinical
dental team experience an increased prevalence
of respiratory infections compared to the
general population or their medical colleagues. 57
Employing polymerase chain reaction methodology,
Legionella spp. have been detected in 68%
of DUW samples and L. pneumophila in 8%. Comparable
prevalence rates were observed in potable
water samples but e significantly from a public
health standpoint e none of the potable samples
had counts of >10 000 legionella/mL whereas 19%
of the DUW samples were in this category. 58 The
magnitude of legionella antibody titres correlated
directly with the duration of time spent carrying
out clinical work, suggesting that aerosols generated
from DUW are the likely source. Conversely,
a survey carried out by the Central Public Health
Laboratory, London found no evidence that previous
dental treatment was a risk factor in patients
with legionellosis. 59 A single case of fatal pneumonia
in a dentist from L. dumoffi has been reported.
L. dumoffi and other Legionella spp. were recovered
from his surgery waterlines, although not
from his domestic potable water supply. 60 Unfortunately,
the isolates were not available for molecular
typing which would have confirmed the link to
the source. 47 However, the possibility still remains
that DUW-associated infections have gone

104 S. Kumar et al.
unrecognised or unreported because of the failure
to associate exposure to DUW or aerosols with the
development of specific infections. Sporadic infections
such as Pontiac fever, also caused by Legionella
spp., are less likely to be investigated or
notified to health authorities (www.bda.org). 61
Risk factors identified in domestic acquisition of
Legionnaires’ disease are of relevance in preventing
infection in the dental surgery. Multivariate
analysis showed an increased risk of infection
following recent plumbing repairs, the use of an
electric rather than a gas water heater, smoking,
and working >40 h per week. 61
Approaches to risk reduction
DUW cleaning and disinfection systems
DUW treatment agents can be introduced into
DUWs from independent reservoir bottles or from
disinfectant delivery devices connected to the DCU
water supply inlet. In the case of DCUs connected to
a municipal water mains supply, it is imperative
that the connection is turned off prior to DUW
treatment to prevent contamination of mains water
with treatment agent. After treatment, DUWs
should be flushed thoroughly using clean water
before DCUs are used for patient treatments.
DCU manufacturers have developed a new generation
of DCUs with integrated DUW cleaning
systems that facilitate and simplify DUW disinfection.
Indeed other DCU manufacturers are likely to
follow suit, as the dental community becomes
more aware of the problem of DUW contamination.
12,13 WCSä (Water Cleaning System) was
found to be very effective at eradicating DUW biofilm
in these DCUs when used with Sanosilä (a
highly effective, universally applicable disinfectant
having two main components: hydrogen peroxide
and silver) and consistently provided
output water with bacterial densities below the
ADA recommended threshold of w200 cfu/mL for
up to seven days post disinfection. The microprocessor-controlled
WCS was originally developed
to be retrofitted to existing Planmecaä DCUs having
a mains water supply. Some DUWs contain a microprocessor-controlled
system where waterlines
are filled with disinfectant and subsequently automatically
flushed with clean water during the disinfection
cycle. If the cycle is not completed
properly or a power failure occurs during the cycle,
an error message is displayed on the DCU instrument
console. 13,62 While this error message is
displayed, all DCU functions are locked, preventing
the DCU from being used. The WCS is semi-
automated and requires the operator to fill the
disinfectant container with disinfectant, to turn
off a valve on the mains water supply and to place
DCU instrument hoses into special receivers. DUW
cleaning is activated from a keypad on the DCU
control console and disinfectant is automatically
fed into each DUW. Following this the DCU electrical
supply is switched off to allow the disinfectant
in situ to take effect. When the electrical power is
turned back on, a code message on the DCU control
console display prompts the operator to flush
the DUWs. This is initiated by disconnecting the
disinfectant container module and turning on the
mains water supply switch. The longer-term
goal should be for fully automated disinfection systems
that are validatable. Recently Planmeca
developed a more advanced microprocessor-controlled
DUW cleaning system called the Water Management
System (WMS), a fully integrated and
automated DUW cleaning system that requires
minimal effort on the part of the operator. The
WMS is more advanced and automated than the
WCS and also contains many additional features,
including an air gap.
Anti-retraction valves and retrograde
aspiration of oral fluids
The implicated source of contamination was reaspiration
of fluid from the oral cavity that
occurs when negative pressure is generated on
stopping equipment. Recently, molecular techniques
were employed to demonstrate the recovery
of viral particles and human DNA in
waterlines servicing the air turbine and prophy
angle handpiece. 63 Anti-retraction valves (also
known as check valves) will limit re-aspiration
and are most efficacious when fitted immediately
distal to the handpiece. As with any component
of the water supply line they are subject to clogging
due to biofilm deposition and fatigue. In order
to ensure adequate mechanical functioning,
they require regular maintenance and programmed
replacement. Autoclaving of handpieces
after use and flushing of units for 30 s at the end
of patient treatment and for 2.5 min at the end
of the day will augment the action of the anti-retraction
valve and should help to eliminate any
aspirated fluid. 64 Some manufacturers have incorporated
anti-retraction valves within the handpiece
design, permitting autoclaving of the
valve between patients. Backflow from dental
units to the mains water supply may occur and
it may be necessary to install check-valves to
prevent this occurring. 65

Dental unit waterlines 105
Filtration
Both the original and more recent publications
demonstrated that a high level of recontamination
of DUW occurs within 24 h as a result of trapping
and growth of bacteria on the filters. 48 Therefore
disposable filters are recommended, which must
be changed daily. 66 For maximum efficiency, filters
should be inserted just distal to the point of entry
of water into the handpiece. A pore size of 0.2 mm
is recommended (US Federal Drug Administration).
67 Designs with pleated filters have a larger
surface area for filtration. Only minimal reductions
in flow rate are experienced and are unlikely to be
noticeable in practice. Filters have no impact on
biofilm formation. Although disposable filters are
a promising method of improving water quality,
their clinical effectiveness has not yet been fully
established. Nevertheless, prevention of planktonic
bacteria from entering the handpiece from
the waterline will reduce patient exposure to
harmful pathogens. Filters should also reduce retrograde
contamination.
Flushing
Recommendations for control of DUW contamination
from the CDC, ADA and British Dental Association
all state that waterlines should be flushed
through for ‘several minutes’ at the start of each
clinic day to substantially reduce microbial accumulation
caused by overnight stagnation in the
waterlines. 67 Furthermore, in order to minimise
exposure to aerosols the procedure is best performed
in conjunction with high velocity evacuation
into an enclosed container. Discharging the
stagnant water improves the perceived quality of
the water and reduces the malodour and bad taste
imparted to the water by microbial contamination.
It will also draw through low concentrations of
chlorine (0.1e0.5 ppm) normally present in mains
water. However, it is recognised that flushing provides
only temporary reductions in bacterial load
and has no effect on the biofilm. As a result of
the physics of the laminar flow in the waterline,
the layer in immediate contact with the biofilm is
stationary even during flushing. The effectiveness
of flushing has been challenged by a number of
authors who report bacterial clearance was both
variable and minimal when used for short periods
of time (

106 S. Kumar et al.
Chlorination
Chlorine, as sodium hypochlorite, is the most
commonly employed biocide in water treatment
plants and has proven efficacy in cold water
hospital systems, particularly in controlling
Legionella spp. proliferation. 85 Chlorine can be
added to the water at the central supply intake.
This approach is mainly applicable to larger multiple
surgery practices or in Dental Hospitals and
universities. Independent reservoir clean water
systems can also be used to deliver chlorine flushes
to the dental waterline. The system can be
‘shocked’ or hyperchlorinated intermittently with
high doses of 50 ppm chlorine every six months.
Alternatively a continuous chlorination system
can be installed, with an automatic dosing mechanism
providing 1 ppm chlorine at the chair. Chlorination
was found to be effective in maintaining
drinking water standards in the storage tank and
distribution pipes. Equivocal results have been obtained
from measurements of DUW with some
studies reporting bacterial counts reduced to
a few hundred and others finding only temporary
remission in contamination and no elimination of
L. pneumophila. 86 When legionellae are sequestered
within free-living amoebae there is
a 30e120-fold increase in chlorine resistance,
thus explaining the failure to eradicate the organism
from the system. 87 Resistance to biocides will
develop in the bacterial population with extended
exposure. Potentially, higher doses of 3e5 ppm
could overcome these problems, but high chlorine
levels are unpalatable, and long-term corrosion
damage occurs with free residual chlorine levels
as low as 1 ppm. In addition, high levels of chlorine
are associated with in-vitro formation of trihalomethanes,
which are recognised as potential carcinogens.
88 However, these problems apply to
chlorinated water for clinical use. Lower doses
have little effect on biofilms but higher doses
could be used in the independent water systems
as a flush to remove planktonic bacteria, as the
apparatus is purged of chlorine before patient
use. Corrosion of metal components would still
be a problem.
Glutaraldehyde
This is available for use with an integral, automated
flush system with a contact time of 7 min. 17
Glutaraldehyde is a highly effective disinfectant
with bactericidal action against most vegetative
bacteria, mycobacteria and viruses, but its sensitisation
of the human lung and skin has severely limited
the use of this compound in dentistry except
in situations where exhaust ventilation can be assured.
Bacteria within the biofilm pose a major
stumbling block to the use of biocides. They are
3000-fold less susceptible to hypochlorite and so
they are not readily degraded even by concentrated
solutions of bleach or of other disinfectants
such as glutaraldehyde. Planktonic organisms will
be destroyed but even if the majority of the organisms
in the biofilm are eliminated the architecture
of the biofilm survives and acts as a pre-formed
matrix for renewal of the biofilm. For the future,
‘electro-enhancement’ of biocides producing neutralisation
of the surface charge may be incorporated
into medical equipment to resolve the
problem of biofilm build-up. 89
In 2005, a study aimed to evaluate the enhancement
of the biocidal efficacy of glutaraldehyde
against Pseudomonas fluorescens biofilms by the
application of an electric field. It was observed
that the electric-field-enhanced glutaraldehyde
efficacy reduced the number of surviving cells in
the range of one to four orders of magnitude
with respect to those with only glutaraldehyde
treatment. 90
Peroxide, ozone and ultraviolet light
Hydrogen peroxide and ozone can also be introduced
continuously into the waterlines during
patient treatment. 91 Such measures have the advantage
of maintaining low levels of planktonic
counts throughout treatment which, during complex
restorative procedures, may be prolonged.
Bacteria from the biofilm are shed continually
while the film is in contact with water.
Hydrogen peroxide has been used in dentistry as
a bleaching agent and root canal irrigant, as well
as in dentrifices and mouth rinses. It is used as
a disinfectant (7% solution) for flexible endoscopes,
where the efficacy was found to be
comparable with that of 2% glutaraldehyde, and
in the disinfection of contact lens cases where it
was reported to be more effective against biofilms.
92,93 Unfortunately, the published efficacy
data on hydrogen peroxide and ozone with regard
to purification of DUW is limited at the present
time. A US Food and Drug Administration (FDA)-approved
delivery system for hydrogen peroxide is
commercially available which provides metered,
microprocessor-controlled, continuous release of
stabilised peroxide into the waterline. 67 Silver-catalysed
hydrogen peroxide has been found to be
a highly effective, universally applicable disinfectant
and consistently provided output water with
bacterial densities below the ADA-recommended
threshold of w200 cfu/mL for up to seven days

Dental unit waterlines 107
post-disinfection. UV treatment of water has been
used alone and in conjunction with ozone and
other biocides for control of legionella and reduction
of endotoxins in water cooling towers and water
treatment plants, for example for swimming
pools. UV would appear to be an attractive, nonpolluting
alternative for point of entry of mains
water purification. However, evidence that UV
irradiation alone has a significant effect on reducing
microbial contamination is equivocal due to
the relative resistance of some important waterborne
pathogenic species. 94 A major advantage
of these systems is that they avoid introducing
chemical disinfectants into the effluent water system
with the potential for pollution and destructive
effects on wildlife.
Independent clean water systems
Introducing sterile water rather than contaminated
mains water into the system, even if the concentration
of organisms is low in the mains water, is
a logical first step in initiating a clean if not sterile
chain for DUW delivery. Changes in weather patterns
due to global warming have resulted in
extremes of rainfall from droughts to flood. This
can cause contamination of the municipal water
supply due to backflow into the mains of floodwaters
or of stagnant water from leaks from piping
that becomes depressurised when the distribution
system is cut off. Dentists need to be able to
maintain a constant source of safe water during
periods of interruption to the municipal water
distribution system arising from emergency introductions
of boil notices, drought orders, standpipes
or rota cuts. 95 A separate, pressurised clean water
reservoir system, filled with sterile water plumbed
to the waterlines, bypasses the mains connections
to the municipal water, providing a suitable alternative.
17 Reservoir systems also comply with water
distribution by-laws which state that dental units
should be isolated from the incoming mains water
supply by incorporation of an air gap in the system,
thus preventing back-siphonage. This approach has
a long history of military use in field surgeries when
armies are on manoeuvres.
The cost of fitting clean water systems is approximately
£200 (US $320). The total volume of
water consumed per day as an irrigant is in the
range of 1e2 L per dental unit, thus the use of
a small reservoir is feasible and convenient. Reservoirs
should be used preferably with sterile water or
boiled water that is allowed to cool in a sterile,
sealable container. Water bottles need to be
handled with care as the water can become contaminated
with skin organisms. Water-adapted
organisms such as the opportunistic pathogens
Pseudomonas aeruginosa and Burkholderia cepacia
(causative agent of pneumonia in cystic fibrotic
patients) are able to survive for long periods in distilled
water. 96 These systems are capable, if used
correctly, of delivering a high quality water supply,
but not sterile water, if they are installed on existing
biofilm-contaminated waterlines. 97 The design
is dual purpose and can be used for both water
delivery and regular purging of waterlines with disinfectant.
In order to maintain planktonic counts at
a minimum, the waterlines need to be routinely disinfected
(either daily or weekly) according to manufacturers’
instructions with a suitable diluted
disinfectant such as sodium hypochlorite or hydrogen
peroxide for 10 min and then flushed thoroughly
with sterile water.
Contamination of the system can still occur due
to suck-back of oral fluids through faulty antiretraction
valves and a 30 s flush between patients
is still mandatory. Letting the system drain down
to dry and purging with air or ethanol will help to
prevent biofilm proliferation due to desiccation.
Autoclavable systems
In response to the evolving high standards for
quality control and prevention of DUW contamination,
a fully autoclavable assembly of water
reservoirs, silicon multi-lumen DUWL tubing and
fittings to be sterilised between patients has been
produced and has been cleared for marketing by
the FDA. Such devices should, if manufacturers’
instructions are fully adhered to, remain free from
biofilm build-up, as any contamination from retrograde
aspiration or from skin organisms during
manipulation should be destroyed during autoclaving.
Autoclavable systems may be the solution
to providing secure, sterile water systems.
Water pre-treatment
The poorer the quality of water supplied to DUWs,
the more readily biofilm is likely to form on its
surfaces. Heterotrophic bacteria present in supply
water can convert organic material dissolved in
supply water into biomass locally. 98 The watersoftening
unit is plumbed inline in the DCU water
supply and will require periodic regeneration and
maintenance.
Other water pre-treatment systems are available
that may be of benefit in treating water
destined for DCUs, depending on the quality of the
supply water. These include sediment pre-filters
that remove suspended solids, activated carbon
filters that reduce organic contaminants, and

108 S. Kumar et al.
kinetic degradation fluxion (KDF) filters that remove
some dissolved metals.
ADA guidelines and recommendations
on dental unit waterlines
General recommendations
1. Use water that meets US Environmental Protection
Agency regulatory standards for drinking
water (i.e.

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