Biofilms are emerging as an increasing problem as medical technology advances. Dental practice is no exception and interest in the role of biofilms within dental units as a possible source of cross-infection is intensifying. It is difficult to quantitate the risks associated with aerosolised bacteria for the majority of patients seen in general practice. However, it seems prudent to eliminate this source of infection during treatment of compromised patients. This article attempts to provide a brief overview of current concepts and problems in this area of infection control

Bagg, J.

Burke, F.T.

Hood, J.

Smith, A.J.

English

Enlighten: Publications

12 BRITISH DENTAL JOURNAL, VOLUME 186, NO. 1, JANUARY 9 1999
PRACTICE
cross-infection control
Dental unit water lines are an integral
part of dental surgery equipment, sup-
plying water as a coolant, primarily for
air turbines and ultrasonic scalers. This
operation inevitably leads to a fine spray
which is then inhaled by both the patient
and the dental staff. The large volumes
of water produced are mostly removed
by high volume aspiration although
some is almost certainly swallowed by
the patient. Several reports1–7 and our
own unpublished observations have
suggested that microbial contamination
of dental unit water lines is a common
event. This article discusses the possible
implications of these findings.
The water quality regulations provide
a legal definition of the term ‘whole-
someness’ when applied to potable
water and include a number of chemical
and microbiological parameters.8
Water-borne pathogens such as
Legionella species and coliforms, eg E.
coli, should always be absent from
potable water supplies. The usual stan-
dard for the number of colony forming
units (cfu) accepted in potable water is
less than 10 cfu/ml at 37°C (or less than
100 cfu/ml at 22°C).8
It has been known for more than 30
years that water from dental lines can be
heavily contaminated with microorgan-
isms.9 These findings of high levels of
microbial contamination, including
more recently Legionella species, have
been confirmed in general practice and
hospital units in our own unpublished
studies and by other workers.1–7 One of
the major concerns is contamination of
the water supply with Legionella species,
in particular L. pneumophilia, the
causative agent of Legionella pneumo-
nia (Legionnaires’ disease and non-
pneumonic legionellosis (Pontiac
fever)). Transmission of Legionellae to
humans occurs when water containing
the bacterium is aerosolised in droplets
and inhaled. The risk of infection
depends not only on the size and
amount of the inhaled inoculum, but
also on the susceptibility of the individ-
ual host. It has been suggested that the
presence of Legionella species within
dental line water may contribute to
lower respiratory tract infections
among dental personnel.10 It has been
reported that dental staff have a higher
prevalence of Legionella antibodies
compared to the general public,11–13
suggesting that aerosols generated by
dental equipment are a possible source
of infection. This hypothesis may be
supported by the recent case of a Cali-
fornia dentist who died from Legionel-
losis. Subsequent investigations showed
high levels of the same Legionella
species from his dental units1 although
molecular confirmation of source and
infecting strain was lacking.
Biofilms
The source of bacterial contamination
within the dental unit water supply is
thought to be because of microcolonies
of proliferating bacteria, fungi and pro-
1Lecturer & Honorary Specialist Registrar in Oral
Microbiology, 3Senior Lecturer & Honorary
Consultant in Oral Microbiology, Infection Research
Group, Glasgow Dental Hospital & School, 378
Sauchiehall Street, Glasgow G2 3JZ; 2Consultant
Microbiologist, Bacteriology Department, Glasgow
Royal Infirmary, Castle Street, Glasgow G4 0SF;
4Professor of Primary Dental Care, Dental Primary
Care Unit, Glasgow Dental Hospital & School, 378
Sauchiehall Street, Glasgow G2 3JZ
REFEREED PAPER
Received 12.02.98; accepted 08.07.98
© British Dental Journal 1999; 186: 12–14
Water, water everywhere but not a
drop to drink?
A. J. Smith,1 J. Hood,2 J. Bagg,3 and F. T. Burke,4
Biofilms are emerging as an increasing problem as medical
technology advances. Dental practice is no exception and
interest in the role of biofilms within dental units as a possible
source of cross-infection is intensifying. It is difficult to
quantitate the risks associated with aerosolised bacteria for
the majority of patients seen in general practice. However, it
seems prudent to eliminate this source of infection during
treatment of compromised patients. This article attempts to
provide a brief overview of current concepts and problems in
this area of infection control.
The source of bacterial
contamination within
the dental unit water
supply is thought to
be because of
microcolonies of
proliferating bacteria,
fungi and protozoa on
the inner surface of
the water lines
In brief
l A review of the clinical implications of
microbial contamination of dental unit
water lines
l Highlights the inevitable failure of the
chemical disinfection of dental unit 
water lines
l In the absence of new independent
research, we advise that the BDA
guidelines are followed to reduce the
clinical implications of microbial
contamination of dental unit water lines
l The solutions to the problem of microbial
contamination in dental unit water lines
lies in closer links between dental unit
manufacturers, dentists and
microbiologists to redesign the water
supply lines to discourage biofilm
development
BRITISH DENTAL JOURNAL, VOLUME 186, NO. 1, JANUARY 9 1999 13
PRACTICE
cross-infection control
tozoa on the inner surface of the water
lines. The organisms are embedded in a
matrix of extracellular polymeric sub-
stances forming a biofilm. Biofilms are
important because they protect organ-
isms from the effects of heat and chemi-
cals thus reducing their susceptibility to
disinfection processes. The contact time
required to produce a particular level of
killing of organisms in the biofilm by
chlorine may be hundreds or even thou-
sands of times greater than that
required to produce an equivalent kill
for the organisms suspended in water.
Thus, it is possible for organisms in
biofilms to survive and grow even when
the water contains residual chlorine at
the concentrations employed in drink-
ing water. This reduction in disinfection
efficiency is caused by several factors
inherent in the structure of the biofilm
including reduced penetration, absorp-
tion or changes in bacterial structure.
Disinfection failures may occur through
neglect of this particular facet of infec-
tion control. 
There have been several attempts to
address this problem including auto-
claving of handpieces, handpiece
replacement between patients, flushing
of the unit prior to use, ‘anti-contami-
nation’ devices to prevent retrograde
aspiration of oral secretions into the
water supply line, connection to a sepa-
rate water supply (eg connection to bot-
tles of distilled water), chemical
disinfection of waterlines, ultra-violet
radiation disinfection and the use of in-
line water filters. These have been devel-
oped and implemented in many dental
practices.3,10,14–16 The most commonly
used procedure of flushing the hand-
piece with water prior to use may lower
bacterial counts6,17 but high levels of
microbial contamination can still per-
sist.6,17,18 Nevertheless, the British
Dental Association (BDA)19 and Cen-
ters for Disease Control (CDC)20 rec-
ommend that all water lines should be
allowed to run and discharge water for
several minutes at the beginning of each
day and for a shorter interval between
patient appointments. Research to date
tified,23–25 therefore dental exposure
may represent an unrecognised element
in the history of previous cases. Fur-
thermore, non-pneumonic Legionel-
losis of the Pontiac fever type may occur
in dental personnel or their patients and
cause symptoms clinically indistin-
guishable from other flu-like episodes.
More research needed
Therefore, the implications of heavily
contaminated water from dental units
are not fully understood and very little
research has been undertaken in this
area. Workers have reported that
extended exposure to aerosols from
dental handpieces, air/water syringes
and ultrasonic scalers may have been
responsible for significantly elevated
antibody titres against Legionella
species in dental clinic personnel, when
compared with known negative con-
trols.11,13 In view of the extent of expo-
sure of patients and staff to Legionella
species from a dental source it is sur-
prising that no definitive clinical associ-
ations with community acquired
pneumonia have emerged thus far.2,26
Where contaminated water, eg potable
or nebulised water, has been involved as
a source of infection, such as in hospi-
tals, most patients infected were known
to be immunosuppressed. With a
greater proportion of the general public
living longer, conditions known to
compromise host resistance will
become more common in the commu-
nity and in return this problem may
become more acute in general dental
practice.
Conclusion
Various studies have identified signifi-
cant microbial contamination of dental
unit waters, including Legionella species.
At present, there is little epidemiological
evidence that this constitutes a significant
risk of infection, though collecting the
appropriate data is difficult. Among
immunocompromised individuals there
is undoubtedly a potential for infection
by this route. Therefore, every effort
should be made to ensure that potable
suggests that which ever method is
used, microbial recolonisation of the
waterline is inevitable and repeat disin-
fecting treatments are necessary. We
suspect that this is mainly because of
the complex design of dental chair
equipment, resulting in the stagnation
of water within the equipment lines
where bacteria, including Legionella
species could proliferate within a
biofilm.2
A public health problem?
While many investigations have con-
firmed the types of microbial contami-
nation that can accumulate in modern
dental equipment,21 evidence is lacking
that links this to a major public health
problem. There may be several reasons
for this. Most of the organisms charac-
terised from contaminated dental and
medical water sources are opportunistic
pathogens with low virulence potentials
and probably cause no problems when
immuno-competent hosts are exposed
to them. Secondly, the incidence of
legionella community acquired pneu-
monia varies from 2–15% of all com-
munity acquired pneumonias that
require hospitalisation.22 The sources
of most cases of community-acquired
pneumonic legionellosis are never iden-
Non-pneumonic
Legionellosis of the
Pontiac fever type may
occur in dental
personnel or their
patients and cause
symptoms clinically
indistinguishable
from other flu-like
episodes
14 BRITISH DENTAL JOURNAL, VOLUME 186, NO. 1, JANUARY 9 1999
PRACTICE
cross-infection control
quality water emerges from dental unit
handpieces. This should firstly involve
co-operation between dental unit manu-
facturers, dental practitioners and micro-
biologists to produce equipment that
does not encourage the build up of
biofilms. The importance of the engi-
neering and design issues are highlighted
in a recent publication which indicates
that certain models of dental units are
more prone to contamination with
Legionella species.27 Secondly, a reason-
able protocol for disinfecting and moni-
toring the water supply in busy dental
practices is required urgently, so water
used for dental patient treatment satisfies
accepted, safe public health standards. In
view of the paucity of reliable data on dis-
infection of dental units, we feel it neces-
sary to stand by the BDA and CDC
statements19, 20 on the control of micro-
bial contamination in dental unit water
lines. However, we trust as a result of
ongoing research and evaluation of novel
decontamination regimes, firmer recom-
mendations should be possible in the
near future.
1 Atlas R M, Williams J F, Huntington M-K.
Legionella contamination of dental unit
waters. Appl Environ Microbiol 1995; 61:
1208-1213.
2 Pankhurst C L, Philpott-Howard J N. The
microbiological quality of water in dental
chair units. J Hosp Infect 1993; 23: 167-174.
3 Pankhurst C L, Philpott-Howard J N , Hewitt
J, Casewell M W. The efficacy of chlorination
and filtration in the control and erradication
of Legionella from dental chair water
systems. J Hosp Infect 1990; 16: 9-18.
4 Whitehouse R L S, Peters G, Lindte J, Lilge C.
Influence of biofilms on microbial
contamination in dental unit water. J Dent
1991; 19: 290-295.
5 Williams H N, Kelley J, Folineo D, Williams
G C, Hawley C L, Sibiski J. Assessing
microbial contamination in clean water
dental units and compliance with
disinfection protocol. J Am Dent Assoc 1994;
125: 1205-1211.
6 Williams J F, Johnston A M, Johnson B,
Huntington M K, Mackenzie C D Microbial
contamination of dental unit water lines:
prevalence, intensity and microbial char-
acteristics. J Am Dent Assoc 1993; 124: 59-65.
7 Williams H N, Paszko-Kolva C, Shahamat M,
Palmer C, Pettis C, Kelley T. Molecular
techniques reveal high prevalence of
Legionella in dental units. JADA 1996; 127:
1188-1193. 
8 The microbiology of water 1994. Part 1 —
drinking water. HMSO publications. 
9 Blake G C. The incidence and control of
bacterial infection of dental units and
ultrasonic scalers. Br Dent J 1963; 115: 413-416.
10 Paszko-Kovla C. Risk of infection from
dental handpieces. ASM News 1991; 57: 287.
11 Fotos P G, Westfall H N, Synder I S, Miller R
W, Mutchler BM. Prevalence of Legionella
specific IgG and IgM antibody in a dental clinic
population. J Dent Res 1985; 64: 1328-1385.
12 Paszko-Kolva C C, Shahamat M, Keiser J,
Colwell R R. Prevalence of antibodies against
Legionella species in healthy and patient
populations. In JM Barbaree, RF Brieman
and AP Dufour (eds) Legionella: current
status and emerging prospectives. pp24-26.
Washington DC: American Society for
Microbiology, 1993.
13 Reinthaler F F, Mascher F, Stunzer D.
Serological examination for antibodies
against Legionella species in dental
personnel. J Dent Res 1988; 67: 
942-943.
14 Martin M V. The significance of bacterial
contamination of dental unit water systems.
Br Dent J 1987; 163: 152-154.
15 Scheid R C, Kim C K, Bright J S, Whitely M
S, Rosen S. Reduction of microbes in
handpieces by flushing before use. J Am Dent
Assoc 1982; 105: 658-660.
16 Scheid R C, Rosen S, Beck F M. Reduction of
CFUs in high-speed handpiece water lines
over time. Clin Prev Dent 1990; 12: 9-12.
17 Karpay R I, Puttaiah R, Mills S E, Plamondon 
T J, Dove S B, Levine U. Efficacy of flushing
dental units for different time periods. J Dent
Res 1997; 76: (abstract) 3366. 
18 Gross A, Devine M J, Cutright D E. Microbial
contamination of dental units and ultrasonic
scalers. J Periodontol 1976; 47: 670-673.
19 British Dental Association. Infection Control
in Dentistry: Advice Sheet 1996; A12: 7.
20 Centers for Disease Control. Recommended
infection control practices for dentistry.
MMWR 1993; 42: 1-12.
21 Molinari J A. Practical infection control for
the 1990’s. Applying science to government
regu-lations. J Am Dent Assoc 1994; 125:
1189-1197.
22 Muder R R, Yu V L, Fang G D. Community
acquired legionnaires disease. Semin Respir
Infect 1989; 4: 32-29.
23 Anonymous. Legionnaires disease in
England and Wales, 1997. Com Dis Rep
Weekly 1997; 7: 321-324.
24 Jospeh C A, Harrison T G, Ilijic-Car D,
Bartlett C L R. Legionnaires disease
surveillance: England and Wales, 1996. CDR
Review 1997; 7: 153-159.
25 Oppenheim B A, Sefton A M, Gill O N, et al.
Widespread Legionella pneumophilia
contamination of dental stations in a dental
school without apparent human infection.
Epidemiol Infect 1987; 99: 159-166.
26 Stout J E, Yu V L, Muraca J, Joly N, Troup N,
Tompkins L S. Potable water as a cause of
spo-radic cases of community-acquired
Legionaires disease. N Engl J Med 1992; 326:
151-155.
27 Challacombe S J, Fernandes L L. Detecting
Legionella pneumophila in water systems: A
comparison of various dental units. J Am
Dent Assoc 1995; 126: 603-608.
It has been suggested
that the presence of
Legionella species
within dental line
water may contribute
to lower respiratory
tract infections among
dental personnel


Water, water everywhere but not a drop to drink?

Enlighten

12 BRITISH DENTAL JOURNAL, VOLUME 186, NO. 1, JANUARY 9 1999PRACTICEcross-infection controlDental unit water lines are an integralpart of dental surgery equipment, sup-plying water as a coolant, primarily forair turbines and ultrasonic scalers. Thisoperation inevitably leads to a fine spraywhich is then inhaled by both the patientand the dental staff. The large volumesof water produced are mostly removedby high volume aspiration althoughsome is almost certainly swallowed bythe patient. Several reports1–7 and ourown unpublished observations havesuggested that microbial contaminationof dental unit water lines is a commonevent. This article discusses the possibleimplications of these findings.The water quality regulations providea legal definition of the term ‘whole-someness’ when applied to potablewater and include a number of chemicaland microbiological parameters.8Water-borne pathogens such asLegionella species and coliforms, eg E.coli, should always be absent frompotable water supplies. The usual stan-dard for the number of colony formingunits (cfu) accepted in potable water isless than 10 cfu/ml at 37°C (or less than100 cfu/ml at 22°C).8It has been known for more than 30years that water from dental lines can beheavily contaminated with microorgan-isms.9 These findings of high levels ofmicrobial contamination, includingmore recently Legionella species, havebeen confirmed in general practice andhospital units in our own unpublishedstudies and by other workers.1–7 One ofthe major concerns is contamination ofthe water supply with Legionella species,in particular L. pneumophilia, thecausative agent of Legionella pneumo-nia (Legionnaires’ disease and non-pneumonic legionellosis (Pontiacfever)). Transmission of Legionellae tohumans occurs when water containingthe bacterium is aerosolised in dropletsand inhaled. The risk of infectiondepends not only on the size andamount of the inhaled inoculum, butalso on the susceptibility of the individ-ual host. It has been suggested that thepresence of Legionella species withindental line water may contribute tolower respiratory tract infectionsamong dental personnel.10 It has beenreported that dental staff have a higherprevalence of Legionella antibodiescompared to the general public,11–13suggesting that aerosols generated bydental equipment are a possible sourceof infection. This hypothesis may besupported by the recent case of a Cali-fornia dentist who died from Legionel-losis. Subsequent investigations showedhigh levels of the same Legionellaspecies from his dental units1 althoughmolecular confirmation of source andinfecting strain was lacking.BiofilmsThe source of bacterial contaminationwithin the dental unit water supply isthought to be because of microcoloniesof proliferating bacteria, fungi and pro-1Lecturer & Honorary Specialist Registrar in OralMicrobiology, 3Senior Lecturer & HonoraryConsultant in Oral Microbiology, Infection ResearchGroup, Glasgow Dental Hospital & School, 378Sauchiehall Street, Glasgow G2 3JZ; 2ConsultantMicrobiologist, Bacteriology Department, GlasgowRoyal Infirmary, Castle Street, Glasgow G4 0SF;4Professor of Primary Dental Care, Dental PrimaryCare Unit, Glasgow Dental Hospital & School, 378Sauchiehall Street, Glasgow G2 3JZREFEREED PAPERReceived 12.02.98; accepted 08.07.98© British Dental Journal 1999; 186: 12–14Water, water everywhere but not adrop to drink?A. J. Smith,1 J. Hood,2 J. Bagg,3 and F. T. Burke,4Biofilms are emerging as an increasing problem as medicaltechnology advances. Dental practice is no exception andinterest in the role of biofilms within dental units as a possiblesource of cross-infection is intensifying. It is difficult toquantitate the risks associated with aerosolised bacteria forthe majority of patients seen in general practice. However, itseems prudent to eliminate this source of infection duringtreatment of compromised patients. This article attempts toprovide a brief overview of current concepts and problems inthis area of infection control.The source of bacterialcontamination withinthe dental unit watersupply is thought tobe because ofmicrocolonies ofproliferating bacteria,fungi and protozoa onthe inner surface ofthe water linesIn briefl A review of the clinical implications ofmicrobial contamination of dental unitwater linesl Highlights the inevitable failure of thechemical disinfection of dental unit water linesl In the absence of new independentresearch, we advise that the BDAguidelines are followed to reduce theclinical implications of microbialcontamination of dental unit water linesl The solutions to the problem of microbialcontamination in dental unit water lineslies in closer links between dental unitmanufacturers, dentists andmicrobiologists to redesign the watersupply lines to discourage biofilmdevelopmentBRITISH DENTAL JOURNAL, VOLUME 186, NO. 1, JANUARY 9 1999 13PRACTICEcross-infection controltozoa on the inner surface of the waterlines. The organisms are embedded in amatrix of extracellular polymeric sub-stances forming a biofilm. Biofilms areimportant because they protect organ-isms from the effects of heat and chemi-cals thus reducing their susceptibility todisinfection processes. The contact timerequired to produce a particular level ofkilling of organisms in the biofilm bychlorine may be hundreds or even thou-sands of times greater than thatrequired to produce an equivalent killfor the organisms suspended in water.Thus, it is possible for organisms inbiofilms to survive and grow even whenthe water contains residual chlorine atthe concentrations employed in drink-ing water. This reduction in disinfectionefficiency is caused by several factorsinherent in the structure of the biofilmincluding reduced penetration, absorp-tion or changes in bacterial structure.Disinfection failures may occur throughneglect of this particular facet of infec-tion control. There have been several attempts toaddress this problem including auto-claving of handpieces, handpiecereplacement between patients, flushingof the unit prior to use, ‘anti-contami-nation’ devices to prevent retrogradeaspiration of oral secretions into thewater supply line, connection to a sepa-rate water supply (eg connection to bot-tles of distilled water), chemicaldisinfection of waterlines, ultra-violetradiation disinfection and the use of in-line water filters. These have been devel-oped and implemented in many dentalpractices.3,10,14–16 The most commonlyused procedure of flushing the hand-piece with water prior to use may lowerbacterial counts6,17 but high levels ofmicrobial contamination can still per-sist.6,17,18 Nevertheless, the BritishDental Association (BDA)19 and Cen-ters for Disease Control (CDC)20 rec-ommend that all water lines should beallowed to run and discharge water forseveral minutes at the beginning of eachday and for a shorter interval betweenpatient appointments. Research to datetified,23–25 therefore dental exposuremay represent an unrecognised elementin the history of previous cases. Fur-thermore, non-pneumonic Legionel-losis of the Pontiac fever type may occurin dental personnel or their patients andcause symptoms clinically indistin-guishable from other flu-like episodes.More research neededTherefore, the implications of heavilycontaminated water from dental unitsare not fully understood and very littleresearch has been undertaken in thisarea. Workers have reported thatextended exposure to aerosols fromdental handpieces, air/water syringesand ultrasonic scalers may have beenresponsible for significantly elevatedantibody titres against Legionellaspecies in dental clinic personnel, whencompared with known negative con-trols.11,13 In view of the extent of expo-sure of patients and staff to Legionellaspecies from a dental source it is sur-prising that no definitive clinical associ-ations with community acquiredpneumonia have emerged thus far.2,26Where contaminated water, eg potableor nebulised water, has been involved asa source of infection, such as in hospi-tals, most patients infected were knownto be immunosuppressed. With agreater proportion of the general publicliving longer, conditions known tocompromise host resistance willbecome more common in the commu-nity and in return this problem maybecome more acute in general dentalpractice.ConclusionVarious studies have identified signifi-cant microbial contamination of dentalunit waters, including Legionella species.At present, there is little epidemiologicalevidence that this constitutes a significantrisk of infection, though collecting theappropriate data is difficult. Amongimmunocompromised individuals thereis undoubtedly a potential for infectionby this route. Therefore, every effortshould be made to ensure that potablesuggests that which ever method isused, microbial recolonisation of thewaterline is inevitable and repeat disin-fecting treatments are necessary. Wesuspect that this is mainly because ofthe complex design of dental chairequipment, resulting in the stagnationof water within the equipment lineswhere bacteria, including Legionellaspecies could proliferate within abiofilm.2A public health problem?While many investigations have con-firmed the types of microbial contami-nation that can accumulate in moderndental equipment,21 evidence is lackingthat links this to a major public healthproblem. There may be several reasonsfor this. Most of the organisms charac-terised from contaminated dental andmedical water sources are opportunisticpathogens with low virulence potentialsand probably cause no problems whenimmuno-competent hosts are exposedto them. Secondly, the incidence oflegionella community acquired pneu-monia varies from 2–15% of all com-munity acquired pneumonias thatrequire hospitalisation.22 The sourcesof most cases of community-acquiredpneumonic legionellosis are never iden-Non-pneumonicLegionellosis of thePontiac fever type mayoccur in dentalpersonnel or theirpatients and causesymptoms clinicallyindistinguishablefrom other flu-likeepisodes14 BRITISH DENTAL JOURNAL, VOLUME 186, NO. 1, JANUARY 9 1999PRACTICEcross-infection controlquality water emerges from dental unithandpieces. This should firstly involveco-operation between dental unit manu-facturers, dental practitioners and micro-biologists to produce equipment thatdoes not encourage the build up ofbiofilms. The importance of the engi-neering and design issues are highlightedin a recent publication which indicatesthat certain models of dental units aremore prone to contamination withLegionella species.27 Secondly, a reason-able protocol for disinfecting and moni-toring the water supply in busy dentalpractices is required urgently, so waterused for dental patient treatment satisfiesaccepted, safe public health standards. Inview of the paucity of reliable data on dis-infection of dental units, we feel it neces-sary to stand by the BDA and CDCstatements19, 20 on the control of micro-bial contamination in dental unit waterlines. However, we trust as a result ofongoing research and evaluation of noveldecontamination regimes, firmer recom-mendations should be possible in thenear future.1 Atlas R M, Williams J F, Huntington M-K.Legionella contamination of dental unitwaters. Appl Environ Microbiol 1995; 61:1208-1213.2 Pankhurst C L, Philpott-Howard J N. Themicrobiological quality of water in dentalchair units. J Hosp Infect 1993; 23: 167-174.3 Pankhurst C L, Philpott-Howard J N , HewittJ, Casewell M W. The efficacy of chlorinationand filtration in the control and erradicationof Legionella from dental chair watersystems. J Hosp Infect 1990; 16: 9-18.4 Whitehouse R L S, Peters G, Lindte J, Lilge C.Influence of biofilms on microbialcontamination in dental unit water. J Dent1991; 19: 290-295.5 Williams H N, Kelley J, Folineo D, WilliamsG C, Hawley C L, Sibiski J. Assessingmicrobial contamination in clean waterdental units and compliance withdisinfection protocol. J Am Dent Assoc 1994;125: 1205-1211.6 Williams J F, Johnston A M, Johnson B,Huntington M K, Mackenzie C D Microbialcontamination of dental unit water lines:prevalence, intensity and microbial char-acteristics. J Am Dent Assoc 1993; 124: 59-65.7 Williams H N, Paszko-Kolva C, Shahamat M,Palmer C, Pettis C, Kelley T. Moleculartechniques reveal high prevalence ofLegionella in dental units. JADA 1996; 127:1188-1193. 8 The microbiology of water 1994. Part 1 —drinking water. HMSO publications. 9 Blake G C. The incidence and control ofbacterial infection of dental units andultrasonic scalers. Br Dent J 1963; 115: 413-416.10 Paszko-Kovla C. Risk of infection fromdental handpieces. ASM News 1991; 57: 287.11 Fotos P G, Westfall H N, Synder I S, Miller RW, Mutchler BM. Prevalence of Legionellaspecific IgG and IgM antibody in a dental clinicpopulation. J Dent Res 1985; 64: 1328-1385.12 Paszko-Kolva C C, Shahamat M, Keiser J,Colwell R R. Prevalence of antibodies againstLegionella species in healthy and patientpopulations. In JM Barbaree, RF Briemanand AP Dufour (eds) Legionella: currentstatus and emerging prospectives. pp24-26.Washington DC: American Society forMicrobiology, 1993.13 Reinthaler F F, Mascher F, Stunzer D.Serological examination for antibodiesagainst Legionella species in dentalpersonnel. J Dent Res 1988; 67: 942-943.14 Martin M V. The significance of bacterialcontamination of dental unit water systems.Br Dent J 1987; 163: 152-154.15 Scheid R C, Kim C K, Bright J S, Whitely MS, Rosen S. Reduction of microbes inhandpieces by flushing before use. J Am DentAssoc 1982; 105: 658-660.16 Scheid R C, Rosen S, Beck F M. Reduction ofCFUs in high-speed handpiece water linesover time. Clin Prev Dent 1990; 12: 9-12.17 Karpay R I, Puttaiah R, Mills S E, Plamondon T J, Dove S B, Levine U. Efficacy of flushingdental units for different time periods. J DentRes 1997; 76: (abstract) 3366. 18 Gross A, Devine M J, Cutright D E. Microbialcontamination of dental units and ultrasonicscalers. J Periodontol 1976; 47: 670-673.19 British Dental Association. Infection Controlin Dentistry: Advice Sheet 1996; A12: 7.20 Centers for Disease Control. Recommendedinfection control practices for dentistry.MMWR 1993; 42: 1-12.21 Molinari J A. Practical infection control forthe 1990’s. 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