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The Bug Blog
Page updated 9 October 2006
Dental unit waterlines (DUWL) are colonised by bacteria derived from the incoming mains water and to a lesser extent by oral bacteria that enter the waterlines via suck back through the handpiece. In untreated dental units overnight stagnation and infrequent patterns of use result in amplification of the incoming organisms to form a biofilm on the inner surface the DUWL. This effect is further amplified as the DUWL acts as dead leg on the plumbing system. If the units are not drained or flushed, water remains stagnant in the dental unit waterlines for considerable periods of time as most dental units are not in active use for an average of 131 hours per week.
The biofilm lining the inner surface of the waterline, like dental plaque forms rapidly and within a week can be shedding large numbers of bacteria into the water with recorded counts of up to 10 4-6 colony forming units (cfu) /ml. The American Dental Association (ADA) recommends that DUWL should contain £ 200 cfu/ml of aerobic bacteria 1 . Both ADA and the BDA advise that DUWL should comply with drinking water regulations. European drinking water directives are more stringent than the American and require that drinking water should contain £ 100 cfu/ml aerobic bacteria.
In some DUWL opportunistic respiratory pathogens such as Legionellae spp have been recovered. In dental units where this is the case then there is the potential of an occupational risk to the dental team from exposure to contaminated DUWL aerosols 2 . High bacterial counts in the waterlines have been associated in one study with an increased prevalence of occupational asthma 3 .
Likewise vulnerable patients such as the immunocompromised and those persons suffering from chronic respiratory disease, alcoholics, and diabetics may potentially be at increased risk of respiratory infection or colonisation from inhaling contaminated aerosols during dental treatment 4 . Fortunately, the health risks associated with inhaling contaminated DUWL aerosols are thought to be exceedingly low in healthy individuals. UK national surveillance data found no association between dental treatment and Legionnaires' disease. But there is no room for complacency as a dentist in the USA died from Legionnaires' disease as a result of exposure to Legionellae in his surgery's dental unit waterlines.
European Union regulations state that there should be a physical gap or type A air gap separating the dental unit waterlines from the mains water supply in order to prevent back-siphonage of clinical material into the municipal drinking water supply. Compliance with the regulations can be achieved either by installing a pressurized storage tank plumbed with a regulation size air gap or an independent water reservoir bottle to supply water directly to the dental unit waterlines and handpieces.
Under the auspices of the Health and Safety at Work Act 1974 the Health and Safety Commission issued "The control of legionella bacteria in water systems. Approved code of practice and guidance" which came into force in January 2001. In order to comply with their legal duties dentists should identify and assess sources of risk that are likely to encourage the organisms found in DUWL and dental surgery plumbing to multiply and become aerosolized. They should prepare a scheme for preventing and controlling the risk. A brief summary of the guidelines pertaining to dental practice is outlined in table 16. For a full explanation of the guidelines, please consult the Code of Practice, which is available from HSE books.
(Based on HSC (2000). The control of legionella bacteria in water systems. Approved code of practice and guidance.)
Hot water plumbing
Cold water plumbing
Recommended monitoring and recording
Most dental unit waterlines incorporate anti-retraction valves to prevent back-siphonage up stream into the surgery plumbing. Similarly anti-retraction valves or their equivalent design features are an integral component of airturbine handpieces. In the handpieces the valve reduces suck back of fluids from the oral cavity.
Flushing the waterlines for 2 minutes at the start of the day and for 20-30 seconds between patients reduces the bacterial count by approximately 97% but in DUWL where this method is used as the sole means of water quality management flushing is unlikely to provide water of drinking water standard i.e. with a total bacterial count of £ 100 cfu /mL, nor will flushing remove the biofilm. However, in dental units, which are not drained down at night, flushing at the start of the day will help to reduce the bacterial load caused by overnight water stagnation. Flushing between patients helps to prevent cross contamination by removing any suck-back of oral fluids that have bypassed the anti-retraction valve.
The Health and Safety commission recommend daily draining down and cleaning of the waterlines to reduce biofilm build up. The biocide (disinfectant) can be introduced with a pressurized pump or via an independent reservoir bottle.
Independent bottled water systems: are either manufactured as an integral part of the dental unit or can be installed or retro-fitted separately. They have the advantage of bypassing the mains system and avoiding contamination with Legionella spp and other waterborne respiratory pathogens. They should not be filled with tap water as this will introduce opportunistic respiratory bacteria into the DUWL and rapidly lead to biofilm formation.
A variety of different commercially available biocides can be used to improve the quality of water issuing from the DUWL. They can be used intermittently either daily or weekly to disinfect the waterlines according to the manufacturer's instructions or continuously via automated dosing systems that delivers biocide whenever the waterline is used
Products containing one of the following active agents: - hydrogen peroxide (e.g. Dentisept, Sterilex ultra) citric acid (e.g. Alpron), iodine, alcoholic chlorhexidine (Bio Blue), UV, ozone, electrochemically activated water (e.g. Sterilox) will produce water of drinking water standard. Not all products completely remove the biofilm, so regular dosing according to the manufacturer's instructions is required to control the bacterial count. Only use a disinfectant that is recommended as compatible with your particular dental unit as unsuitable products can damage internal components resulting in leaks. Water of drinking water standard or better can also be achieved by the fitting of p oint of use disposable microbial filter immediately adjacent to the handpiece . This prevents suspended bacteria entering the handpiece, but does not remove or reduce the biofilm upstream in the waterlines.
Sterile water delivered in a separate delivery system is required for minor oral surgery (MOS) procedures, periodontal and implant surgery . A variety of delivery systems are commercially available incorporating pump and bagged sterile solutions or fully autoclavable assemblies of reservoirs and hand piece tubing to be used with sterile water.
Even when sterile water is used in the reservoir organisms from the hands, the environment or sucked back through the DUWLs can contaminate the reservoir bottle. The organisms rapidly divide in the warm environment in the surgery to form a biofilm. Therefore, reservoir bottles should be disinfected before refilling to prevent this happening.
At the end of the working day the water should be drained down from the waterlines. The reservoir bottle should be disinfected (e.g. with diluted 1: 10 [approx.0.5%] household bleach solution or with the approximately equivalent 5,000 ppm solution of Haz tabs or Precept) or with a commercial product (for example: Alpron, Dentisept, Sterilex Ultra, Bio Blue or Sterilox), rinsed with sterile water and stored dry and inverted overnight. This will prevent biofilm formation in both the bottle and the waterlines. Only use a product that is compatible with your dental chair