Dental unit waterlines - water quality in dental practice

This first article, in a two-part series on water quality in dental practice by Professor Laurence Walsh on behalf of the Infection Control Subcommittee, provides useful guidance for all practitioners managing water quality in their practices.

The second part is published under the article Water quality in dental practice – Understanding water quality. 

Biofilms within dental unit waterlines (DUWL) have been linked to respiratory infections and wound infections, involving pathogens such as Pseudomonas aeruginosa, Legionella pneumophila and nontuberculous mycobacteria. These bacteria are widespread in natural sources of freshwater and are found commonly in manmade water systems. If tap water with no additional chemical additives is fed into a dental chair through a mains water connection or via a bottle, there will be rapid development of biofilm in the waterlines. Even with chemical treatments, growth of biofilms will occur more rapidly when there is stagnation (from periods of non-use such as weekends and holidays), and when the ambient temperature is higher.

Dental units connected directly to mains water must have backflow prevention devices, such as reduced pressure zone (RPZ) valves, to prevent water from the dental practice running back into the reticulated water system.

Many dental chairs now use self-contained bottled water systems, with the bottle located internally or externally. It is essential to check the dental chair manufacturer’s instructions regarding the correct water type and the appropriate chemical additives. When a water bottle contains a continuously acting antimicrobial agent (e.g. silver, iodine, chloramine), the bottle can be left on the chair overnight. However, if any visual contamination is present, the bottle must be removed for thorough washing.

For dental chairs that use short acting antimicrobial agents (e.g. ozone, hydrogen peroxide), the manufacturer may advise removing the water bottle at the end of the day, then flushing the lines with water, purging them with compressed air, and keeping the bottle stored inverted and dry on the bench overnight. As before, if visual contamination is present, remove the bottle for thorough washing.

Regardless of the system used for biofilm control, ADA recommends flushing all waterlines at the start of the working day for two minutes (with no handpieces attached).

A range of chemical agents are available for biofilm prevention and control, including hypochlorites, chloramine T, peroxides, ozonation, silver and iodine. Systems for delivering these agents include external dosing, internal dosing, tablets, liquid dosing, and slow release collection straws. If using tablets, remember that it is necessary to wait two minutes for the tablets to fully dissolve before using the dental unit.

There are special products made for long term ‘mothballing’ of low use dental units, to prevent biofilm growth over several months. These are left sitting in the waterlines, and are flushed out before the chair is brought back into service.

MICROBIAL MONITORING

Commercial tests can be used to assess bacterial levels in water coming from DUWL. These measure the levels of heterotrophic microorganisms using a nutrient agar. A typical test involves incubation at room temperature for 3-5 days, after which colonies are counted. Ideally, there should also be a second sample incubated at body temperature for two days. However, this requires an appropriate incubator.

The objective is to keep bacterial numbers as low as possible. The ADA recommended threshold is no more than 500 colony forming units per mL (CFU/mL) for non-surgical dental procedures since this is a widely used international limit for safe drinking water. When treating medically compromised patients, it is recommended that the water from the dental unit waterlines contain less than 200 CFU/mL. The specimen container used to collect the water sample must be sterile. If there is chlorine present, a small amount of sodium thiosulfate (X-ray film fixer) should be added to the sample to neutralise any residual chlorine.

Frequency of water testing depends on the test results. A starting point would be to assess exit water from a dental chair as a baseline. If results are below 200 CFU/mL, then re-test after three months, and if they remain low, then extend that to every three or six months. With a new dental chair installation, it is advisable to test exit water quality on installation, then three months later. Annual testing would be a workable recommendation for dental chairs in most clinics that are using chemical treatment agents routinely, on the proviso that any indications of water that is turbid or cloudy entering or leaving the dental unit should trigger further testing.

If test results indicate a bacteria level greater than 200 CFU/mL, then undertake a shock treatment to reduce bacterial contamination from the waterlines in that dental chair. Whenever applying a shock treatment, be sure to adhere to the product instructions provided by the manufacturer, as well as the instructions for the dental chair, since not all shock treatment products are compatible with all brands of dental chair. For example, only some control blocks can withstand sodium hypochlorite shock treatment without suffering corrosion. After a shock treatment, flush the agent out and continue normal chemical treatments, then re-test levels of microorganisms. Some types of additives to water bottles come with recommendations for regular dental unit water monitoring and shock treatment.

 

Disclaimer

Insurance issued by Guild Insurance Limited ABN 55 004 538 863, AFS License No. 233791 and subject to terms, conditions and exclusions. This article contains information of a general nature only, and is not intended to constitute the provision of legal advice.  Republished from the ADA News Bulletin, December 2017 No 471 and February 2018 No 472 with the kind permission of the Australian Dental Association. Guild Insurance supports ADA through the payment of referral fees. Please refer to the policy wording and policy schedule for details. For more information call 1800 810 213.

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