Dental Unit Water Lines (DUWL)

A variety of organisms are found in dental water lines, including bacteria, fungi and protozoa.

Dental Unit Waterlines

Dental Unit Waterlines (DUWL) can be contaminated by microbes coming from three different sources:

  • Incoming mains water which is a possible source of legionella and mycobacteria spp.
  • Suck-back of oral bacteria via the dental handpiece.
  • From hands and the surgery environment during filling and handling of the self-contained water bottles. 

Key Disease Causing Microbes in DUWL

A variety of organisms are found in DUWL including bacteria, fungi and protozoa. Of particular concern to health are the respiratory disease causing bacteria such as legionella spp, atypical mycobacterium spp and pseudomonas species.

The high speed rotation of the airturbine aerosolises the DUWL microbes providing a route of transmission particularly for respiratory bacteria.

Contaminated aerosols can be inhaled (portal of entry described in the chain of infection) by both the dental team and patients during treatment.

The dental team is exposed to contaminated aerosols on a daily basis over the long term, and patients are exposed in the short term during treatments.

The overall microbial quality is also of importance as high levels of bacteria in DUWL have been associated with occupational asthma in dentists.

As patients, including those who may be of increased susceptibility to legionella, are likely to be exposed to aerosols during the course of their dental treatment, and the water which produces that aerosol is likely to be above 20°C, it is essential that the biofilm and bacteria including legionella are not allowed to proliferate in the water system.

Biofilm Formation in the Dental Unit Waterlines

In DUWL that are not treated with a biocide such as disinfectant, Sterilox or a UV light system, the contaminating bacteria are able to multiply and form a biofilm on the inner surface of the DUWL.

Biofilms form rapidly, and within a week start shedding large numbers of bacteria into the waterline.

Under the Health and Safety Act and associated regulations e.g. The Control of Substances Hazardous to Health (COSHH) Regulations 2002 (as amended), a risk assessment must be carried out in order to identify favourable conditions for bacteria proliferation within the water systems. A combination of factors promote biofilm formation:

  • DUWL are made of micro-bore tubing that has a large surface area to volume ratio.
  • Overnight stagnation.
  • Relatively low levels of water usage.
  • When not in use Dental Unit Waterlines (DUWL) are a dead leg on the plumbing system.
  • Unused or sporadically used outlets.

DUWL Quality and Management

Water quality in dental premises is regulated under the ‘Code of Practice: Legionnaires disease’, ‘The Control of Legionella Bacteria in Water Systems (L8)’, ‘HTM 01-05’ (England & Northern Ireland) and ‘WHTM 01-05’ (Wales), and ‘HTM 04-01’.

Water supply to dental unit waterlines (DUWL) should be of drinking water quality with a total viable count (TVC) expected to be between 100 and 200 cfu/ml of bacteria.

A separate supply of sterile water or saline should be used to cool rotary instruments used during minor oral surgery in order to prevent surgical wound infections.

As DUWL can be contaminated via three different routes, a variety of methods are required to prevent contamination and biofilm formation.

Methods of Maintaining Water Quality

In order to prevent back-siphonage of clinical material into the municipal water supply, all dental equipment that is supplied by mains water must have a ‘Type A’ air gap separating it from the mains water.

A legionella risk assessment should be carried out on the DUWL.

The following combination of methods are used to maintain drinking water quality in the DUWL:

  • Flush the DUWL at the start of every day for two minutes, and between patients for 20-30 seconds. Manufacturer guidelines should be followed for the end of day procedure. Flushing reduces the number of free floating bacteria but has no impact on biofilm formation. Therefore flushing is insufficient on its own to control the bacterial count in the DUWL.
  • Use continuous or short purges of disinfection with a biocide (daily or weekly according to the manufacturer’s instructions). Only use a product that is compatible with your dental chair.
  • Service and maintain dental handpiece anti-retraction valves and DUWL waterlines check valves.
  • Use distilled or reverse osmosis water in the self-contained water bottle. Tap water is to be avoided as this will introduce respiratory bacteria into the DUWL.
  • Disinfect reservoir bottles with a biocide before refilling to prevent biofilm formation in both the bottle and the waterlines.
  • Drain down the DUWL at the end of the day. Self-contained water bottles should be removed, disinfected with a compatible biocide, flushed with distilled or reverse osmosis water and left open to air dry. Store inverted overnight. If you are using a system where the manufacturers don’t recommend the removal of the bottle then this advice should be followed.
  • Flush dental chairs that are not used, or sporadically used, at least twice a week.

Where a control scheme is put into place it is essential that the control measures are validated to ensure that they are effective and that ongoing monitoring takes place to verify their continued effectiveness. Monitoring for microbiological parameters such as the heterotroph count can be useful for validation.

In accordance to HTM 01-05 and WHTM 01-05 “All microbiological measurements should be by approved methods and / or be carried out by United Kingdom Accreditation Service (UKAS) accredited laboratories. Dip slides are not acceptable” (Paragraph 19.68). The bacterial standard required for the water being used in the DUWL being that for drinking water quality.

The frequency of these checks in a dental practice is to be risk assessment led. The frequency for legionella in cooling water for example is quarterly and this might be considered a reasonable frequency for dental systems. TVCs taken at the same time could be used to help reassure the responsible person that the hygiene regime is adequately maintaining the water quality within normal drinking water parameters. Should legionella or other bacteria be found then the correct response (borrowing from ‘L8’ principles) might be to:

  1. Check the regime
  2. Undertake a sterilisation – using a method written specifically for the dental practice

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