How safe are your dental unit water lines?

The main purpose of DUWLs is to supply water for instruments connected to the dental chair unit, including ultrasonic scalers, ‘three-in-one’ air/water syringes, high-speed turbine handpieces and slow-speed conventional handpieces. This water is used to irrigate and cool tooth surfaces during dental treatment, preventing the harmful effects of heat generation on both instruments and vital tissues[i].

Studies have revealed that DUWLs are often contaminated by numbers of different microorganisms, including bacteria, fungi, protozoa, and viruses[ii]. This means that the aerosols and droplets produced by instruments connected to DUWLs may contain pathogens that put patients at risk as well having detrimental effects on dental professionals’ health outcomes.

Potential risks

The main risk to staff and patients from DUWL contamination arises from opportunistic respiratory pathogens that include Pseudomonas aeruginosa that can cause several harmful infections, Legionella pneumophila, the primary causative agent for Legionnaires’ disease, and non-tuberculous Mycobacteria, a pathogen responsible for respiratory infections[iii]. Pathogens may also directly contaminate surgical wounds or exposed surgical sites, leading to possible wound/surgical site infection[iv].

Legionnaires’ disease (the most severe form of Legionellosis) is a serious pneumonia caused by inhalation (and rarely aspiration) of legionella bacteria. The pneumonia can cause long-term health problems for about 10% of community acquired cases[v].

It has been suggested that legionellae within DUWLs may contribute to respiratory illness amongst the dental team[vi]. A recent study has shown that even though a few individual cases of Legionella pneumophila infections in dentists and patients have been reported so far, the antibody prevalence against Legionella in dentists and dental staff is significantly higher compared with the general population[vii].

The majority of organisms isolated from DUWLs are gram-negative water bacteria, which are a potent source of endotoxin associated with inflammatory reactions in periodontal disease and systemic illness, e.g. septic shock. A consequence of endotoxin exposure is the triggering or exacerbation of asthma, and endotoxins of DUWL origin may account for reports of an increase in occupational asthma suffered by dentists exposed to contaminated waterlines[viii].

The problem of biofilm

Evidence of occupational exposure from DUWLs was first described in the 1960s[ix]. Studies have since demonstrated that DUWLs provide a favourable environment for microbial proliferation and biofilm formation that can cause health problems for those exposed, especially those who are immunocompromised.

Biofilms thrive in DUWLs because they provide an ideal environment for microbial growth for a variety of reasons. These include:

• A complex network of narrow-bore flexible polyurethane or polyvinyl chloride (PVC) plastic tubing. Being a closed system, this means that any contamination cannot usually be seen.
• The high surface area-to-water volume ratio and intermittent use of DUWLs leading to stagnation of water for extended periods during the day causes minimal disruption to growing biofilm.
• Low flow rate (typically 30ml/min) encourages biofilm formation.
• The warm surgery environment provides the ideal temperature range for bacteria to flourish.

Microorganisms can access DUWLs through incoming municipal water, contaminated independent water reservoirs (e.g. water bottles), retrograde movement of output water and saliva into dental handpieces, and water heaters that may encourage microbial growth[x].

Despite the integration of anti-retraction valves in dental couplings and handpieces that are designed to prevent backflow, issues such as clogging and irregular operation of these can occur, leading to further biofilm development as additional contaminates enter the system[xi].

Control of biofilm

If biofilm formation is not strictly controlled in DUWLs, the water quality will soon deteriorate, resulting in a failure to comply with DUWL management guidelines and pose a health risk to patients and staff. For water used in DUWLs, the current recommended standard for England, Wales and Northern Ireland is 100 to 200 colony forming units (CFUs) per millilitre of water [cfu/ml], as detailed in HTM 01-05 section 6.79.

By improving the quality of DUWL output water and minimising biofilm formation, the risk of infection presented by waterborne microbial pathogens can be reduced. Water quality should therefore be routinely monitored, or tested, as recommended by the chemical treatment manufacturer to ensure that treatments are working effectively and that water used in dental procedures meets safety standards.

Controlling biofilm to maintain water quality can be achieved either by a regular purge treatment, by continual dosing into the chair’s water bottle, or a combination of the two. Products should always be used according to manufacturers’ instructions.

• Purge: If water quality indicates biofilm is present in the DUWL, then purge cleaning should be carried out. Following the manufacturer’s instructions, leave the chemical for the advised contact time, drain and rinse the line. The chair should be continually monitored and the action repeated weekly. Purge cleaning also applies to new dental chairs before commissioning as they are always ‘wet tested’ and then left stagnant, allowing the possible formation of biofilm, and after longer periods of annual leave when a chair might be left stagnant.
• Continual dosing: Continually dosing the chair with a low-level water maintenance solution reduces the planktonic bacteria count in water flowing through the unit, minimising the potential for biofilm formation.

It is important to note that a maintenance solution alone is not usually strong enough to eliminate established biofilm and therefore a combination of these methods, which would normally include performing a purge every three months alongside continual dosing, is the recommended option.

HTM 01-05 in England, WHTM 01-05 in Wales and SDCEP in Scotland all provide similar guidance in the ways in which water should be treated and stored in DUWLs to minimise the issues of biofilm. The prevention measures include:

• Flush lines for at least 2 minutes at the start and end of each working day.
• Flush lines for 30 seconds between patients.
• Flush/drain lines after prolonged periods of non-use/inactivity (e.g. holidays and in part-time surgeries).
• Use chemical water treatments (according to manufacturer’s instructions) in DUWLs to minimise biofilm formation.
• Continually monitor all DUWLs within the practice.

Importance of monitoring

While microbiological monitoring for total viable counts (TVCs) in DUWLs is not considered essential (apart from situations where there are indications from taste and odour)[xii], biological dip slides are a straightforward, in-house method of identifying microbial contamination of water systems.

It is recommended to use dip slides to test both the DUWL input and output (3in1) samples and tests should be regularly monitored and maintained[xiii]. If this indicates bacterial counts are building, then a disinfectant purge treatment (also known as a ‘shock’ treatment) should be undertaken using a suitable CE/UKCA (UK Conformity Assessed) marked disinfectant treatment.

The frequency of dip slide testing and monitoring should always follow the chemical manufacturer’s instructions. Most responsible manufacturers recommend the use of a monitoring dip slide every quarter and more frequently if elevated risk factors (e.g. extended stagnation periods) occur.

As stated in HTM 01-05 6.78: “some companies and other institutions offer comprehensive water-purification services that include periodic microbiological sample monitoring. Such services, provided they are quality-controlled, may contribute usefully to risk reduction in this area”. If dip slides can be assessed independently by a third party in a laboratory environment that supplies reports and certificates of compliance for each registered surgery that passes the criteria for clean water, this ensures a practice can stay compliant with infection control protocols.

Ongoing management

Although the number of proven published cases of infection resulting from exposure to water from DUWLs is limited, these instances are noteworthy to prevent further cases and improve management of dental water supplies[xiv]. The aim for dental teams is to control and inhibit biofilm formation on a continual basis using effective cleaning, disinfection and preventive maintenance strategies along with regular monitoring. This forms an intrinsic part of practices’ infection control protocols, providing valuable evidence to satisfy inspectors and give peace of mind to those ultimately responsible for the wellbeing of patients and staff.