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Safe enough to drink

Safe enough to drink

Maintaining good-quality dental treatment water should be a practice priority. This article explains why water becomes contaminated, how to test it, and ways to improve it. - By Chris H. Miller, PhD

According to the Centers for Disease Control and Prevention (CDC), dental unit water entering a patient's mouth should have a microbial quality of no more than 500 colony-forming-units per milliliter (CFU/mL).1 Each CFU can be considered as one or a very few bacterial cells, and an mL has the volume of about 1/4 of a teaspoon. This 500 CFU/mL is the same as the EPA's standard for drinking water. In other words, water used for routine dental care such as rinsing or cooling should at least be the same microbial quality as drinking water.

Why is dental unit water contaminated?

The water coming into the dental unit from municipal supplies is supposed to be drinking water quality, although this may not always be the case. While the water leaving the water treatment facility is of good quality, the quality can decrease as it enters distribution systems and passes through faucets in buildings. So the water going into the dental unit does contain varying levels of bacteria and this causes a continuous seeding of the inside of the dental unit waterlines. Many waterborne bacteria have developed mechanisms that allow them to attach to hard surfaces, where they multiply, forming a tightly bound biofilm that coats the surfaces over which the water passes. A biofilm is defined as a mass of microbes tightly attached to a surface in a moist environment. A typical example of a biofilm is oral biofilm (dental plaque). Another example is the biofilm that forms on the inside walls of dental unit waterlines

Figure 1. Scanning electron micrograph of dental unit waterline biofilm from a highspeed handpiece hose shows a level greater than the recommended 500 CFU/mL. Original magnification was 6,000X.

In fact, waterlines provide an ideal site for biofilm formation. Due to the lines' small diameter, they have a large surface area-to-volume ratio. In other words, there is ample surface to which bacteria can attach and multiply. The water in these lines also has significant periods of stagnation (e.g., overnights, weekends, and between patients) that facilitate biofilm formation. Thus, biofilms can and do form in dental unit waterlines.

As water passes over the dental unit waterline biofilm, it picks up bacteria from the surface of the biofilm and carries them into patients' mouths through the highspeed handpiece, air-water syringe, or ultrasonic scaler. The levels of bacteria in dental unit water that has not been treated usually are well above the 500 CFU/mL limit, and often are in the tens to hundreds of thousands per mL.2

One study demonstrated that the untreated dental water microbial counts can reach about 200,000 CFU/mL within just five days of installing a new dental unit.3 Although some of the naturally occurring waterborne bacteria (e.g., Pseudomonas aeruginosa and Legionella pneumophila) have pathogenic potential—usually in compromised hosts—there is no evidence of widespread disease being caused by dental unit water.1 Nevertheless, the use of dental water that is below the quality of drinking water hardly seems reasonable.

Improving the quality of dental unit water

It once was thought that flushing the lines would improve the overall quality of dental unit water. Flushing may temporarily reduce the planktonic bacteria (those unattached bacteria in the water itself), and it may bring in more chlorinated municipal water. However, it does not remove the biofilm that is the immediate source of the bacteria in water entering the patients' mouths. In 1995, the American Dental Association (ADA) challenged the dental industry to develop systems that yielded good-quality treatment water.4 Since then, several products have appeared that address this issue. These products involve one or more of the following approaches:

Initial and/or periodic waterlines decontamination. This directly attacks the biofilm accumulated in lines. It usually involves filling the waterlines with a chemical solution, and then letting them sit overnight for one or more nights.

Adding an antimicrobial agent directly to treatment water. This maintains continuous antimicrobial activity in the treatment water itself. This approach usually is preceded by initial decontamination of the waterlines, and sometimes is used in conjunction with periodic line decontamination.

Controlling the quality of incoming water by physical means of heat or filters. This helps ensure high quality water enters the dental unit. The approach may be preceded by initial decontamination of the waterlines, and sometimes is used in conjunction with periodic line decontamination.

Disconnecting from "city water" and providing treatment water in a separate water reservoir. This provides a mechanism to better control the quality of the water entering the unit.

USAF-tested products

The United States Air Force Dental Evaluation and Consultation Service, Division of Infection Control & Safety (https://decs.nhgl.med.navy.mil), headed by Dr. Jennifer Harte, has tested and prepared detailed information on many products.

Some products involve antimicrobial agents added directly to the treatment water after the waterlines are initially decontaminated (see sidebar "Continuous antimicrobial treatment" in related links below). Initial decontamination is needed to reduce the level of waterline biofilm that has accumulated in the unit, giving the continuous antimicrobial treatment a better chance to work.

Other products provide periodic decontamination of the waterlines without adding anything to the treatment water (see sidebar "Periodic line decontamination" in related links below).

(Editor's note: The USAF information is summarized in the sidebars and may not include all available products in this area. These lists are not all-inclusive—only tested products are mentioned—and the order of presentation or any omissions do not denote endorsement or disapproval by the author or by DPR.)

Testing for water quality

If nothing ever has been done to a dental unit to improve the microbial quality of its water, then it almost always will be highly contaminated and testing would not be necessary. When something has been done to improve the water quality, it may be appropriate to test the water both before and after the treatment. This will help ensure the system is working and has been installed properly. Subsequent testing can determine whether the system is being used properly and is being adequately maintained. The CDC recommends checking with the manufacturer of the dental unit or the water delivery system about monitoring frequency. Some of the manufacturers/distributors of the products listed in the sidebars recommend testing the water at periods ranging from weekly to every six months.

Clearly a concern

Essentially all untreated dental units emit water that is not of drinking water quality. This occurs because biofilm forms inside the dental unit waterlines, and this biofilm continually contaminates the water as it passes through the lines. So even if good quality water enters the dental unit, it will become contaminated before it enters the patient's mouth if the internal biofilm is not properly managed. Various products now are available to help reduce the level of biofilm present and improve the quality of treatment water.

1. CDC. Guidelines for infection control in dental health-care settings—2003. MMWR 2003;52(N0 RR-17):1-68.

2. Miller CH. Microbes in dental unit water. Calif Dent Assoc Journal 1996;24(1):47-52.

3. Barbeau J, Tanguay R, Faucher E, et al. Multiparametric analysis of waterline contamination in dental units. Appl Environ Microbiol 1996;62:3954-9.

4. Shearer BG. Biofilm and the dental office. JADA 1996;127:181-9.

5. USAF. USAF Dental Evaluation & Consultation Service. Synopsis of dental unit waterline treatment products and devices.


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