How Microbes Contaminate Your Dental Unit Waterlines

Jim Chandler


Every day, water used during routine dental procedures exposes patients to millions of micro-organisms. Treated city water typically enters a dental office with very few viable bacteria present. But by the time that water runs through pipes, tubing, and dental delivery systems, micro-organisms have been given the opportunity—and sometimes, prime conditions—to grow exponentially in dental unit waterlines. 

Bacterial Growth

The design and typical usage behaviors associated with dental delivery systems are major contributors to bacterial growth. Small-diameter waterlines, low flow rates, long periods of stagnation, waterline termination (dead legs), and even occasional “suck back” from patients all contribute to creating an ideal environment for microbial growth.

The microbiological quality of water used in the dental industry is quantified by heterotrophic plate count (HPC). This time-tested method of gauging water quality dates to the 19th century, and it provides a good general measure of how well a water system is being maintained by counting the number of colony-forming units (CFU) of bacteria that a water sample contains.

HPC testing counts environmental bacteria that are generally not harmful, and therefore this method provides only an indirect indication of water quality. But because HPC testing does give a representative perspective on how well a water system has been maintained, both the Centers for Disease Control and Prevention (CDC) and the ADA have set guidelines for the dental industry based on HPC counts. The CDC and ADA both recommend that water used in nonsurgical dental procedures contains fewer than 500 CFU/mL.

As previously mentioned, municipal water generally arrives at a practice with very low levels of viable bacteria. But the pipes, tubing, and dental unit waterlines in the facility narrow progressively to create increasingly higher surface-to-water ratios that contribute to the growth of any micro-organisms present. Incidentally, this increasingly smaller-diameter tubing also allows cold water from the municipal supply to warm to room temperature more quickly, helping to incubate any living organisms.

Narrow tubing creates what’s known as laminar flow in the waterline. Laminar flow refers to water’s tendency to flow freely in the center of tubing, while allowing the flow along the outside of the tubing to remain relatively stagnant. This flow stagnation enables micro-organisms to grow virtually undisturbed along the edges of the tubing, which can lead to the development of biofilm.

Biofilm is essentially a stubborn layer of bacteria that grows on the inside surface of tubing. Biofilm also is “smart.” Once it develops, it can use a process called quorum sensing to protect itself from chemicals or other threats in the water, which makes it difficult to kill. Quorum sensing is a complex process that enables bacteria to send signals downstream to other bacteria to warn of threats such as antimicrobials or antibiotics. Downstream bacteria can actually respond by changing its DNA to protect itself.

Questions to Ask

Conditions relatively unique to dental environments can contribute to microbiological growth. Once established, micro-organisms can develop into biofilms that are difficult to kill and control. Next, we must answer a couple of questions that are vitally important to understand: how much of a “threat” does this microbiological contamination really pose, and what should practitioners do about it?

To answer the first question we must distinguish between common (and typically harmless) types of environmental bacteria and more serious types of pathogenic micro-organisms. Pathogenic or disease-causing micro-organisms found in water include bacteria such as Escherichia coli and Salmonella, viruses such as norovirus and rotaviruses, and protozoans such as Giardia and Cryptosporidium.

The good news is that city water facilities typically manage pathogenic microbial contamination fairly easily. But remember that dental unit waterlines can provide the perfect environment for microbial growth. Even a single micro-organism that survives filtration or chemical treatment has the potential to grow exponentially in a dental waterline. Also, typically harmless types of bacteria—when present in large numbers—can have negative effects on young children and immunocompromised individuals.

In the News

A recent and terribly unfortunate example of this reality occurred in September 2016, when dozens of children were hospitalized after undergoing pulpotomy procedures at a facility in Anaheim, Calif. These children were diagnosed with severe infections of the mouth and jaw from an environmental bacterium called Mycobacterium abscessus. The children had to receive antibiotics intravenously for several months, and many lost adult teeth as a result of the infection. Ultimately, approximately 500 patients were required to return for evaluations.

Although the first instinct in a situation like this is to blame the infection on a disregard for sanitation and surgical protocols, which very well might have been the case, we must remember that even following guidelines to a tee does not guarantee zero risk of infection. I share this example—and unfortunately, there are more like it—only to underscore the potential seriousness of the issue of microbial contamination in dental waterlines.

What Should You Do?

Dental practitioners should follow the protocols outlined by the CDC and various dental organizations for flushing all water-bearing lines for at least 2 minutes at the beginning of each workday and for at least 20 seconds in between patients. Beyond this, due to the nature and complex design of dental delivery units, the periodic use of waterline cleaners and antimicrobials is extremely important for proper waterline care.

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Mr. Chandler has nearly 40 years of experience in water treatment technologies. He is the founder and president of Vista Research Group, which provides a complete line of water treatment, purification, and steam processing solutions used by thousands of dental practices throughout the United States and Canada. He holds multiple patents; a bachelor’s degree with concentrations in conservation, biology, and chemistry; and a master’s of higher education administration degree from Kent State University. And, he is the author of The Book on Dental Water, which is available now in multiple formats at He can be reached at

Disclosure: Mr. Chandler is the author of The Book on Dental Water and the president of Vista Research Group, which manufactures several products designed to meet the water treatment, purification, and processing needs of dental practices.

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