Dental Unit Waterlines: Check Your Dental Unit Water IQ
By Helene S. Bednarsh, RDH, MPH; Kathy J. Eklund, RDH, MPH; and Shannon Mills, DDS Reprinted from Access Vol. 10, No.9, copyright ©1997 by the American Dental Hygienists' Association
"Water, water everywhere and not a drop to drink." The water being delivered through dental unit waterlines (DUWLs) during oral healthcare procedures is suddenly of public concern. Is the water that travels through the dental unit handpiece and the air-water syringe safe for public consumption? The real answer - what this article seeks to present - lies somewhere between scientific investigation and public perception.
The news media have been increasingly interested in this subject. In several metropolitan areas, television news programs have aired segments focusing on the risk of exposure to contamined water in the dental office. The powerful effect that media can have on public policy in healthcare is well demonstrated by the aftermath of the Florida HIV transmission case several years ago. The public, having a previously heightened concern that dentistry, may again have reason to question the safety of dental care.
At first glance, it may seem that dentistry is reliving the public outrage associated with dental handpiece sterilization in 1992. With the handpiece dilemma, however, density had the technology available to address the problems and merely had to use it to be accountable to the public. The DUWL and handpiece dilemmas are similar on two points but differ on a third.
- With both issues: the problems are still being researched and need further explanation and definition.
- With both issues: Although a risk has been identified, no public health problem has been identified; and there is no epidemiological evidence available to qualify the risk.
- With DUWLs: Available technology does not yet provide complete answers, or fully manage the problem.
Researchers have know for many years that water delivered by dental units frequently contain high levels of bacteria. Despite the fact that contamination of dental treatment water often exceeds levels permissible in municipal water supplies or in recreational waters, there is no current evidence of widespread public health problems. The growth in concern is due to at least three major factors.
- There has been a general increase in awareness on the part of both oral healthcare workers and clients regarding infection control issues in dentistry.
- The number of both scientific and public media reports of relatively high levels of potentially pathogenic microorganisms in the dental treatment water has increased.
Case reports have been published associating illness with dental water contamination.
The Problems Related to Dental Water Quality
The types of bacteria found in the dental unit are for the most part, the same as are found in drinking/tap water. The numbers of bacteria are, however, almost always higher in the dental water unit. Municipal water treatment authorities usually try to keep the numbers of waterborne bacteria - often referred to as heterotrophic bacteria - below 500 colony forming units (CFU) per milliliter of water.
A CFU is a bacteria cell or clump of cells which will form a distinct colony on an agar growth plate when cultured. Usually this number is referred to as CFU/ml. Most studies of water from dental handpieces and air-water syringes have demonstrated contamination levels which are hundreds or even thousands of times greater than is permissible in drinking water supplies.
The types of microorganisms found in the dental unit water is mainly bacteria and some protozoa and fungi. Two types of microbial communities exist in the dental unit waterline: 1) planktonic (free-floating) microbes found in the water itself; and 2) microbes in a sessile form attached to the inside walls of the waterlines called biofilm.
The key to understanding the DUWL problem is to understand not only the bacteria, but the biofilms and their relationship to the geometry of small bore tubing used in dental units. Microbial biofilms are found in nature virtually anywhere that moister and a suitable solid surface for bacterial attachment exist. Biofilms consist primarily of naturally occurring, slime-producing bacteria and fungi which for complex cooperative communities on wet surfaces. Dental plaque is one of the best know examples of biofilm. Just as plaque forms on the surface of teeth, bacterial biofilms form on the walls of small-bore plastic tubing in dental units which deliver coolant water from high-speed dental handpieces and air-water syringes used in dental treatments. But biofilms also form in water mains, so there existence alone is insufficient to explain fully the problem with dental water quality.
Biofilm formation is enhanced by many factors(see figure 1 and figure 2). Water stagnation occurs when the dental unit is not in use, which facilitates accumulation of bacteria. Even when the water in the tubing is flowing during use, it is not under high pressure, and the flow rate very low near the tubing walls. Microbes continually enter the tubing from the water supply and from "suck-back" that occurs during handpiece and air-water syringe use. The bacteria that enter the tubing are able to adhere to the lining of the tubing and then multiply. Once on the tubing, the bacteria have a continuous source of nutrients entering from from the source water that supports the accumulation of the microbes in the biofilm. Biofilm begins to form in new tubing within a few weeks. Bacteria are continuously released from the surface of the biofilm into the flowing water in the line. Patients and clinical staff are then exposed to these planktonic microorganisms.
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Dental Unit Waterline Biofilm Formation
- Microbes enter the tubing from incoming water and to a lesser degree from dental clients during treatments
- The microbes have the ability to adhere to the surfaces and to the inside walls of the dental tubing within hours.
- The attached microbes begin to multiply and start to form a spreading film on the tubing walls.
- Additional microbes from the incoming water continue to attach and multiply.
- The microbes produce polysaccharide that coats the cells, forming a slime layer.
- Within a few weeks, the biofilm has covered most of the inside walls of the tubing.
- Microbes are continuously released from biofilm into the flowing water.
Conditions that Facilitate Biofilm Formation in Dental Unit Waterlines
- Low numbers of microbes are continually entering the tubing.
- Nutrients are continually being supplied in the incoming water.
- Stagnation of the water in the tubing facilitates accumulation.
- The water's natural flow rate is low near the tubing walls.
- Water in the tubing is not under high pressure.
- The tubing's smaller diameter creates a large surface-to-volume ratio.
The design of DUWL is the key to understanding the surface-volume ratio problem. The larger the diameter of the tube, the more water there is in relation to the surface area available for bacterial colonization. For example, imagine two segments of tubing, each capable of holding 100 liters of water. The first segment is a 10-inch water main. A length of water main sufficient to hold 100 liters of water is less than one inch in length and has a total internal surface area of about two square inches - about the size of a folded gauze sponge. That available surface area is the limiting factor in determining the amount of biofilm that can form.
In comparison, a segment of 1.8 millimeter diameter dental waterline with an equivalent 100 liter capacity is over 100 feet in length and has a total surface are available for colonization which is equal to a page and a half of a typical daily newspaper. This proportional increase in the amount of potential biofilm relative to a given water volume is one of the major factors influencing dental water quality in unrelated systems.
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Health Risk to Oral Healthcare Workers and Clients
Although some bacteria recovered from dental waterlines are traceable to the dental client, most of the organisms that pose health risk to both oral healthcare workers and their clients are naturally occurring, heterotrophic water bacteria. Predominantly gram-negative, many of these bacteria once were considered nonpathogenic. As many as nine potentially pathogenic organisms associated with opportunistic wound and respiratory infections have been isolated from dental unit water systems.
Organisms of concern include: Pseudomonas, Klebsiella, Legionella, and non-tuberculosis Mycobacterium species. Pseudomonas is a well-known opportunistic pathogen and a common contaminant in dental unit water. Pseudomonas species (e.g. P. aeruginosa, P. cepacia) are commonly found in soil and water. They can survive on a meager nutrient supply and often exhibit resistance to antibiotics and antimicrobial chemicals.
In 1987, Martin reported in the British Dental Journal that dental unit water was a source of postoperative wound infection with P. aeruginosa in two immunocompromised cancer patients. (1) Martin also identified 78 normal patients exposed to P. aeruginosa in dental unit water who remained colonized for four to ten weeks. (1)
Legionella (L. pneumophila and 30 other species) is commonly found in natural and domestic waters. It frequently exists inside of amoebae and causes Legionnaires' disease (pneumonia either from inhalation from the outside source or aspiration from colonized oropharynx). Studies have detected Legionella in the following. (2-6)
- Ten percent of 42 dental units in Austria.
- Three of five dental units in a London Clinic.
- Four percent of 194 dental unites in a London hospital.
- Several dental units in an Ohio dental school.
- Eight percent of 38 offices in California, Oregon, Washington State and Michigan.
In 1995, Atlas, et al., reported in Applied Environmental Microbiology a case report of a possible death of a California dentist resulting from Legionnaires' disease secondary to exposure to dental unit water. (7) It must be noted that Legionella also was isolated from the dentist's home shower head. Two studies have demonstrated elevated levels of antibodies to Legionella bacteria - suggestive of occupational exposure - in oral healthcare workers compared to demographically similar nondental populations. (8-10)
The lack of epidemiological evidence of defined risk is not surprising given the nature of dental practice. Most potential postoperative complications in density are self-limiting, and very few dentist routinely obtain bacterial cultures or report post-treatment infections. Since most dentists are in small groups or single practitioner settings, there is little opportunity to conduct large-scale retrospective studies. Any prospective study would be complicated by ethics and informed consent. It is highly unlikely that oral healthcare clients would voluntarily accept the risk of treatment with water known to be contaminated with tens or hundreds of thousands of bacteria.
The isolation of Legionella bacteria, as well as several host species of amoebae, in waterlines has done much to raise the level of concern in the research and in the public policy communities. The presence of the organisms which causes Legionnaires' pneumonia, combined with the possibility that a dentist's death was in some way associated with the dental unit water, expands the focus of apparent risk from wound infection in clients to include respiratory infections in healthcare workers. The common practice of heating dental water systems alleviate client discomfort may well further add to biofilm formation.
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Current Guidelines and Regulations for Dental Water Quality
There are currently no laws or regulations that set specific standards for the quality of water used in oral healthcare. Proposed federal regulations would set a national standard of 500CFU/ml for heterotrophic bacteria in drinking water. This is the current standard used by many states and community water systems. Although it may seem prudent to perform dental procedures with water that is at least "fit to drink," these rules would only apply to public water treatment works.
The 1993 Recommendations for Infection Control in Dentistry from the Centers for Disease Control and Prevention (CDC) require that sterile irrigating solutions be used for all dental procedures which involve the cutting of bone (Figure 3). The only recommendation in this document which addresses dental unit water is to flush the dental units at the beginning of each day and between patients. This recommendation, based on information available at the time the guidelines were completed, was never intended as a means to control or eliminate biofilms in waterlines.
In 1995, The American Dental Association (ADA) Council on Scientific Affairs published a statement on dental unit waterlines that challenged the industry to produce systems that can reduce the level of bacteria used in dental treatment to 200 CFU/ml or fewer by the year 2000. Neither the ADA statement nor the CDC guidelines require any specific action on the part of oral healthcare workers to control biofilms in waterlines or improve the quality of dental unit water.
The scientific basis for the ADA's recommendation of 200CFU/ml as a maximum for contamination of water used in dental treatment was based on several factors. The ADA's expert panel on dental waterlines felt that a specific goal was necessary for the dental equipment and products industry to benchmark their progress in improving the quality of water provided by dental delivery systems. Although complete elimination of biofilms and bacterial contamination is obviously ideal, there is evidence that this may not be economically or technically feasible with present technology. Since proposed drinking water standards set contamination limits at 500CFU/ml the panel felt that water used for nonsurgical dental procedures which may generate respirable aerosols and are often minimally invasive, should be lower.
The available scientific literature suggest that the recommended limit can be achieved in the clinical setting, albeit with conscientious compliance on the part of the oral healthcare practitioner. Although there is no epidemiologic basis for the recommendation in the dental setting, the literature on use of water in hemodialysis units has shown an increased rate of treatment complications when colony counts exceed 200 CFU/ml. This does not suggest that dental procedures are similar in risk potential to hemodialysis. It does, however, lead to a body of scientific methodology and technology already in place, which can be readily adapted for use in dentistry.
In June 1996, the Office of Sterilization and Asepsis Procedures (OSAP) Research Foundation convened a Scientific Symposium on Dental Water Quality. The symposium was followed by a special workgroup which was prepared to draft OSAP position paper on dental water quality to be finalized and released by the end of 1996.
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Current Guidelines and Recommendations Regarding Dental Water Quality
- Flush water lines at the beginning of the day for 30 seconds (may temporarily reduce the level of microbes in the water).
- Flush air/water through handpieces for 20 seconds after each patient (helps reduce any patient-borne microbes that may have entered the handpiece and were "sucked back" down the dental unit line).
- Do not use dental unit water for any procedure that involves cutting to the bone.
- "Encourages industry and the research community to improve the design of dental equipment so that by the year 2000, water delivered to patients during nonsurgical dental procedures consistently contains no more than 200 CFU/ml at any point in the time in the unfiltered output of the dental unit."
- "Encourage manufacturers of dental equipment to develop accessory components that can be retrofitted to dental units currently in use, whatever the water source (public or independent), to aid in achieving this goal."
- "Urges industry to ensure that all dental units manufactured in the United States in the future have the capacity to be equipped with a separate water reservoir independent of the public water supply."
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Four methods are now widely advocated to reduce the level of bacterial contamination in dental unit water. The methods are:
- flushing waterlines for several minutes at the beginning of the day and after periods of disuse;
- using an independent water reservoir system that is separate from the municipal water source (sterile water);
- use of an independent water reservoir system combined with period or continuous application of chemical germicides; and
- use of microfiltration to trap microbes before they reach the dental client.
Although flushing recommendations are found in a number of published infection control guidelines, there is little scientific support for this practice(see table). Studies have shown that biofilms cannot be removed by flushing alone, and that biofilm bacteria can quickly recontaminate treatment water. Flushing between clients, however may be beneficial to eliminating retracted client material.
The largest number of published studies have investigated the use of separate water reservoirs that isolate the dental water supply from the public water system. Since the unit itself is the main source of bacterial contamination, germicidal treatment is necessary to suppress or remove biofilms. Several studies have shown varying degrees of success using a number of chemicals and protocols.
Microbial filtration of ultrasonic and dental handpieces was first reported in 1978 by Dayoub, et al.(12) More recent research has been published on point-of-use microfiltration in dentistry. This technology is widely used in medicine and industry and has the potential to reduce the levels of bacteria in treatment water. The use of filters is effective in reducing the CFU/ml to close to zero; however because they are placed at the end of the line, they have no effect on the formation of biofilms in waterlines.
Both separate water reservoir systems and microfiltration devices have been commercially available for a number of years. Most dental unit manufacturers now offer reservoir systems as optional equipment. Retrofittable systems are also available to add to existing dental units. These devices are of no value, however, without germicidal treatment, even when sterile water is used in the reservoirs. Users must follow routine maintenance protocols to control biofilm formation. These protocols, as well as safety and compatibility information, should be supplied by the manufacturer of the system.
An inline disposable microfiltration cartridge also is on the market. This device must be inserted as close to the water-using instrument as possible, and should be replaced at least daily on each line. Although this technology can dramatically reduce bacterial contamination in dental coolant and irrigating solutions, the biofilms that colonize the unit are unaffected. Filtration cartridges can be used in combination with water reservoirs to assure improved water quality.
Other Technological approaches, including continuous chemical treatment devices and antimicrobial tubing, are currently under development. All devices and chemicals that claim to improve the quality of water used in dental treatment and/or control biofilms are subject to regulation as medical devices by the Food and Drug Administration (FDA). To be assured of a device's safety and efficacy, users should select products that have been cleared by FDA to market.
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Comparison of treatment methods and devices for control of dental unit waterline contamination.
(minutes per day)
(per unit installed)
||Less than 5 minutes
(once a week)
(based on the degree of automation)
(based on the degree of automation)
|Point of use microfilters
(up to 15 minutes per patient)
|Up to $1200
All estimates are based on treating 8 patients per day, and assume the use of bottled sterile water (USP) and 2 water-carry lines.
- Disinfectant By-Products (DBP) may form when chemicals used for waterline treatment reach with biofilms or water system materials.
- May produce DBPs if incoming water is hyper-chlorinated.
- Use of reservoirs alone will have no effect on biofilms or system components without chemical treatment.
- Continuous treatment may inactivate planktonic forms but may have only biostatic effects on biofilms. This may result in increased potential for formation of DBPs.
- No product in these classes are presently cleared to market for control of dental water contamination.
- Cost to replace existing tubing will vary by complexity of water system. Frequency of replacement interval will also be a factor.
- Surgical sterile water delivery devices provide sterile, non-pyrogenic pathway for delivery of sterile water or other surgical irrigants. All components are heat sterilizable or single-use disposable.
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Chemical Treatment of Biofilms
Over the past three decades, different cleaners, antiseptics, and disinfectants have been proposed or evaluated for their effectiveness in controlling microbial contamination. These solutions can be employed in two ways.
- Periodic or "shock" treatment with (hopefully) biocidal levels of chemicals.
- Continuous application of chemicals at levels (again, hopefully) which can control or eliminate biofilms, but are below threshold levels for toxicity in humans.
Other approaches being researched involve the use of cleaning agents that remove, rather than kill, biofilms; and the use of waterline materials which resist microbial adhesion.
Although the remarkable ability of biofilms to resist chemical attack has been repeatedly demonstrated, several chemicals have shown promise. The most extensively evaluated substances are chlorine compounds, including sodium hypochlorite (household bleach). Unfortunately, many of the compounds which have shown promise in controlling biofilms can also corrode or degrade dental unit materials. Moreover, these chemicals can react with dental unit materials, or with the biofilms, to produce disinfectant by-products which may have unanticipated effects on dental materials, clients, and healthcare workers.
Before initiating any chemical treatment regimen, the user should contact the dental equipment manufacturer to obtain treatment recommendations and safety precautions. All devices and chemicals which clam efficacy for the purpose of controlling biofilms or improving dental water quality are subject to FDA regulation as adjunctive medical devices. Select products cleared by FDA for marketing ensures users of their safety and efficacy.
The same chemical treatment regimens and technology used in the dental unit should be effective with the ultrasonic scalers and air polishing equipment, which also pose risk of bacterial contamination via biofilm in tubing. The user should contact the manufacturer for information on compatibility with the equipment and any chemical disinfectant or antimicrobial used. Microfiltration also has shown promise in clinical evaluations with these devices. Some of the ultrasonic scalers now on the market permit the use of sterile irrigants and have autoclavable or disposable waterlines and handpieces.
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Water Sources and Water Quality
Even the most effective chemical treatment protocol will be of little value if the water placed in the reservoir bottles is contaminated. Although sterile water is not necessary for routine dental procedures, the cleaner the water entering the system, the better (see figure 4). For this reason, sterile water is the ideal for use in a reservoir system. Freshly distilled water, or even tap water will have very low levels of viable bacteria. Since bacteria such as Pseudomonas can grow very quickly even in distilled water, frequent cleaning and disinfecting of distillers and storage containers also is required. Although untreated tap water may be of good quality in many areas, it can be unpredictable. Incidences of "boil water" emergencies in cites around the nation are being reported with disturbing regularity.
Similarly, the microbiologic quality of bottled drinking water can vary widely. Distributors of bottled drinking water will not generally assume responsibility for illness associated with the medical or dental use of their products. Home water filtration or reverse-osmosis devices are prone to bacterial contamination and are of questionable value as a source of dental treatment water.
Ultraviolet germicidal irradiation (UVGI) may be able to provide water of acceptable quality but can not control biofilm. Recent mass mailings to dentists and displays at national meetings by well intentioned purveyors of UVGI systems have suggested otherwise.
The scientific literature suggest that virtually all dental units which connect to public water supplies, and have not been treated or are not using point-of-use filtration, will greatly exceed 200 CFU/ml as recommended in the current ADA guidelines for dental water quality. For this reason, there is little to be gained by initial testing of units in the clinical setting to see if they are contaminated. Clinical monitoring procedures should be designed to assess compliance with recommended water treatment protocols. If devices and protocols have been properly evaluated for safety and efficacy by the manufacturer, then there is no need for revalidation of the safety and efficacy by the user. Monitoring procedures should be designed to identify technique errors or noncompliance on the part of the user. By implementing protocols to eliminate biofilms and attain colony counts in unfiltered output which are as low as reasonably achievable, the need for routine identification of specific organisms in dental treatment water is eliminated.
Testing can be preformed by microbiology labs in most metropolitan areas. In-office test kits, used to test water in hemodialysis centers and the food industry, also are available. If testing is done, records should be maintained as part of the clinical quality assurance documentation.
No matter which type of water is used, care must be taken in handling reservoir bottles and filters. Careless handling, combined with lax handwashing discipline, can lead to colonization of water systems with enteric or skin bacteria. There is also the potential exposure and contamination to the user that does not wear the appropriate personal protective equipment.
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Approaches to Improve Dental Unit Water Quality
- Water Source-Improve the quality of the incoming water.
a. Use a nonmunicipal water source (separate reservoir), or use a hand syringe for irrigation.
b. Municipal water or well water
- Waterlines-Control biofilm in the tubing.
a. Routinely replace and decontaminate the lines.
b. Air purge lines and dry (let set empty) overnight.
- Output Water-Control water quality as it leaves the tubing.
a. Microbial filtration (e.g. 0.22 micrometer pore size filter)
b. Continue to use high volume evacuation (HVE) with all water sprays.
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A major question among oral healthcare professionals is whether the scientific evidence justifies taking action irrespective of the cost. The majority of infection control recommendations, especially in dentistry, are not epidemiologically based. Recommendations to wear gloves, clean and sterilize dental instruments, or to provide antibiotic prophylaxis for clients at risk for bacterial endocarditis are based on an understanding of the etiology of disease transmission - and a knowledge that sufficient numbers of pathogens may be present in the dental environment to cause disease in susceptible host.
In the case of DUWLs, these same conditions apply, despite the absence of numerous documented cases of illness or death. At least two portals of entry - the respiratory and vascular systems are available to opportunistic pathogens such as Pseudomonas, Legionella, and their microbial kin. The numbers, often exceeding hundreds of thousands in aggregate, have been sufficient to result in outbreaks of disease in other settings, such as hospitals, hotels, grocery stores and cruise ships.
A more knowledgeable public, already sensitized to previous media exposes of lax dental infection control practices and lingering questions about the general quality of drinking water in the U.S., may have little patience with the oral healthcare professional on this issue. Consequently, before the cost of equipment and overhead are considered, professional ethics and public trust must be weighed.
The cost to retro-fit existing equipment with separate water systems start at less than $100 per dental unit. Since most protocols currently recommended by manufacturers use commercial bleach, cost for chemicals may be negligible. The use of sterile water for irrigation - United States Pharmacopoeia (USP) can raise cost as compared to water prepared on-site. In most cases, water reservoirs need to be refilled only once or twice a day. A typical maintenance protocol requires 15 to 20 minutes per unit once a week. Microfiltration devices can be installed at minimal cost, but devices must be changed at least daily on all water carrying lines at a cost of about $3 per filter. Filter replacement, however, will be less time-consuming for the user.
Used in combination, microfiltration and self-contained, chemical treated units may reduce the need to change filters on a daily basis, thus reducing the cost of microfiltration. Separate/self-contained water systems can be ordered at minimal cost as an option on a new dental unit. Further research is needed to investigate this combined technology and cost implications.
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- Martin MV; The significance of the bacterial contamination of dental unit water systems. British Dental Journal 1987;163;15204.
- Reinthaler FF, Masher F; Legionella pneumonia in dental units. Zbl Bakt Hyg B 1986;183:86-88
- Oppenheim BA, Sefton AM, et al.; Widespread Legionella pneumophila contamination of dental stations in dental school without apparent human infection. Epid Int 1987;99:159-166
- Challacombe SJ, Fernandes LL; Detecting Legionella pneumophila in water systems: A comparison of various dental units. Journal of the American Dental Association 1995;126:603-608
- Schold RC, Rosen S, Beck FM; Reduction of CFUs in high-speed handpiece waterlines over time. Clinical Preventive Dentistry 1990;12(2):9-11
- Williams JF, Johnston AM, et al.; Microbial contamination of dental unit waterlines. Journal of the American Dental Association 1993;124:59-65.
- Atlas RM, Williams JF, et al.; Legionella contamination of dental unit waters. Applied Environmental Microbiology 1995;61;1208-1213.
- Foto PG, Westphal HN, et al.; Prevalence of Legionella specific IgG and IgM antibody in a dental clinic population. Journal of Dental Research 1985;64:1382-1385.
- Reinthaler FF, Mascher F, Stunzer D; Serological examination for antibodies against Legionella species in dental personnel. Journal of Dental Research 1988;67:942-943.
- Dayoub MB, Rusilko DJ, et al.; A method of decontamination of ultrasonic and high-speed handpieces.
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