JADA Continuing Education
Evaluation of compressed air used in the dental operatory
J. SEAN HUBAR, D.M.D., M.S.,
WILLIAM PELON, Ph.D. and
DIANA M. GARDINER, Ph.D.
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ABSTRACT
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Background. The authors report the findings obtained when they quantitatively examined compressed air samples from air-water syringes located in different dental operatories at the Louisiana State University School of Dentistry for the presence of microbial contaminants.
Methods. Streams of air of 30 seconds’ duration from air-water syringes were forced through sterile modified stainless steel membrane filter holders (Millipore, Millipore Corp., Bedford, Mass.), each containing a membrane filter (average pore diameter = 0.45 micrometers). Each filter was aseptically removed, placed onto the surface of a Petri dish containing sheep blood agar and incubated under increased carbon dioxide tension at 37 C for 48 hours. The authors performed a count of the resultant microbial colonies, after which they microscopically examined the gram-stained organisms.
Results. Bacteria were detected in 24 percent of the samples. The number of colonies observed on the filters varied among the dental units. The air from only one of the dental units sampled repeatedly was found to be free of bacterial contaminants. This contrasted with other units for which one or more samples were found to be positive for microorganisms. The majority of colonies observed were pigmented. Microscopic examination of organisms from representative colonies revealed that most were either gram-positive cocci or gram-negative diplococci and tetrads. The results of the one-sample t test were found to be significant (t = 5.6, df = 98, P = .0001). The 95 percent confidence interval was 0.15 to 0.32.
Conclusion. The results suggest that, at least statistically, a percentage of air lines will have bacteria present.
In air-turbine–driven dental handpieces and air-water syringes, compressed air is forced through separate tubing identical in composition to the adjacent polyurethane tubing that delivers water directly into the unit. Air line tubing leads directly into high-speed and slow-speed handpieces and into air-water syringes. Biofilms occurring in dental unit water-lines have been reported as sources of potential infections in the literature over the last several decades.1–8 The American Dental Association and dental equipment manufacturers are aware of this problem and are exploring corrective measures. Grenier9 has reported that routine dental procedures can produce aerosols, which cause a significant increase in the level of bacterial air contamination. The Centers for Disease Control and Prevention has recommended that dentists minimize aerosol sprays discharged from high-speed handpieces with the use of high-velocity evacuation, because immunocompromised patients may be at risk of developing an infection from potential pathogens introduced in this manner, owing to their diminished host-immune response.10,11
The results of this study suggest, at least statistically, that it would be reasonable to expect that a percentage of air lines will have bacteria present.
Typically, a single, central source distributes compressed air throughout an entire clinical facility. At the Louisiana State University School of Dentistry, or LSU-SD, the air supplying the air syringe is compressed to a pressure of 40 pounds per square inch, or psi. In humid climates, the accumulation of condensate in a compressed air tank and its connected air lines during periods of inactivity (for example, overnight or over weekends) would allow the bacterial contaminants that may be present within the lines to proliferate.12–15 As clinic activities resume, the microbial progeny may be transported through the air lines and then introduced into dental patients’ oral cavities, as well as the surrounding environment where clinic personnel could be at risk of becoming exposed. Consequently, in addition to the bacteria-laden aerosols generated from patients’ saliva (Grenier9), aerosols containing organisms that originated from air-line biofilms could further compound the problem.
In this cursory article, we present the findings obtained when we quantitatively examined compressed air samples from air-water syringes located in different dental operatories at LSU-SD for the presence of microbial contaminants.
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METHODS
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LSU-SD has 285 operatories, the majority of which are distributed among the different clinics located on four floors of the clinic building. For the purposes of this study, we randomly selected 10 dental units for sampling. They were located in the oral diagnosis and patient screening clinics.
To quantitate the microbial populations present in the air lines, we examined air from air-water syringes mounted on the side arms of dental units. We directed an unobstructed 30-second air stream from each air-water syringe equipped with a clean disposable tip into a modified low-pressure, stainless steel membrane filter holder (Millipore, Millipore Corp., Bedford, Mass.), type 304 stainless steel that contained a sterile membrane filter (average pore density = 0.45 micrometers) (Figure 1
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Figure 1. Modified membrane filter holder with disposable tip. Image reproduced with permission of Millipore Corp., Bedford, Mass.
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We sterilized each filter apparatus and filter used as a unit by autoclaving it at 10 psi for 10 minutes. The disposable tips were used only once for each sample taken. To prevent any of the compressed air from escaping the entry port of the filter apparatus, we trimmed the disposable pipette tip and fitted it with rubber tubing. This was tightly inserted into the open end of the filter holder leading into the collection cylinder that contained the sterile filter (Figure 2).
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Figure 2. The lower portion of the modified membrane filter holder is open. Each membrane filter was positioned on the support screen before closure and autoclaving. Image reproduced with permission of Millipore Corp., Bedford, Mass.
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After a 30-second stream of air was passed through the assembly, we aseptically removed the membrane filter and placed it onto the surface of a Petri dish containing sheep blood agar (Figure 3A and Figure 3B). The Petri dish containing the filter was incubated under increased carbon dioxide tension at 37 C for 48 hours. A count of the resultant microbial colonies (Figure 3B) was performed, followed by Gram-staining and microscopic examination.
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Figure 3. Air sample exposed membrane filters on Petri dish containing sheep blood agar before (A) and after (B) incubation at 37 C for 48 hours. Multiple colonies are seen growing on the membrane filter after incubation.
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We obtained air samples from the air-water syringes of 10 different units, with samplings being repeated on 10 different occasions. We used a one-sample t test using an SPSS statistical package (Version 10.1, SPSS Inc., Chicago) to determine if the number of occurrences of bacteria in the total number of samples was significantly different from a test value of 0.
To confirm the effectiveness and reliability of the modified membrane filter holder technique, we first evaluated this same system with water from air-water syringes on dental units located in the different clinics. We directed water through the membrane filters for five seconds after which time, we removed the membranes from the filter holders and incubated them for bacterial isolation and quantitation as described. This was repeated 10 times.
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RESULTS
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Table 1 shows that 23 of 99 air samples were positive for bacterial contaminants. The number of colonies detected varied among the dental units. Only one of the dental units sampled (D) repeatedly was found to be free of bacterial contaminants. This was in contrast to other dental units, in which one or more samples were found to be positive for microorganisms.
All of the water samples that were tested with the Millipore system showed evidence of extensive microbial contamination with resultant colonies that were too numerous to count.
The majority of colonies we observed were pigmented. Microscopic examination of the organisms from the colonies revealed most to be either gram-positive cocci or gram-negative diplococci and tetrads. For the statistical analysis of all of the samples using the one-sample t test, we considered the presence of any bacterial colony to be positive. Using this criterion, the results of the one-sample t test were found to be significant (t = 5.6, d = 98, P = .0001). The 95 percent confidence interval was 0.15 to 0.32.
In contrast to the findings with the air samples, all of the water samples that were tested with the Millipore system showed evidence of extensive microbial contamination with resultant colonies that were too numerous to count. This corroborates the findings from earlier reports regarding the presence of microorganisms in the waterlines.1–5,7
Table 2 supports the findings of other reports, particularly those that have sampled water from air-water syringes after periods of inactivity.6,12 Similarly, when air lines were not in use, we found that there was a tendency for bacterial populations within the lines to increase in number. This was evident from the data in Table 2. When air samples were collected from the same air line sources early in the morning and late in the afternoon, 45 percent of the morning samples were positive for bacterial isolates, while all of the afternoon samples were negative.
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DISCUSSION
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Microbial films are ubiquitous in nature and can be found virtually anywhere there is moisture and a solid surface for attachment. In dental units, biofilms form on small-bore plastic tubing that delivers coolant water for dental handpieces and in air-water syringes used in patient care.
The objective of this study was to assess the nature of air quality emitted from the air-water syringes with respect to microbial contaminants. First, we performed tests to determine the reliability of the apparatus to trap microorganisms by directing water from air-water syringes instead of air. Large numbers of bacteria were detected in the waters we tested, as evidenced by the colonies that developed on the membrane filters after incubation. Quantitatively, these were too numerous to count. These data support the effectiveness of modified filter apparatus systems for trapping microorganisms.
In contrast to the results obtained with water samples from air syringes, the numbers of air contaminants detected for a given sample generally were low. Only a single colony was detected with six samples, while four samples yielded two colonies each. These results raise the question as to whether such isolates were merely aerial contaminants. While this cannot be denied, 77 percent of the samples tested also were completely negative. It may be argued that if aerial contamination was a problem, this number should have been smaller. Furthermore, the Petri dish containing medium had a diameter of 100 millimeters, while the diameter of the membrane filter was 47 mm. This left more than 50 percent of the agar surface exposed. Yet, all bacterial growth observed occurred only on the membrane surfaces and not on the exposed agar surface.
We explored the possible lethal effect of air under pressure on organisms entrapped on the filter membrane. We deposited a dilute Staphylococcus aureus suspension in equal volumes on the membrane filters within each of two filter holders. We introduced nitrogen gas at a pressure of 40 psi into one filter holder for 30 seconds, while the second holder was left untouched. No differences in colony counts were observed on the two membrane filters after their incubation on sheep blood agar.
The findings shown in Table 2
support the observations of others,12–15 namely that inactivity encourages bacterial population increases, even in the air lines of air-water syringes. While such accumulations of contaminants may not be as massive as those seen in samples from water-lines, these observations suggest that routinely blowing out air lines before the start of the day’s activities may have merit.
While the number of microbial colonies detected varied, the presence of any colony was considered a positive result for the purposes of this study. One sample of the 100 samples collected was contaminated—evidenced by an overgrowth of mold on the surface of the membrane and agar—and, therefore, was not included in the statistical analysis. Therefore, based on the 99 air-line samples in this study, the results of the analyses reveal that the significant t value obtained is significantly different from zero. We detected bacteria in 24 percent of the samples. The confidence interval (15 to 32 percent) indicates that the population mean fell within this range and that we could be 95 percent confident of this estimation.
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CONCLUSIONS
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Biofilms commonly occur in dental unit water-lines and act as a source of potential infection particularly for the immunocompromised patient. Typically, identical polyurethane tubing separately delivers compressed air into air-turbine driven handpieces and air-water syringes. The results of this study suggest, at least statistically, that it would be reasonable to expect that a percentage of air lines will have bacteria present and act as another source of potential infection to the patient and personnel in the general vicinity of the dental unit being used.
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FOOTNOTES
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Dr. Hubar is a professor, Department of Oral Diagnosis, Medicine and Radiology, Louisiana State University School of Dentistry, 1100 Florida Ave., New Orleans, La. 70119, e-mail "jhubar{at}lsuhsc.edu". Address reprint requests to Dr. Hubar.
Dr. Pelon is a professor, Department of Microbiology, Immunology and Parasitology, Louisiana State University School of Dentistry, New Orleans.
Dr. Gardiner is a professor and the director of Institutional Services, Planning and Accountability, Louisiana State University School of Dentistry, New Orleans.
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REFERENCES
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ABSTRACT
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