The correlation of DUWL frequency of use with effluent contamination

Objective: No previous studies have examined the correlation of dental unit waterline (DUWL) frequency of use with effluent contamination. Methods: Six functioning dental units with self-contained water systems in an outpatient dental clinic were used. For four weeks, three units were chemically treated with freshly mixed chlorine dioxide (Group A) according to manufacturer's instructions, and three units had no chemical treatment (Group B). For a further four weeks, a crossover design was used, and units that were initially A became B, and vice versa. The frequency of use of individual lines; high-speed handpiece; slow-speed handpiece; unit air/water; assistant air/water; and ultrasonic line was recorded after each clinical session. Weekly water samples were collected aseptically from the five lines on each unit in bottles that contained sodium thiosulphate. Triplicate ten-fold serial dilutions in phosphate buffer solution were made and agitated vigorously for 15 seconds. One-tenth of a milliliter of each sample was plated on R2A agar using the spread-plate technique and incubated at room temperature for 7 days. Results: A total of thirty-six source tap water samples and 180 DUWL samples were taken. The unit air/water syringe line was the most frequently used; the ultrasonic line was the least frequently used. The B high-speed handpiece line was the most highly contaminated; the A assistant air/water syringe was the least contaminated. There was no correlation between all lines and frequency of use (p<0.10); a negative correlation in Group B (p<0.015); and no correlation in Group A (p>0.50). Conclusion: Increased frequency of use of DUWL did not affect lines that were chemically treated, but was associated with less contamination of untreated DUWL.