ObjectiveThe aim of this study was to count anaerobic bacteria before and after the use of dental equipment and to study the influence of chlorhexidine on the dental unit reservoir water.MethodsSterile swabs were used to collect bacterial samples from the cuspidor, lights, syringes, low- and high-speed handpieces, and dental chairs (arms and backrest) before and after the placement of barriers. Blood agar plates were placed on the patient’s and dentist’s forehead and by the patient’s nose and shoulder and exposed to aerosols without (Group 1) and with 0.5% (Group 2) and 1.0% (Group 3) chlorhexidine generated by the high-speed handpiece. Ten aerosol samples were collected for each group. A sample of 1mL of the dental unit reservoir water was collected before and after the use of the high-speed handpiece. The anaerobic bacterial counts were compared by the Wilcoxon and Kruskal-Wallis tests.ResultsThe contamination on the high-speed handpiece (p=0.0431) and cuspidor (p=0.0117) increased significantly after use. Contamination in the dental unit reservoir water also increased significantly after use of the high-speed handpiece. The most contaminated area was the patient’s nose.ConclusionThe addition of 0.5% and 1.0% chlorhexidine in the dental unit reservoir water reduced the microbial contamination in the dental office significantly.Indexing terms: Bacteria, Anaerobic. Contamination. Dental equipment. Chlorhexidine.