Introduction:
The quality of dental unit water is of considerable importance since patients and dental staff are regularly exposed to water and aerosols generated from the dental unit. Following ADA instruction, this study was performed to control the contamination of Dental Unit Water Line (DUWL) to less than 200 CFU/ml. The purpose of this study was to evaluate the microbiology of DUWL at dental offices and dental faculty of Babol.
Materials and Methods:
In this laboratory study, DUWL of dental offices from different areas as well as that of the dental faculty of Babol was microbiologically evaluated. An amount of five ml water from the syringes and high speed hand pieces before and 2 minutes after flushing and drinking water of units and tap water were gathered in three different sterile polyethylene dishes. Then the samples were cultured on the specific media and the number of the bacterial colonies were counted after keeping at 37°^c for 48 hours. The data were analysed by SPSS software and chi-square, Fisher’s exact and Paired t-tests as well as ANOVA were used.
 Results:
33.3% of all species samples were positive for presence of bacteria.
Microorganisms isolated were as follow: Staphilococcus aureus, coliform, ecoli, pseudomonas aeroginosa, streptococcus (except  β hemolytic group A), klebsiella, coagulase negative staphylococcus and proteus.
The number of the colonies were more than the standard limit. Contamination of the water of hand pieces was reduced after flushing. Contamination of tap water compared to the water of handpieces was less and mean contamination of the samples gathered from the dental offices was higher than those gathered from the dental faculty.
Conclusion:
Flushing decreases the contamination of DUWL, but in surgeries and in persons with immunedeficiency, the use of other methods of DUWL control and decontamination is recommended.