Metal Analyses of Dental Unit Water Systems

Microbial deposits in dental unit water lines (DUWL) are a potential health problem. Flushing with sodium hypochlorite is one possible procedure for controlling biofilm in DUWL; however, strong oxidizing agents, such as 5000 ppm sodium hypochlorite or 5000 ppm sodium hypochlorite in 1% acetic acid, may attack metal components in a dental unit. Dental units in five clinics equipped with separate water reservoir systems (SWS) were analyzed for metal content in the water. Source water for these units was either municipal water (MW), distilled water (dH2O), or sterile deionized water (sdH2O). Except for calcium, there were no major differences in the metal content of source water drawn from any of the units. The units using dH2O and sdH2O had been treated with either 500 or 5000 ppm bleach from several weeks to several years. A metal profile of the effluent obtained during treatment with hypochlorite showed no major dissolution of metal components (average Cu = 0.56, Ni = 0.17, Zn = 0.91 mg/L). The final water sample flushed from the units after hypochlorite treatment with SWS contained less metal than the water from the units connected to the local municipal water system. The metal ions in the effluent from units being treated with hypochlorite in 1.0% acetic acid were approximately 10 times greater than those from units treated with sodium hypochlorite alone (Cu = 20, Ni = 1.5, Zn = 22 mg/L). Flushing DUWL with 5000 ppm bleach and using sdH2O in the SWS caused less corrosion and resulted in lower metal concentration in the water than the use of municipal water.