Table 1.

Prevention of contamination of dental unit Hydraulic System (HS).

Indipendent water supply	Distilled or sterile water coming from independent tanks (sterilizable or disposable) can solve the problem of contamination of the dental unit but does not solve the problem of retrograde contamination.
Filters at the dispensing taps	0.22 micron filters at taps do not avoid retrograde contamination, compromise the efficiency of the dental unit, resulting in drips and loss of flow.
Flushing	Forced rinsing for few minutes (30″ −2′) after each patient; forced rinsing at the beginning and at the end of the day; both procedures do not remove the biofilm.
UV Ray	They can reduce the contamination of water origin at the entrance but it is difficult to guarantee its effectiveness.
Electrolictic treatment of water	Biocidal oxidizing radicals such as chlorine derivatives, can be released into the water. However they have a corrosive action on water tubes of the dentist unit.
Continuous treatment with disinfectants	The use of a low concentration of disinfectant in the water is not harmful for patients or for operators but have modest effectiveness and select resistant microorganisms.
Discontinuous treatment with disinfectants	Use of medium or high concentration of disinfectants added to water between one patient and another at night or once a week overnight involves particular attention to the possible deterioration of the implant (valves, tubes, connections) of the dental unit. Different types of treatment have been tried: peracetic acid, sodium hypochlorite at different concentrations (1500–5000 ppm), chlorine dioxide, glutaraldehyde, hydrogen peroxide, phenol, povidone iodine, isopropanol, chlorhexidine, ethanol and chlorhexidine, phosphoric acid, ozone. Sperimentations were carried out on pilot dental unit with the objective of reducing the bacterial charge and reducing or eliminating the biofilm. The results are encouraging on the reduction of bacterial charge, but not on the elimination of the biofilm; the most important problem seems to be the maintenance of the bacterial reduction. In no case the experiments are sufficiently conclusive, although the use of peracetic acid seems to be among the most encouraging.
Antireflux valves	Prevent retro-aspiration but may contaminate and accumulate biofilms or incrustations that make them less efficient. They involve periodic maintenance and replacement.