Dental unit waterlines: biofilm forms in the tubing and ends up in the patient's mouth

Dental unit waterlines are the point in the practice where contamination is least visible and most readily overlooked, because the water coming out looks clean.

The problem is physical before it is hygienic. The tubing has a very small diameter, and in a narrow tube the flow velocity near the walls is almost nil.

That zone of stagnation is the ideal environment for biofilm to form, organising on the inner wall and releasing bacteria into the water passing through.

Investigations on untreated units have found bacterial counts far above the limits set for drinking water, and this in systems supplied with compliant mains water.

The composition of the biofilm includes environmental organisms, generally of little consequence for a healthy person, but not for an immunosuppressed patient or a frail older adult.

Legionella deserves specific attention, because the aerosol produced by rotary instruments is the typical transmission route for this organism.

Flushing at the start of the day and between patients reduces the count in stagnant water but does not remove adherent biofilm, and on its own it is not sufficient.

Continuous disinfection systems introduce a low-concentration agent into the supply water and prevent biofilm reforming after removal.

The periodic shock treatment removes biofilm already present and is performed with the products the manufacturer specifies, because some disinfectants damage the circuits.

Periodic microbiological testing is the only way to know whether the protocol works. Without sampling there is no information at all: contaminated water is indistinguishable from compliant water.

Suction poses a different problem, because the risk is backflow of material from the tubing into the patient's mouth, which can occur if they close their lips around the tip.

Disinfecting the suction lines at day's end and cleaning the filters regularly is the maintenance most often neglected, because the result is never seen.

In summary: biofilm forms in the tubing through wall stagnation, flushing does not remove it, continuous disinfection and periodic shock treatment are both needed, microbiological testing is the only verification, and suction lines carry their own backflow risk.