Bacterial growth is logarithmic in nature, and generation times for some waterborne organisms are measured in minutes. As such, untreated or improperly treated units can be quickly re-colonized from small numbers of "survivor" organisms.
When clinical testing shows rebound contamination in a treated dental unit, the USAF found that the higher levels of bacteria could be attributed to one of the following human errors.
These troubleshooting tips have been adapted from the USAF Dental Investigation Service's document "Troubleshooting Dental Water Quality Problems"
- Non-compliance. Dental units must be treated according to the established and recommended schedule. Failure to treat when indicated will result in rapid regrowth of the biofilm.
Untreated units left idle for various reasons usually exhibit re-
growth of biofilms. If a unit is to be out of service for an
extended period of time, treat the unit, completely air purge
the lines, and store it dry.
For short periods of disuse, continue weekly treatment. All
units left unused for more than the standard retreatment
period should be re-treated before clinical use.
2. Incomplete treatment. All lines (including ultrasonic scaler and air/water syringe lines) capable of carrying water have biofilm, whether they are routinely used or not. Untreated
lines provide a refuge for bacteria and other organisms that
can quickly re-contaminate the entire system.
In newer dental units with separate water reservoirs, use the
purge mechanism to facilitate simultaneous treatment
3. Failure to air purge. Many intermittent chemical treatment
protocols are more effective when water is removed from the
lines both before and after introduction of chemical
The first air purge assures minimal dilution of chemical agent.
The second purge following disinfectant treatment enhances
the effectiveness of the water flush to remove residual
chemicals. A final air purge, to leave the unit dry when not in
use, discourages regrowth of biofilm bacteria.
4. "Sewage in, sewage out." Since no dental unit can provide water that is cleaner than the water that enters it, take care
to assure that source water used to fill independent water
reservoirs is of acceptable quality (less than 200 CFU/ml of
heterotrophic mesophilic bacteria).
Practices using water distillers should note that it is equally
important to properly maintain water distillers and storage
Disinfect large storage vessels and individual water
reservoirs at least weekly.
If bottled sterile water is used to fill water reservoirs, date
the bottle when opened, re-cap it immediately, and use as
quickly as possible.
5. Inadequate contact time. For most chemical treatments, the effects of shorter than recommended treatment times are
unknown but unlikely to be beneficial.
6. Improper dilution. Dilution can influence treatment efficacy.
As an example, studies have shown 5.25% bleach diluted
1:10 (that is, 1 part bleach to 9 parts water) to be more
effective than a 1:100 dilution in controlling bacteria in
However, just as dilution that is too weak is likely to be
ineffective, dilution that is too strong is likely to cause
deterioration of dental equipment that contains metal
Note: The recommended 1:10 dilution is based on using commercial bleach packaged by the gallon at 5.25% (52,500 ppm) sodium hypochlorite, which dilutes to a 5,250 ppm bleach solution. Commercial bleach supplied in a 96-ounce bottle contains the same amount of sodium hypochlorite, therefore at a higher concentration (about 7%, or 70,000 ppm). To correct for the higher concentration, the proper dilution for waterline treatment is approximately 1:13 (1 part bleach and 12 parts water), which yields a 5,385 ppm sodium hypochlorite solution.
"Note" on NaOCl concentration in Item 6 provided by Richard Karpay, DDS, MPH.