Monitoring of copper-silver in hospital water systems?

We have had a copper-silver ionization system on our hot water system for more than a year now. No cases of hospital-acquired Legionnaires’ disease have occurred since installation, but the copper and silver ion levels are sometimes less than the recommended range by the manufacturer (copper 0.2-0.8 ppm; silver at 0.02-0.08 ppm (20-80 ppb). Is this cause for alarm or justification to perform additional disinfection like a superheat and flush?

Our experience indicates that ion levels slightly below the manufacturer’s recommended levels are still effective in systems that have been operational for awhile. For example, we have had copper levels in the range of 0.1-0.2 ppm and silver at 0.01-0.02 ppm (10-20 ppb). Our environmental cultures remain well below our threshold of 30% positivity (0-10% most of the time). I would request either cleaning of the electrodes or adjustments in amperage if the level of copper is at or below 0.1 ppm copper or 0.01 (10 ppb) silver.

We would not recommend that a decision to perform a superheat and flush be based on ion levels. This procedure is labor intensive and should be performed in response to either detection of a hospital-acquired case of Legionnaires’ disease or an increase in colonization of the system to 30% or above.