Int. J. Environ. Res. Public Health 2012, 9 2761 numerous changes had been implemented in methods used for disinfection and monitoring and improvements made to the equipment used in the production and distribution of ultrapure dialysis water, particularly in dialysis units adopting online convective procedures with production of high volumes of infusate, such as online hemodiafiltration techniques. 2. Materials and Methods Characteristics of the units: five dialysis units were investigated over a 16 year 3 month period (195 months) from May 1996 to April 2012. In total 945 samples were taken (minimum three samples for each distribution ring every three months in steady state equipment). The observation periods are illustrated in Tables 1–3. 2.1. Use of Polyvinyl Chloride (PVC) Piping: Two Centres—See Table 1 PVC piping was used in two successive periods: (1) Use of irregular/occasional microbiological controls, chemical disinfection and SRO. (2) Use of regular microbiological controls, chemical disinfection and SRO in polyamide. The use of PVC however was discontinued and it was replaced with PEX rings in one unit and AISI INOX 316L steel rings in the other. Student’s T-test was used for comparing data. Results were considered significant when p was <0.05. Table 1. PVC piping. Occasional chemical disinfection Regular monthly Two dialysis centres (average every quarter) disinfection Reverse osmosis SRO TRO 29 a Technical dialysis posts 29 Observation time, months 60 36 Sampling, total numbers 120 72 CFU, mL–1, average, bacteria * 238.7 ± 374.3 153.9 ± 160.7 CFU, mL–1, average, mycetes * 19.2 ± 34.4 b 5.2 ± 7.5 c CFU, mL, medianbacteria 122.2 25.5 vCaFluUe,smbLac,tmeriinaimum and maximum 7–1,773 0–1,500 a Six posts switch to INOX and another 23 switch to PEX piping (Tables 2 and 3); the PVC piping ring was abandoned due to its inadequate safety; b vs. c: p < 0.05; (*) values represent averages ±1 standard deviation from (heterotrophic plate count) HPC 22 °C measurements. Average and median are both derived from bacteria and mycetes at HPC 22 °C. 2.2. Use of AISI INOX 316L Piping: Three Centres—See Table 2 We used high corrosion resistance stainless steel piping in three successive periods: (1) Use of AISI INOX 316L steel distribution ring with irregular/occasional microbiological controls, chemical disinfection and one stage reverse osmosis (SRO) system in polyamide;