Abstract: A dual cartridge system for purifying water supplied to a dental unit and dynamic dental instrument has two disposable elements, each having an iodinated anionic exchange resin containing a continuous fixed-rate release iodinated resin I.sub.(n). Iodine is released from the resin at a specified and consistent rate as water passes through the resin to maintain dental water purity. One cartridge is located in the dental unit water supply tubing. A second cartridge is located in the water supply tubing adjacent the connection to the water supply system. The iodinated biocidal resin in each cartridge neutralizes and kills disease-causing bacteria, virus, protozoa, aspirated oral flora and prevents microbial back-growth to eliminate cross-infection between dental patients.