The PF-4 is for use with Berkey Ceramic Filters – If you use Black Berkey filters, you will need the PF-2 *** PF-4 reduction elements are designed for use in conjunction with Berkey Ceramic Filters to adsorb the following unwanted elements found in drinking water. Fluoride, Arsenic V and pre oxidized Arsenic III, Other residual heavy metal ions. FLUORIDE: Testing for fluoride was based on 20-30ppm of the ion in the influent aqueous solution at a flow rate of no more than 3 gpm per cubic foot of media. Results of < 1ppm of the fluoride ion in the effluent were typical for the media (95% reduction). Under optimum conditions, effluent concentrations of less than 50 ppb were readily achieved (99.75% reduction). ARSENIC: This product uniquely targets the entire family of arsenic oxide anions as well as the arsenic cations. REPLACEMENT: Under normal conditions it is recommended that each set of two PF-4 elements be replaced after 1,000 gallons.