The ultimate test of whether a PFAS water filter is working is laboratory measurement of filtered water quality compared to unfiltered source water. Before-and-after testing provides definitive performance verification that no alternative approach can replicate, giving households direct evidence of actual protection.
When to Conduct Before-and-After Testing
Testing both source and filtered water immediately after filter installation provides a baseline for future comparison. Retesting every 2 to 3 years — or after any system issue such as a filter running past replacement date, an unusual taste or odor event, or a change in source water quality — maintains ongoing performance documentation. For households in high-PFAS areas where even small failures in filter performance could result in exposure above regulatory limits, annual testing of filtered water is a worthwhile investment in protection certainty.
Sample Collection Protocol
Collect source water from a tap upstream of any filtration — typically a cold water tap before the point where the filter system connects, or the unfiltered supply side connection on the filter valve. Collect filtered water from the filter’s dedicated output tap. Follow the laboratory’s exact collection instructions for sample volume, container type, and preservation to ensure results reflect actual filter performance rather than sampling artifacts.
Interpreting Results
Calculate the percentage reduction for each detected PFAS compound: (source concentration minus filtered concentration) divided by source concentration, times 100. Compare this calculated removal rate to the filter’s rated performance for the detected compounds. Removal below the certified rate may indicate filter saturation, incorrect installation, or system malfunction requiring investigation. Consistent performance within rated specifications provides confidence that ongoing protection is being delivered.