A quantitative microbial risk assessment of activated carbon point-of-use filters and the risk of Legionella pneumophila

Activated carbon point-of-use (POU) filters have become increasingly popular. Often certified to remove lead and improve water taste, these filters are not certified to remove bacteria. In this study, we examined the effects of using POU filters on the risk posed by Legionella pneumophila (L. pneumophila) in a quantitative microbial risk assessment model. Release rates of L. pneumophila were measured for new and artificially aged (ripened) POU filters. Additionally, we measured aerosol concentrations produced by water fixtures that might have a POU filter (e.g., drinking fountain bubbler, faucet). We modeled scenarios without filters (with and without residual chlorine) and with filters (new and ripe filters), four water usage activities (drinking from a drinking fountain, handwashing, dishwashing, and showering), and three age groups (children, adults, and elderly). Systems with the highest to lowest risk were those without filters and without chlorine, those with ripe filters, those without filters and with chlorine, and finally, those with new filters. The risk of illness decreased for activities with shorter exposure times, such as drinking and handwashing. The L. pneumophila concentrations, aerosol concentrations, and exposure time had the largest impacts on the risk of illness. The maximum allowable concentration of L. pneumophila needed to achieve a 10^-4 target annual risk of illness was 0.02 copies/mL. This study showed that POU filters may reduce L. pneumophila risk but only if the filter is new (≤ 1 week old before the filter ripens significantly), and other strategies for reducing L. pneumophila concentrations or decreasing exposure time may be needed to further decrease the risk of illness.