Quantitative microbial risk assessment of pathogen exposure from rainwater used in high-pressure vehicle washing

A literature-based quantitative microbial risk assessment (QMRA) was performed for the fit-for-purpose use of roof-collected rainwater in high-pressure vehicle washing. Our exposure assessment combined estimates of enteric pathogens in roof runoff (available for <i>Salmonella</i>, <i>Campylobacter</i>, and <i>Giardia</i> spp.) with an experimental study that directly measured vehicle washing exposure doses via a conserved tracer. For dose-response modeling, we considered a disability-adjusted life year (DALY) endpoint to capture the disease burden of potential pathogen infections. Annual risks for domestic and occupational scenarios were compared to a 10^-6 DALY per person per year (ppy) benchmark using either untreated water or water treated to achieve previously reported log reduction targets (LRTs) for other forms of non-potable use. Combined across pathogens, vehicle washing using untreated roof-collected rainwater resulted in 95th percentile risks of 10^-1.4 and 10^-2.4 DALY ppy for occupational and recreational exposures, respectively, exceeding the selected benchmark. Treatment following indoor use or irrigation LRTs met the benchmark for domestic but not occupational use, suggesting that home vehicle washing can be included with other non-potable uses following existing treatment guidances. We also calculated new setting-specific LRTs for both scenarios (1.0-3.5 for domestic and 3.0-5.5 for occupational depending on pathogen), providing explicit risk-based treatment guidance for these applications.