Quantitative microbial risk assessment of bioaerosols from a manure application site

Abstract

Bioaerosol concentrations and deposition were monitored at the edge of a dairy manure application site in northern New York State. Total bacteria, fecal indicator bacteria (Enterococcus spp. and Escherichia coli), and select bacterial pathogens (Salmonella spp., Campylobacter spp., and E. coli O157:H7) were measured in the manure and air by real-time quantitative PCR (qPCR). The 8-h average bacterial air concentration measured by liquid impingement following manure application was 7.89 × 105 copies m−3, one order of magnitude greater than mean background measurements (6.35 × 104 copies m−3; n = 6). Eight-hour ambient concentration of Enterococcus spp. was 1.54 × 104 copies m−3; E. coli and pathogens were less than their respective limits of detection. The measured deposition flux of bacteria was 1.08 × 103 copies m−2 s−1, corresponding to bulk deposition velocity of 0.15 cm s−1. Using inverse dispersion modeling with the US Environmental Protection Agency’s AERMOD, the emission of bacteria from the manure-amended field was estimated to be 1.27 × 105 copies m−2 s−1. AERMOD was also used to model downwind bioaerosol concentrations; the greatest modeled 8-h average downwind bacteria concentrations were 8.00 × 105 copies m−3 above background at 100 m and 3.95 × 103 copies m−3 above background at 1,000 m. Potential health risks associated with these bioaerosols were estimated by quantitative microbial risk assessment based on AERMOD results using measured pathogen concentrations in land-applied manure and emission rate estimates for total bacteria. Median risks of infection over an 8-h exposure period were 1:500 at 100 m and 1:100,000 at 1,000 m; peak risks (95th percentile) were 1:250 and 1:50,000, respectively.