Applications of Quantitative Microbial Risk Assessment to Respiratory Pathogens and Implications for Uptake in Policy: A State-of-the-Science Review.

BACKGROUND: Respiratory tract infections are major contributors to the global disease burden. Quantitative microbial risk assessment (QMRA) holds potential as a rapidly deployable framework to understand respiratory pathogen transmission and inform policy on infection control. OBJECTIVES: The goal of this paper was to evaluate, motivate, and inform further development of the use of QMRA as a rapid tool to understand the transmission of respiratory pathogens and improve the evidence base for infection control policies. METHODS: We conducted a literature review to identify peer-reviewed studies of complete QMRA frameworks on aerosol inhalation or contact transmission of respiratory pathogens. From each of the identified studies, we extracted and summarized information on the applied exposure model approaches, dose-response models, and parameter values, including risk characterization. Finally, we reviewed linkages between model outcomes and policy. RESULTS: We identified 93 studies conducted in 16 different countries with complete QMRA frameworks for diverse respiratory pathogens, including SARS-CoV-2, Legionella spp., Staphylococcus aureus, influenza, and Bacillus anthracis. Six distinct exposure models were identified across diverse and complex transmission pathways. In 57 studies, exposure model frameworks were informed by their ability to model the efficacy of potential interventions. Among interventions, masking, ventilation, social distancing, and other environmental source controls were commonly assessed. Pathogen concentration, aerosol concentration, and partitioning coefficient were influential exposure parameters as identified by sensitivity analysis. Most (84%, n=78) studies presented policy-relevant content including a) determining disease burden to call for policy intervention, b) determining risk-based threshold values for regulations, c) informing intervention and control strategies, and d) making recommendations and suggestions for QMRA application in policy. CONCLUSIONS: We identified needs to further the development of QMRA frameworks for respiratory pathogens that prioritize appropriate aerosol exposure modeling approaches, consider trade-offs between model validity and complexity, and incorporate research that strengthens confidence in QMRA results. https://doi.org/10.1289/EHP12695.

Novel Integrated Biotrickling Filter-Anammox Bioreactor System for the Complete Treatment of Ammonia in Air with Nitrification and Denitrification.

An integrated biotrickling filter-anammox bioreactor system for the complete treatment of ammonia in air with conversion to nitrogen gas without the supply of an extraneous electron donor for denitrification was established. Partial nitritation (i.e., conversion of ammonium to nitrite) was successfully achieved in the biotrickling filter (BTF) through free ammonia (FA) and free nitrous acid (FNA) inhibition on nitrite-oxidizing bacteria (NOB). During transients, while increasing nitrogen loading, FA was the main inhibitor of ammonia-oxidizing bacteria (AOB) and NOB, while during a steady state, it was mainly FNA, which was responsible for inhibitory effects due to the accumulation of nitrite. Ammonia removal by the BTF reached 50 gN m-3 h-1 with 100% removal at an inlet concentration of 404 ppmv and a gas residence time of 21 s. Average removal of ammonia during stable operation was 95%. The anammox bioreactor was slightly undersized compared to the BTF and could remove 75% of total nitrogen discharged by the BTF when the two reactors were connected and liquid was in one-pass mode. This undersizing caused accumulation of nitrite in the system when liquid was circled in a quasi-closed loop, which gradually inhibited the activity of anammox bacteria. Overall, this study demonstrates that ammonia in air can be effectively treated and converted to harmless nitrogen gas without an external electron donor supply using a biotrickling filter combined with an anammox bioreactor.