ABSTRACT This proposal is being submitted as an Emergency Competitive Revision for the University of North Carolina (UNC) Superfund Research Program (SRP) in the context of the Coronavirus Disease 2019 (COVID-19) pandemic. We focus on inorganic arsenic (iAs), the #1 contaminant of the Agency for Toxic Substances Registry (ATSDR) contaminating drinking water around the globe. iAs is toxic to many organs in the body, acting as a carcinogen, diabetogen, neurotoxicant, and immunosuppressant. Notably, in several of the North Carolina counties where iAs levels in drinking water are high, the prevalence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection is also high, indicating that a co-exposure to iAs and SARS-CoV-2 likely occurs. This co-exposure to iAs on the susceptibility to SARS-CoV-2 infection and COVID-19 severity has never been studied. The goal of this proposal is to characterize the interaction between iAs exposure and SARS-CoV- 2 infection using differentiated primary human nasal epithelial cells, an established in vitro model for respiratory infections, and a novel humanized hAS3MT mouse strain, in which the metabolism of iAs and disposition of its metabolites resemble those in humans. In a strong, transdisciplinary approach, we will use differentiated primary human nasal epithelial cells and a humanized hAS3MT mice to test the hypothesis that chronic exposure to iAs enhances susceptibility to SARS-CoV2 infection and severity of COVID-19. While integrating human and mouse models, we will also examine the role of sex and genetic background on the disease outcome. The two new additional Aims include: (Aim 1) Identify the effects of iAs and its metabolites on SARS-CoV-2 infection in differentiated primary human nasal epithelial cells; and (Aim 2) Characterize the effects of iAs exposure on SARS-CoV-2 MA infection and COVID-19 outcomes in hAS3MT mice. In Aim 1, we test the hypothesis is that exposure to iAs or its metabolites enhances susceptibility to and severity of SARS-CoV-2 infection in the human nasal epithelial cells. In Aim 2, we will compare immune response, viral titer, lung pathology and mortality in hAS3MT mice exposed to iAs in drinking water (0, 40, or 400 ppb) and infected with a mouse-adapted SARS-CoV-2 that has been recently created at UNC Chapel Hill. To assess the role of sex and genetics, we will use male and female hAS3MT mice with C57BL/6NCrl and 129S6/SvEvTac backgrounds. The proposed research utilizes novel laboratory models that are available only at UNC and are uniquely suited for studies of the interaction between environmental iAs exposure and SARS-CoV2 infection. The proposed research addresses a critical need for understanding of the interaction between a widespread environmental exposure and a pandemic infection that affects hundreds of millions of people worldwide.