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Background : Masking, which is known to decrease the transmission of respiratory viruses, was not widely practiced in the United States until the COVID-19 pandemic. This provides a natural experiment to determine whether the percentage of community masking was associated with decreases in emergency department (ED) visits due to non-COVID viral illnesses (NCVI) and related respiratory conditions. Methods: Observational study of ED encounters in a 11-hospital system in BLINDED during 2019-2020. Year-on-year ratios for all complaints were calculated to account for ‘lockdowns’ and the global drop in ED visits due to the pandemic. Encounters for specific complaints were identified using the International Classification of Diseases, version 10. Encounters with a positive COVID test were excluded. Linear regression was used to determine the association of publicly available masking data with ED visits for NCVI and exacerbations of asthma and chronic obstructive pulmonary disease (COPD), after adjusting for patient age, sex, and medical history. Results: There were 285,967 and 252,598 ED visits across the hospital system in 2019 and 2020, respectively. There was a trend towards an association between the year-on-year ratio for all ED visits and the BLINDED stay-at-home order (parameter estimate=-0.0804, p=0.10). A 10% percent increase in the prevalence of community masking was associated with a 17.0%, 8.8%, and 9.4% decrease in ED visits for non-COVID viral illness and exacerbations of asthma exacerbations and COPD, respectively (p<0.001 for all). Conclusions: These findings may be valuable for future public health responses, particularly in future pandemics with respiratory transmission or in severe influenza seasons.

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