question: The current study evaluated ozone levels through passive samplers installed in 4 different points in a medium-sized city (Rio Grande, Brazil) with naturally low NO2 levels during a week of COVID-19 lockdown. Additionally, we evaluated the consequences of this response with regard to human health risk assessment and reduction of hospital admissions and ozone-related deaths. The reduction in ozone levels, one month after the implementation of containment measures, varied between 26 and 64% (average of 44%), in the different studied sites. The reduction of human mobility during the pandemic reduced the levels of ozone in Rio Grande city and consequently will bring benefits to health services in the municipality. This unexpected reduction in O3 levels must be related to the low 'natural' levels of NO2 in the city, which make the contribution of other precursors important for the fluctuation of O3 levels.