COVID-2019 (COronaVIrus Disease 2019), discovered at the end of December 2019, has spread around the world, becoming a pandemic. To prevent its spread, many governments implemented strict lockdown measures, including the closure of factories and limitations on travel. These measures led to a decrease in human activity, affecting air pollutant emissions. This study evaluates air quality during the partial lockdown of Shanghai, the largest city in China, and the broader Yangtze River Delta region. It was found that daily concentrations of PM_2.5, PM₁₀, SO₂, NO₂, and CO during the lockdown period were reduced by 9%, 77%, 31.3%, 60.4%, and 3% respectively, compared to the same period in 2019. Even with decreasing concentrations of PM_2.5 and PM₁₀, the overall values are still more than four times higher for the lockdown period than the World Health Organization (WHO) recommends for safe concentrations (10 μg/m³ and 20 μg/m³). This indicates the existence of other background sources that continue to make a significant contribution to air pollution in the region even with severe reductions in human activity. This study may be used to guide environmental policy, as it demonstrates to what extent the control of pollution sources can improve air quality.