impact of COVID-19 lockdown on air pollution in Europe and North America: a systematic review.

Background Multiple studies report reductions in air pollution associated with COVID-19 lockdowns. Methods We performed a systematic review of the changes observed in hazardous air pollutants known or suspected to be harmful to health, including nitrogen dioxide (NO₂), nitrogen oxides (NO_x), carbon monoxide (CO), sulfur dioxide (SO₂), ozone (O₃) and particulate matter (PM). We searched PubMed and Web of Science for studies reporting the associations of lockdowns with air pollutant changes during the COVID-19 pandemic in Europe and North America. Results One hundred nine studies were identified and analyzed. Several pollutants exhibited marked and sustained reductions. The strongest was NO₂ (93% of 89 estimated changes were reductions) followed by CO (88% of 33 estimated pollutant changes). All NO_x and benzene studies reported significant reductions although these were based on fewer than 10 estimates. About three-quarters of PM_2.5 and PM₁₀ estimates showed reductions and few studies reported increases when domestic fuel use rose during COVID-19 lockdowns. In contrast, O₃ levels rose as NO_x levels fell. SO₂ and ammonia (NH₃) had mixed results. In general, greater reductions appeared when lockdowns were more severe, as well as where baseline pollutant levels were higher, such as at low-elevation and in densely populated areas. Substantial and robust reductions in NO₂, NO, CO, CO₂, PM_2.5, PM₁₀, benzene and air quality index pollution occurred in association with COVID-19 lockdowns. O₃ levels tended to increase, while SO₂ and NH₃ had mixed patterns. Conclusions Our study shows the profound impact of human activity levels on air pollution and its potential avoidability. [ABSTRACT FROM AUTHOR]