Table 1.

Summary of published studies examining changes in air pollution attributable to COVID-19 related interventions in the US and globally.

Citation	Geographic locations	COVID19 related intervention	Confounding adjustment	Statistical approach	Results
Berman et al.22	United States (all counties in the U.S. with both NO2 and PM2.5 monitors)	Reduced traffic and mandated business closures between March 13-April 21. March 13th being when U.S. reported cases exceeded 2000 and the first enacted state-wide social distancing order	None	Two-sided t-tests paired by county (α = 0.05)	25.5% reduction (4.8 ppb) in NO2 was observed during the COVID-19 period
					NO2 decline was statistically significant regardless of when mandated business closures were implemented
					11.3% statistically significant reduction (0.7 μg/m3) of PM2.5 in counties from states that instituted early non-essential business closures
Gillingham et al. 28 (Commentary)	United States (785 monitors)	Shutdowns	Weather and seasonality	Global polynomial and a two-step local regression	PM2.5 concentrations have decreased by around -0.5 μg/m3 since the start of the shutdowns
					Estimated 11% NOx decrease in daily local emissions
					There is insufficient evidence to prove that there was a significant decrease in PM2.5 concentrations in the U.S
Goldberg et al.24	20 cities in North America	COVID-19 Physical distancing measures (lockdown) (15 March to 30 April post-covid-19 period)	Solar zenith angle and meteorological conditions over very short time scales	Average differences	Adjusted for seasonality and meteorology, NO2 had a median drop of 21.6% before and after COVID‐19 physical distancing
Karaer et al. 25	Florida	COVID-19 social distancing behaviors (March 2020)	Population density and income	A cross-correlation based dependency analysis	The decrease in NO2 concentrations and vehicle miles travelled (VMT) started 2 weeks before the official stay-at-home order and resulted in 54.07% and 59.68% decrease in NO2 and VMT by the end of the month, respectively
Miech et al. 27	Phoenix	COVID-19 Stay at home orders (pre-COVID-19: Jan 6-March 6 & Post-COVID-19: March 13-April 8)	Meteorological parameters (horizontal wind speed, temperature, precipitation, and planetary boundary layer height)	Linear regression model	No uniform decrease was found in CO or NO2 across the three sites studied
					There was a significant decrease (45%) in PM10 at all the sites compared to the past two years
Parker et al. 26	Southern California	Stay-At-Home orders (19 March-30 June of the last 5 years)	Meteorological differences	Average differences	Concentrations of PM2.5 and NOx showed an overall reduction (10–45% and 13–40%, respectively) across the basin in 2020
					O3 concentrations decreased (9 ppb or 22%) in the western part of the basin and increased (8 ppb or 15%) in the downwind areas
Venter et al. 36	34 countries	Lockdown (Jan 1- May 15)	Meteorological variability	Linear regression models	11 μg/m3 reduction in NO2 (on average 60% reduction)
					12 μg/m3 reduction in PM2.5 (on average 31% reduction)
					4 μg/m3a increase in O3 (4% increase)
Fu et al.35	20 selected major cities around the world	Lockdown (lockdown period in each city compared to same period in the past 3 years)	Meteorological variability	ANOVA and Tukey’s HSD tests	NO2 decreased significantly in all cities relative to the past 3 years
					PM2.5 decreased in all cities and found a significant decrease in 9 cities relative to each of the 3 years
Benchrif et al. 50	21 selected cities around the world	Lockdown	None	Descriptive statistics	PM25 and NO2 concentrations declined considerably in different cities during lockdown period
Hammer et al.51	China, Europe, and North America	Lockdown (Jan – Apr 2020)	None	Descriptive statistics and simulation study	PM2.5 concentrations decreased in all study locations compared to same period during 2018 and 2019