Changes in air pollution due to COVID-19 lockdowns in 2020: Limited effect on NO 2 , PM 2.5 , and PM 10 annual means compared to the new WHO Air Quality Guidelines.

Background: Lockdowns have been fundamental to decreasing disease transmission during the COVID-19 pandemic even after vaccines were available. We aimed to evaluate and compare changes in air quality during the first year of the pandemic in different cities around the world, investigate how these changes correlate with changes in mobility, and analyse how lockdowns affected air pollutants' annual means.
Methods: We compared the concentrations of NO ₂ , PM _2.5 , and PM ₁₀ in 42 cities around the world in the first months of the pandemic in 2020 to data from 2016-2019 and correlated them with changes in mobility using Human Development Indexes (HDIs). Cities with the highest decreases in air pollutants during this period were evaluated for the whole year 2020. We calculated the annual means for these cities and compared them to the new World Health Organization (WHO) Air Quality Guidelines. A Student's t-test (95% confidence interval) was used to evaluate significant changes.
Results: Highest decreases in NO ₂ , PM _2.5 , and PM ₁₀ were between -50 and -70%. Cities evaluated for the whole year 2020 generally showed a recovery in air pollution levels after the initial months of the pandemic, except for London. These changes positively correlated with year-long mobility indexes for NO ₂ and PM _2.5 for some cities. The highest reductions in air pollutants' annual means were from -20 to -35%. In general, decreases were higher for NO ₂ , compared to PM _2.5 and PM ₁₀ . All analysed cities showed annual means incompliant with the new WHO Air Quality Guidelines for NO ₂ of 10 μg/m ³ , with values 1.7 and 4.3 times higher. For PM _2.5 , all cities showed values 1.3 to 7.6 times higher than the WHO Guidelines of 5 μg/m ³ , except for New Delhi, with a value 18 times higher. For PM ₁₀ , only New York complied with the new guidelines of 15 μg/m ³ and all the other cities were 1.1 to 4.2 times higher, except for New Delhi, which was 11 times higher.
Conclusions: These data show that even during a pandemic that highly affected mobility and economic activities and decreased air pollution around the world, complying with the new WHO Guidelines will demand a global strategical effort in the way we generate energy, move in and around the cities, and manufacture products.
Competing Interests: Disclosure of interest: The authors completed the ICMJE Disclosure of Interest Form (available upon request from the corresponding author) and disclose no relevant interest.
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