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. 2021 Dec 5;33(1):134. doi: 10.1186/s12302-021-00575-y

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© The Author(s) 2021

Open AccessThis article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 4 — Percentage change of AQI PM_2.5, PM₁₀, NO₂ and SO₂ between 2020 during the lockdown period and 2019 and 2021 in different cities in the world. A AQI-PM_2.5 variation, B AQI-PM₁₀ variation, C AQI-NO₂ variation and D AQI-SO₂ variation