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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. 6 — Box and whiskers plot depicting AQI variations for 87 cities in the World. The data show average of the first 4 months of 2018–2021 (January 1st–April 30th). A AQI-PM_2.5 variation, B AQI-PM₁₀ variation, C AQI-SO₂ variation, D AQI-CO variation, C AQI-NO₂ variation and C AQI-O₃ variation. **P < 0.01, *P < 0.05, NA > 0.05