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

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Fig. 5 — Percentage change of AQI-CO and O₃ between 2020 during the lockdown period and 2019 and 2021 in different cities in the world. A AQI-CO variation and B AQI-O₃ variation