Toward Better and Healthier Air Quality: Global PM_2.5 and O₃ Pollution Status and Risk Assessment Based on the New WHO Air Quality Guidelines for 2021.

China and India exhibit a decreasing trend in the number of highly polluted days (> 75 μg m −3 ) for PM.

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If the variable changes smoothly, the slope is likely to be similar to the rate of change and can reflect the trend of that data set, with significantly positive values of the slope indicating an increasing trend in the number of exceedance days and significantly negative values of the slope reflecting the opposite trend.

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