Toward Better and Healthier Air Quality: Global PM2.5 and O3 Pollution Status and Risk Assessment Based on the New WHO Air Quality Guidelines for 2021

Abstract To reduce the high burden of disease caused by air pollution, the World Health Organization (WHO) released new Air Quality Guidelines (AQG) on September 22, 2021. In this study, the daily fine particulate matter (PM2.5) and surface ozone (O3) data of 618 cities around the world is collected from 2019 to 2022. Based on the new AQG, the number of attainment days for daily average concentrations of PM2.5 (≤ 15 µg m−3) and O3 (≤ 100 µg m−3) is approximately 10% and 90%, respectively. China and India exhibit a decreasing trend in the number of highly polluted days (> 75 µg m−3) for PM. Every year over 68% and 27% of cities in the world are exposed to harmful PM2.5 (> 35 µg m−3) and O3 (> 100 µg m−3) pollution, respectively. Combined with the United Nations Sustainable Development Goals (SDGs), it is found that more than 35% of the world's cities face PM2.5‐O3 compound pollution. Furthermore, the exposure risks in these cities (China, India, etc.) are mainly categorized as “High Risk”, “Risk”, and “Stabilization”. In contrast, economically developed cities are mainly categorized as “High Safety”, “Safety”, and “Deep Stabilization.” These findings indicate that global implementation of the WHO's new AQG will minimize the inequitable exposure risk from air pollution.

Table S3.Statistics on the number of cities in each continent under the new AQG PM2.5 (μg/m 3 ) Targets.The figure displays a significant decreasing trend in the number of cities with PM2.5 concentrations exceeding 70μg/m 3 from winter to summer, with the percentage of these cities decreasing from 34.15% in winter (d), to 27.53% in spring (a), and finally to 11.85% in summer (b).Specifically, Asia (Figure S2) are the regions that best highlight the seasonal variations that characterize PM2.5.Cities in Asia with PM2.5 concentrations exceeding 70μg/m 3 accounted for 88.78% (Winter), 87.34% (Spring), 67.65% (Summer), and 90.77% (Autumn) of the number of cities with PM2.5 concentrations exceeding 70μg/m 3 in the world, respectively.And we find that PM2.5 concentrations are generally higher in central and eastern cities of China than in the southern and coastal ones.As an extreme example, in winter, the PM2.5 concentrations in Shijiazhuang (148.27μg/m 3 ) is almost twice as high as that in Haikou (64.8μg/m 3 ).
In addition, both Mumbai and Delhi in India exhibit PM2.5 concentrations exceeding 140μg/m 3 during winter, categorizing them as highly polluted regions.By comparison, we found that Japan has the lowest PM2.5 concentrations (37.97μg/m 3 ) in Asia and is closer to the average levels in Europe (37.99μg/m 3 ) and the United States (31.79μg/m 3 ).

Figure
Figure S1displays the quarterly average concentration values of PM2.5 throughout the investigation period.The figure displays a significant decreasing trend in the number of cities with PM2.5 concentrations exceeding 70μg/m 3 from winter to summer, with the percentage of these cities decreasing from 34.15% in winter (d), to 27.53% in spring (a), and finally to 11.85% in summer (b).Specifically, Asia (FigureS2) are the regions that best highlight the seasonal variations that characterize PM2.5.Cities in Asia with PM2.5 concentrations exceeding 70μg/m 3 accounted for 88.78% (Winter), 87.34% (Spring), 67.65% (Summer), and 90.77% (Autumn) of the number of cities with PM2.5 concentrations exceeding 70μg/m 3 in the world, respectively.And we find that PM2.5 concentrations are generally higher in central and eastern cities of China than in the southern and coastal ones.As an extreme example, in winter, the PM2.5 concentrations in Shijiazhuang (148.27μg/m 3 ) is almost twice as high as that in Haikou (64.8μg/m 3 ).In addition, both Mumbai and Delhi in India exhibit PM2.5 concentrations exceeding 140μg/m 3 during winter, categorizing them as highly polluted regions.By comparison, we found that Japan has the lowest PM2.5 concentrations (37.97μg/m 3 ) in Asia and is

Figure
Figure S3displays the quarterly average concentration values of O3 throughout the investigation period.The global average O3 concentrations were 47.02μg/m 3 and 47.58μg/m 3 in spring and summer, respectively.Unlike PM2.5, which displays seasonal variation, O3 pollution is concentrated during the summer.The figures indicate that the percentage of the number of cities with O3 concentration values below 40μg/m 3 decreased from 43.56% in spring (a), decreased to 34.84% in summer (b), and increased to 80.68% in winter (d), showing a decreasing and then increasing trend.And similar to PM2.5, the area affected by O3 pollution also spreads across central and eastern cities in China.Specifically, the average summer O3 concentrations in China were 73.96μg/m 3 (FigureS4).Of the cities studied, 63.64% had average O3 concentrations exceeding 60μg/m 3 .As a result, China has the highest O3 concentrations in the Asian region.Comparatively, Thailand and Japan had the lowest average O3 concentrations in Asia, measuring at 19.35μg/m 3 and 50.57μg/m 3 , respectively.From an urban perspective, during the summer in China, the cities of Shijiazhuang, Anyang, and Zhengzhou have the highest O3 pollution, with concentrations of 142.02μg/m 3 , 121.27μg/m 3 , and 119.29μg/m 3 , respectively.In addition, during the summer, average O3 concentrations in Delhi, Taiyuan, and Jinan exceed 100μg/m 3 .While the average O3 concentrations in Europe and America were 33.09μg/m 3 and 38.93μg/m 3 , respectively.Even in summer, only 1.16% of European cities and 18.92% of American cities had average O3 concentrations that exceeded 60μg/m 3 .Similarly, it was observed that the highest concentration of O3 tends to happen during the summer months, as shown in FigureS5.Specifically, the percentage of cities with maximum O3 concentrations exceeding 100μg/m 3 decreased from 24.62% in the summer, to 17.8% in the fall, and ultimately to 11.74% in the winter.In terms of spatial distribution (FigureS6), cities with high O3 pollution (maximum O3 > 100μg/m 3 ) during summer are concentrated mainly in China and India.e.g.Shijiazhuang (253.83μg/m 3 ), Mumbai (261.85μg/m 3 ).During summer, high levels of O3 pollution affect more than 50% of Chinese cities, while less than 10% of European and American cities with similar levels of O3 pollution.

Figure S7 .
Figure S7.Statistical test for analyzing trends in PM2.5 and MDA8 O3 concentration using the Mann-Kendall method.

Table S1 .
Number of days per year when the concentrations of PM2.5 exceeds the threshold.

Table S2 .
Number of days per year when the concentrations of MDA8 O3 exceeds the threshold.