World can now breathe easier but further mitigation still needed, air pollution study finds

Ambient fine particulate matter (PM_2.5) is the world's leading environmental health risk factor. At only 2.5 micrometers or smaller, these particles are small enough to be inhaled and cause respiratory problems, such as asthma and bronchitis, and cardiovascular issues, including heart attacks and high blood pressure. For children, PM_2.5 can cause lifelong developmental issues, and for the general population, PM_2.5 is associated with premature death.

To ameliorate these wide-ranging negative impacts resulting from exposure to PM_2.5, which is produced largely through traditional energy sources such as burning fossil fuels or wood, several countries have taken steps to reduce exposure to PM_2.5. But how effective have these mitigation efforts been, and what region or regions are most responsible for driving global PM_2.5 reduction?

Researchers working with Randall Martin, the Raymond R. Tucker Distinguished Professor in the McKelvey School of Engineering at Washington University in St. Louis, examined PM_2.5 data from 1998–2019 to find out.

"The importance of PM_2.5 as a leading risk factor for human health motivates assessment of its long-term changes," Martin said. "We sought to analyze our satellite-derived PM_2.5 estimates for insight into global and regional changes in PM_2.5 exposure and its health effects."

The team's analysis, published in Nature Communications, showed that global, population-weighted PM_2.5 exposure, related to both pollution levels and population size, increased from 1998 to a peak in 2011, then decreased steadily from 2011 to 2019, largely driven by exposure reduction in China and slower growth in other regions.

"Before this work, there was a knowledge gap regarding quantitative local/regional contributions to global population exposure to PM_2.5 and its changes," said Chi Li, first author on the study and a staff scientist in Martin's research group. "We developed a new regional decomposition approach that jointly considered pollution level and population size, and from that we depicted the first-ever time series of regional contributions to global PM_2.5 air pollution."

Li found that many regions have exhibited decreasing exposure since 2011, including continuous reductions in North America and western Europe. He described recent emerging decreases in China as particularly striking.

"Rigorous air quality management in China, which has been most pronounced since 2013, turned out to be the biggest contributor to this global reversal," Li said. "More than 90% of the reduction of global mean exposure from 2011 to 2019 is from China, according to our regional attribution. This result was astonishing when it was derived, but it could be explained well by the rapid reduction of PM_2.5 concentrations due to China's mitigation efforts, which benefit nearly one-fifth of the global population."

Benefits from PM_2.5 exposure reduction include 1.1 million fewer premature deaths in China alone between 2011 and 2019, as well as improved health more generally. Future interventions to reduce PM_2.5 exposure will have even greater impacts for an aging and growing global population, Li said.

"By combining PM_2.5 data with health data and exposure-response models, we also revealed that despite the recent sustained reduction of global PM_2.5 pollution, population aging and growth are now the main challenges in alleviating PM_2.5 health impacts," Li said. "Reducing the same amount of PM_2.5 now will have stronger health benefits than it would have 20 years ago, a globally prevalent phenomenon highlighted in our study."

The team calculated that in 2019, there were still millions of premature deaths worldwide that could be attributed to PM_2.5, highlighting the urgent need for continued reduction to PM_2.5 exposure. Careful monitoring, especially in regions that are currently poorly monitored but highly populated, including South Asia and the Middle East, will be critical to ongoing improvements in air quality and to evaluate the effectiveness of mitigation efforts, Martin said.

"There is need to continue to sustain and develop global monitoring capabilities for PM_2.5 both from satellite, but also from ground-based measurements," Martin said. "The successes in PM_2.5 reductions serve to demonstrate the benefits of PM_2.5 mitigation efforts, and to motivate further mitigation."