Health Risks Linked to Air Pollution at Levels Below WHO Targets

A short-term rise in nitrogen dioxide air pollution could be tied to a bump in local mortality rates the next day, researchers showed in a large international study.

Across 398 cities (located mostly in the U.S., Europe, and Asia), each 10 μg/m³ increase in ground-level NO₂ concentration was associated with increases in mortality the next day that persisted after adjustment for other air pollutants:

• Total mortality up 0.46%
• Cardiovascular mortality up 0.37%
• Respiratory mortality up 0.47%

Concentration-response curves, based on data from 1973 to 2018, were almost linear with no minimum threshold for harm, reported Haidong Kan, MD, PhD, of Fudan University in Shanghai, and colleagues in an epidemiological study published online in The BMJ.

“This result suggests that NO₂ is associated with considerable health risks even at levels below health based standards and guidelines, including the current WHO air quality guidelines,” the group wrote.

Cities participating in the study had a median average annual nitrogen dioxide concentration of 26.9 μg/m³. The WHO sets a target of 40 μg/m³ in annual mean concentration of NO₂ in air quality guidelines.

Ambient nitrogen dioxide primarily comes from vehicle and power plant emissions. The U.S. Environmental Protection Agency concluded in 2016 that there is a causal relationship between short-term exposure to NO₂ and respiratory effects; evidence was deemed insufficient to blame exposure for total mortality and cardiovascular effects, however.

“Our findings from this multi-location analysis add to the supporting evidence for causal associations between short term exposure to NO₂ and non-respiratory endpoints,” Kan and colleagues wrote.

“Although reduction of NO₂ to zero is infeasible, our analysis provides insight into the public health benefits of substantial reductions in NO₂, suggesting considerable health benefits from stricter control of NO₂ emissions and tightening of the regulatory limits of NO₂ in future revisions of WHO air quality guidelines,” they noted.

For their epidemiological study, the investigators drew upon the Multi-City Multi-Country Collaborative Research Network. Mortality data, obtained from local authorities within each country, captured 62.8 million total deaths from 1973 to 2018.

That most data were obtained from select developed areas precluded global generalization of the findings, Kan’s team cautioned.

Furthermore, their database was subject to potential exposure misclassification and coding errors.