It’s Thursday, September 23, and the World Health Organization has tightened its air quality guidelines.

On Wednesday, the World Health Organization, or WHO, published an update to its air quality guidelines — the international rubric that sets non-binding standards for safe levels of air pollution for the world’s countries. The update, which is the organization’s first since 2005, says that air pollution, alongside climate change, is one of the biggest environmental threats to human health. It sets higher standards for almost every pollutant the organization monitors. Adhering to these new guidelines, the WHO said, “could save millions of lives.”

In the 16 years since the WHO last updated its air quality guidelines, evidence of the harms of indoor and outdoor pollutants has only grown. The new report notes that exposure to air pollutants causes 7 million premature deaths each year.

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The guidelines set new levels for four pollutants: tightening the safe air quality standards for nitrogen dioxide and two types of particulate matter — PM2.5 and PM10 — but loosening the standard for sulfur dioxide. They also created new standards for peak season ozone and carbon monoxide — neither of which had been included in the WHO’s 2005 guidelines.

Ken Lee, a research associate at the University of Chicago and a coauthor of a report on PM2.5 and life expectancy, lauded the WHO for its stricter guidelines. Reducing the threshold for PM2.5 “has pretty major implications,” Lee said. “The health benefits of meeting that more stringent guideline are even greater.”

Joseph Winters and Zoya Teirstein

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