WHO revises air quality guidelines for the first time in more than 15 years

The World Health Organization tightened its global air quality guidelines Wednesday in its first revision since 2005. The organization said air pollution is one of the "biggest environmental threats to human health."

"Clean air is fundamental to health," the WHO said. "Compared to 15 years ago, when the previous edition of these guidelines was published, there is now a much stronger body of evidence to show how air pollution affects different aspects of health at even lower concentrations than previously understood."

Under the guidelines, the WHO lowered recommended exposure levels to key pollutants including ozone, nitrogen dioxide, sulfur dioxide, carbon monoxide, PM 2.5 and PM 10. They were adjusted to account for the latest evidence that shows the health effects from exposure. The study found PM 2.5 and PM 10 are particularly dangerous, having the ability to travel deep into either the lungs or the bloodstream.