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 and particulate matter. They were adjusted to account for the latest evidence that shows the health effects from exposure. The study found particulate matter “equal or smaller than 10 and 2.5 microns (µm) in diameter” are particularly dangerous, having the ability to travel deep into either the lungs or the bloodstream.

According to the WHO, exposure to air pollution results in 7 million premature deaths every year, as well as the loss of millions of more healthy years of life.

“In children, this could include reduced lung growth and function, respiratory infections and aggravated asthma,” the WHO said. “In adults, ischaemic heart disease and stroke are the most common causes of premature death attributable to outdoor air pollution, and evidence is also emerging of other effects such as diabetes and neurodegenerative conditions. This puts the burden of disease attributable to air pollution on a par with other major global health risks such as unhealthy diet and tobacco smoking.”

The organization said the effects of air pollution are more prevalent in low income communities and countries. 

“Disparities in air pollution exposure are increasing worldwide, particularly as low- and middle-income countries are experiencing growing levels of air pollution because of large-scale urbanization and economic development that has largely relied on the burning of fossil fuels,” the WHO said.

The group called for countries to make “gradual progress in improving air quality, marked by the achievement of interim targets,” with the ultimate goal of staying under recommended pollution levels.

“We know the magnitude of the problem and we know how to solve it,” WHO Regional Director for Europe, Dr. Hans Henri P. Kluge said in a statement Wednesday. “These updated guidelines give policy-makers solid evidence and the necessary tool to tackle this long-term health burden.”