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The World Health Organization — which has previously found that indoor and outdoor air pollution killed a shocking 7 million people globally in 2012 — released a new analysis Tuesday underscoring the extent of the risk, which seems to grow worse and worse the more we learn about how damaging tiny airborne particles can be to our health.

Most strikingly, the new report, which combines local data with a global model to determine the extent of deadly air pollution across the planet even in places where there are no instruments recording it, finds that 92 percent of people suffer under pollution levels that are worse than WHO standards (as of 2014). The vast majority of deaths are in developing countries. The document calls air pollution the “largest environmental risk factor.”

Of greatest concern is a form of pollution called PM2.5, referring to particles smaller than 2.5 micrometers. The global health agency believes that a concentration greater than 10 micrograms per cubic meter of these fine particles in the air qualifies as dangerous. The great risk is that the particles are so small that they can be inhaled, travel into the lungs, and enter the bloodstream.

“People think of air pollution as a respiratory disease,” said Carlos Dora, who heads the WHO’s air pollution team. “And in fact, it’s heart disease, strokes and cardiovascular. Because there’s very small particles that go into the blood. … The damage air pollution does to the vessels is similar to the damage that cholesterol or high blood pressure do. That has changed a lot the picture.”

Dora said 10 years ago, few would have reached such conclusions about the severity of air pollution. But since then, he said, it is becoming clear that there are few causes of death that take a larger toll each year, including malaria and tuberculosis. The new report credits air pollution with “about one in every nine deaths annually.”

The new model is allowing WHO to give increasingly fine-grained data about air pollution risks, by combining together over 3,000 actual ground measurements from across the world with satellite observations and an understanding of how air flows around the planet from place to place.

“Based on the modelled data, 92% of the world population are exposed to PM2.5 air pollution concentrations that are above the annual mean” WHO guidelines, the report concluded. “With the exception of the region of the Americas, all regions … have less than 20% of the population living in places in compliance with” WHO standards.

And of course it is actually worse than this, because air pollution contains more deleterious elements than just PM2.5, such as larger particles (PM10) and ozone. Moreover, the same emissions sources that are driving air pollution in many cases are also worsening global climate change.

Air pollution is generated both from vehicles and also power generation and many industrial installations. In many developing countries it is also generated inside of homes from the burning of biomass or kerosene. Some of this pollution also floods outside and adds the burden on others.

In general, developed nations such as the United States have managed to clean their air substantially in recent years, but WHO has found that in developing countries the burden remains quite high. A previous report from earlier this year from the agency found that the Indian capital city of Delhi had annually averaged PM2.5 levels of 122, or more than 12 times the safe level.

“Air pollution is improving in rich countries, but it’s still getting worse in most developing countries,” said Dora.

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