HONG KONG — The World Health Organization said Tuesday that 92 percent of people breathe what it classifies as unhealthy air, in another sign that atmospheric pollution is a significant threat to global public health.

A new report, the W.H.O.’s most comprehensive analysis so far of outdoor air quality worldwide, also said about three million deaths a year — mostly from cardiovascular, pulmonary and other noncommunicable diseases — were linked to outdoor air pollution. Nearly two-thirds of those deaths are in Southeast Asia and the Western Pacific region, compared with 333,000 in Europe and the Americas, the report said.

“When you look out through the windows in your house or apartment, you don’t see the tiny little particles that are suspended in the air, so the usual perception is that the air is clean,” Rajasekhar Balasubramanian, an air quality expert at the National University of Singapore who was not involved in the study, said in a telephone interview on Tuesday.

“But the W.H.O. report is a clear indication that even in the absence of air pollution episodes, the concentrations of particles suspended in the air do exceed what’s considered to be acceptable from a health viewpoint,” he said.

In previous studies, the W.H.O. estimated that more than eight in 10 people in urban areas that monitored air pollution were breathing unhealthy air and that about seven million deaths a year were linked to indoor and outdoor pollution.

The new study reduced the second estimate to 6.5 million deaths. But María P. Neira, director of the W.H.O.’s Department of Public Health and Environment, said in a telephone interview that “the trends are still going in the wrong direction.”

“Somebody has to pay for those health systems to sustain the treatment and the care for those chronic patients, and this is something that countries need to balance when they make decisions about the sources of energy they are selecting or the choices they make in terms of public transport,” Dr. Neira said. “These economic costs of health have to be part of the equation.”

The W.H.O. study was conducted by dozens of scientists over 18 months and was based on data collected from satellites, air-transport models and ground monitors in more than 3,000 urban and rural locations, agency officials said Tuesday.

The agency defined unhealthy air as having concentrations of fine particulate matter, known as PM 2.5, above 10 micrograms per cubic meter, or 35.3 cubic feet, but it did not measure concentrations of ozone, nitrous oxide or other harmful pollutants.

The study said that major drivers of global air pollution included inefficient energy use and transportation but that nonhuman factors, such as dust storms, also played a role.

Professor Balasubramanian said it was an open question whether countries in Southeast Asia, a region that has densely packed cities and struggles to combat cross-border pollution, would choose to improve urban air quality by switching to cleaner fuels in their power plants, as Western European countries did several decades ago.

Prolonging the decisions will probably increase the health risk from air pollution, he said, because the region’s population is rising and demanding more energy.

http://www.nytimes.com/2016/09/28/world/air-pollution-smog-who.html?module=watchingportal&region=c-column-middle-span-region&pgtype=homepage&action=click&mediaid=wide&state=standard&contentplacement=7&version=internal&contentcollection=www.nytimes.com&contentid=http%3a%2f%2fwww.nytimes.com%2f2016%2f09%2f28%2fworld%2fair-pollution-smog-who.html&eventname=watching-article-click&_r=0