Air pollution data recently released by the World Health Organization reveals that the majority of cities breathing the unhealthiest air are located in China and India, with a swath stretching across the Middle East and into Eastern Europe.

The WHO data is not comprehensive—with virtually no data from Russia or large swaths of Africa or South America—but it illustrates the impact of decades of regulation in North America, Europe and Australia, where air quality is generally better.

"One of the great gifts we have [in the U.S.] is the Clean Air Act," said Janice Nolen, vice president of national policy at the American Lung Association. She said it's easy to forget how air pollution in the U.S. was once as bad as it is now in Asia. "When you look at pictures of this country from the 1940s, 1950s and 1960s, [the air] was hazy, just like what you see in these international photos."

The U.S. and Europe have "made a strong push to improve air quality over the past 15 years," said Kirsten Koehler, an environmental health sciences professor at Johns Hopkins University. "So what we see is the impact of those policies that have been put in place."

InsideClimate News used the WHO database to map the cities with the highest annual average concentrations of particulate matter (PM)—a mixture of dust, soot and microscopic particles that cause respiratory and cardiovascular disease.

This sobering picture is expected to get worse with climate change, as wildfires and dust storms driven by rising temperatures release additional pollution. According to Nolen, even developed countries, where PM levels generally meet health guidelines, will struggle to keep PM in check.

PM emissions originate from human activity—including combustion from coal plants, diesel engines and wood—as well as natural sources such as wind-blown mineral dust and sea salt.

The database divides particulate matter into two categories: PM10, which refers to particles that are smaller than 10 micrometers in diameter, and PM2.5, for particles under 2.5 micrometers in diameter. For comparison, the average human hair has a thickness of 70 micrometers.

Click and drag on the maps below to change locations, or zoom in and out using the "+" and "-" buttons. Hover over the individual cities to see more detailed PM data.

The maps only included the most reliable data points (click here for an explanation of our methodology).

Many cities monitor PM10 or PM2.5, but not both. Our maps show all the places where annual PM10 levels are above 70 micrograms per cubic meter, and where annual PM2.5 levels are above 30 micrograms per cubic meter. Those benchmarks are a long way from what the WHO considers "healthy" (PM10 levels at 20 and PM2.5 at 10), but they are the first in a series of levels designated by the WHO as reasonable, intermediate goals for countries trying to clean up their air.

We found that most of the cities with the worst PM2.5 levels were in China, which measures PM2.5 more frequently than it does PM10. India, on the other hand, rarely measures PM2.5, and dominated the list for the worst PM10 cities.

Nolen said Asia's high PM levels are a result of rapid industrialization and the continued reliance on burning wood for energy in rural areas.

In Beijing, average PM10 levels hover around 100 micrograms per cubic meter, five times higher than the WHO guideline for healthy air. But PM concentrations can increase exponentially each spring, when the city is hit with dust storms that originate from Xinjiang, a region in northwestern China with widespread desertification. Last year, a dust storm sent Beijing's PM10 soaring to 1,000 micrograms per cubic meter. The WHO recommends 24-hour peak exposure levels stay below 50 micrograms per cubic meter.

Even countries with generally cleaner air are at risk.

PM2.5 levels in Calgary, Canada reached 186 micrograms per cubic meter last summer due to wildfires in Washington state and British Columbia. Calgary's annual average PM2.5 is roughly 10 micrograms per cubic meter.

In the U.S., southwestern states are particularly vulnerable, as increased heat waves, drought, and decades of poor forest management create ideal conditions for wildfires. According to the National Climate Assessment, global warming was the single largest factor driving western U.S. fires from 1916-2003.

A 2012 wildfire near Fort Collins, Colo., led to such high PM levels that local air monitors "assumed it was wrong" and delayed reporting the data, according to Koehler.

Recent wildfires in southern California triggered air pollution warnings in Las Vegas, 300 miles away.

These episodic PM spikes, though brief, present serious health consequences. Short-term exposure leads to acute effects, including asthma attacks or heart attacks, Koehler said, while long-term exposure is linked to chronic illnesses like lung cancer and cardiovascular disease.

After a series of wildfires in Quebec sent a plume of smoke and particulates into New York, Pennsylvania, and other mid-Atlantic states in 2002, researchers at Johns Hopkins linked the pollution to increased hospitalizations for respiratory and cardiovascular illnesses.

A recent report from the International Energy Agency found that investing an additional 7 percent in energy projects that reduce harmful air emissions could halve global air pollution-related deaths by 2040 and enable CO2 emissions to peak by 2020.