South Korea faces a chronic dirty air problem that makes it one of the most polluted countries in the world. It's common to hear that neighboring China is to blame, but a joint study by NASA and the Korean government has found there's a lot South Korea can do on its own to cut the smog.
On many days of the year, a thick industrial haze blankets the capital city of Seoul, where some 25 million people live in the metropolitan area. The health effects can be seen in hospitals, with patients complaining of wheezing and coughing that won't go away.
Dr. Kim Sang-heon, who practices internal medicine at Hanyang University Medical Center, says since there's a clear link between pollution and respiratory illnesses, he preaches smog avoidance to his patients.
"I usually say stay home if they hear it is high," Kim says.
High concentrations in the air of PM 2.5 — fine particulate matter that can get deep in your lungs — are a relatively common occurrence in Seoul. A ranking released in February shows South Korea had the second worst air quality of all advanced nations in the Organization for Economic Cooperation and Development, with only Turkey faring worse. South Korea's air is more than twice as polluted as the other nations' average.
Seoul's pollution levels on some days rival those of Shanghai and Beijing, major Chinese cities whose pollution problems are well-documented. In 2016, Seoul's air quality index was considered unhealthy for sensitive populations (such as children, the elderly and those with existing respiratory conditions) on 78 days.
By comparison, the Los Angeles metro area, which had some of the United States' highest average PM 2.5 readings in 2016, experienced only two such days, according to an NPR analysis of data released by the Seoul city government and the U.S. Environmental Protection Agency.
Beijing, by contrast, experienced average air quality index levels that were unhealthy for sensitive populations, or even more hazardous, during 231 days last year.
Shanghai reached those levels 201 times, according to data collected by the State Department at its embassies and consulates in those cities.
Although China is an easy target for blame, as its industrial dust does drift across borders, South Korea wanted to know more about its own pollution causes.
So its government teamed up with NASA last year for the most ambitious sampling and study of Korean air quality to date.
Last year, NASA flew planes at various altitudes above the peninsula, chasing dust for a month.
This summer, NASA scientists returned to Seoul to begin sharing preliminary results.
"We can't fly over China. So this is a way to sample China and sample Korea, and the Koreans are very interested in working with us," said Barry Lefer, a NASA scientist and program manager who took part in the study. The U.S. and Chinese governments are rivals when it comes to many military and security issues, which inhibits flyovers.
The big question vexing South Korea is how much of its pollution is homegrown versus carried over from neighboring countries. The answer is complicated.
NASA sampled the air at a time when trans-boundary pollution was low. It cautions it can only model the Korean peninsula's air based on the data gathered from its sampling. But its models did point to some interesting answers.
"Our conclusion was that the local emissions are a strong source of ozone and small particles," Lefer said. "The model said that over half of the air pollution is coming from local sources and the rest is coming from other countries."
Local sources include vehicle emissions, industrial sites and power plants. Lefer says news that a majority of the pollution here is homegrown is actually good in a key way.
"You can't do anything about the trans-boundary pollution, whereas you can do something about your local sources," Lefer says.
The government is taking some action now. South Korean President Moon Jae-in is overseeing a fine dust task force and is shuttering 10 of the country's oldest and most polluting coal plants. The city of Seoul issues fine dust alerts over mobile phones to better inform residents of dangerous days.
Kim, the doctor, believes growing public awareness of pollution is effective in improving the air.
"I hope and I expect some new change will be given to us," Kim says.
Armed with more data about South Korea's pollutants, the battle to curb it can come from a place of knowledge.
Jihye Lee contributed to this story.