If you think about smog, you're probably picturing a major city like Los Angeles, where in the 1960s and ’70s the air was so bad that smog alerts telling people to avoid outdoor activity were regular occurrences. The air has improved in L.A. and other big cities in recent years, thanks to cleaner cars and air-pollution regulation.

But the real capital of air pollution in the U.S. is a farming city that sits to the northwest of L.A.: Bakersfield.

Bakersfield is in the San Joaquin Valley, a major agricultural area that stretches through much of California. The San Joaquin Valley contains some of the richest, most productive agricultural land in the country. But its geography — the valley is surrounded on all sides by mountains — creates a bowl that traps air pollution. Levels of soot and ozone — which in warm weather, which the valley has much of the year, can create smog — are some of the highest in the country. And while air in much of the U.S. has improved, in Bakersfield and other towns in the southern San Joaquin Valley, the air quality is as bad as ever — if not worse.

How bad? School officials in Bakersfield have used colored flags to indicate air quality: green for good, yellow for moderate, orange for unhealthy for sensitive groups and red for unhealthy for all groups. But this winter, the air became so bad that officials had to use a new color on the worst days: purple, even worse than red. Because of high levels of air pollution, asthma is prominent throughout the region, and the bad air can also raise levels of respiratory and cardiovascular disease.

Photographer Lexey Swall grew up in Bakersfield, and in this collection of photographs, she shows the human cost of living in one of the most polluted cities in the country. For Bakersfield residents, there's simply no room to breathe.