Cardiovascular cases: 231,750
Rank in most ozone-polluted cities: 4
Fresno is the biggest city in California's San Joaquin Valley and produces a lot of the surrounding area's air pollution.
The biggest culprit: traffic. A busy Interstate runs the length of the Central Valley and provides a steady stream of cars and trucks. Much of the smog drifts South to cities like Visalia and Hanford, but there is also a phenomenon known as the "Fresno Eddy," said Jaime Holt, chief communications officer for the San Joaquin Valley Air Quality District.
"It's kind of a whirlpool that pushes back some of the pollution back into Fresno air space," she said.
There's also few public transportation options so most everyone drives. And with a sky-high unemployment rate -- 17.3% in February -- many of the passenger cars are old.
"Many of the cars are 20-years old and they are dirtier," said Holt.
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