Tucked in the southeast corner of the San Joaquin Valley, Arvin has the most ozone violations in the region so far this year -- exceeding the state threshold 67 times and the federal 33 times.

It has violated at least one of the standards every day this month, except July 3 and 4. That's pretty typical for this town of 16,000.

It's a hot spot for poor air quality because prevailing breezes take pollution from bigger cities into the area. To fix Arvin, air regulators must clean up the rest of the Valley.

Here's what has happened over the last several years:

Pollution rules -- many of them involving engine and fuel standards -- have focused on metropolitan-area ozone, eliminating a lot of fast-reacting gases that form ozone. The slower-reacting gases are drifting downwind and becoming ozone out in the farm country of Kern County.