Air pollution is a persistent problem in California’s Central Valley. A new study finds that the places that were most polluted nearly 40 years ago generally remain the most polluted today.

LAMONT, CA – AUGUST 12: The sun rises over the Kern Oil and Refining Co. refinery on August 12, 2004 near the town of Lamont, southeast of Bakersfield, California. High levels of air pollution and high temperatures are affecting the area this week with today’s high expected to reach 106 degrees. California’s Central Valley is one of the nation’s most important agricultural and oil producing areas. Mass food production has brought heavy use of chemicals, including pesticides that have sickened hundreds of area workers and residents. In 2002, the last year for which numbers are available, 172 million pounds of pesticides were used on California fields sickening 478 people as airborne chemicals drifted 39 times, according to the state Department of Pesticide Regulation. On May 2, a crew of 100 workers was caught in a drift of pesticide at nearby Arvin that made 19 of them sick, including a woman who was five months pregnant. This spring, state Sen. Dean Florez introduced a bill, the Pesticide Drift Exposure Response Act, to help pay for field workers’ medical care. (Photo by David McNew/Getty Images)