For the first time ever, California’s agriculture sector — the biggest industry in the state — will be required to comply with the federal Clean Air Act, following a settlement reached yesterday by the U.S. EPA and environmental and health advocates. Since 1976, the sector has enjoyed exemptions from some of the act’s most important provisions. Critics have long held that the exemptions were illegal and contributed to the state’s notoriously poor air quality, but the EPA, although empowered to intervene, did not choose to do so until environmentalists, physicians, and concerned citizens filed suit. The plaintiffs cheered yesterday’s settlement, which will help control the dust, ammonia, and smoke from agribusiness that fouls California’s air. Growers, however, complained that their farms would now be regulated by people who live 3,000 miles away.