The U.S. 9th Circuit Court of Appeals in San Francisco ruled that California's new regulation is preempted by federal law. The Clean Air Act allows California to set its own standards for various vehicles and engines if it receives waivers from the U.S. Environmental Protection Agency. The state argued that in this case it didn't technically need a waiver, but the judges disagreed.

Ships sailing into the ports of Long Beach and Los Angeles are considered a major source of particulates, nitrogen oxides and sulfur, pollutants that cause the region to frequently violate federal health standards.

Microscopic soot from diesel engines can lodge in lungs, triggering heart attacks, asthma and other cardiovascular and respiratory problems, scientists say. Diesel exhaust has also been linked to lung cancer.

The ruling is the second setback in two months to California's efforts to combat air pollution rather than wait for federal action.

For four decades, the state has adopted its own regulations for cars, trucks, factories, consumer products and other sources of air pollution, often prompting the federal government to set similar standards.

Microscopic soot from diesel engines can lodge in lungs, triggering heart attacks, asthma and other cardiovascular and respiratory problems, scientists say. Diesel exhaust has also been linked to lung cancer.

The ruling is the second setback in two months to California's efforts to combat air pollution rather than wait for federal action.

For four decades, the state has adopted its own regulations for cars, trucks, factories, consumer products and other sources of air pollution, often prompting the federal government to set similar standards.