A federal appeals court Wednesday rejected a state regulation that reduced emissions from ships, dealing a blow to California's attempt to combat one of the major sources of smog-forming pollution in the Los Angeles region, the Los Angeles Times rerpots.

The ruling means that the state must seek federal approval before imposing pollution limits on the thousands of cargo ships, cruise ships and other marine vessels that visit its ports.

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.

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