The air quality in neighborhoods around the Port of Oakland is bad enough to cause health problems for residents three times the state average. But a new study shows that steps taken by state air regulators to rid the port of old, polluting diesel trucks cut the most noxious exhaust emissions in half.

The study, by Robert A. Harley, professor of environmental engineering at UC Berkeley, is based on tests performed from a Seventh Street overpass in West Oakland, one of the main conduits to the Port of Oakland, the fifth-busiest container port in the country.

Harley and his team collected and tested exhaust samples from hundreds of trucks that passed beneath the overpass in November 2009 and again in June 2010 to measure the amount of dangerous particulate matter, soot and nitrogen oxide emitted by the trucks.

The first samples were taken before a January 2010 deadline at the Port of Oakland to retire drayage trucks with engines manufactured before 1994 or install diesel particulate filters on model year 1994-2003 trucks. The filters trap 85 percent of particulate emissions that can get into the lungs and cause serious health problems.

The second samples were taken six months later, after the dirty trucks had been removed or retrofitted with filters.

The results, achieved in eight months, were striking, Harley said.

"The main thing we were so impressed with, we saw a 50 percent reduction in black smoke and 40 percent reduction in nitrogen oxide," Harley said.

Black smoke filled with particulate matter is responsible for severe health problems, including cancer and other lung diseases. Nitrogen oxide is the main contributor to smog.

Without the accelerated replacement program, Harley said, it could have taken 10 years of trucks being voluntarily retired to achieve the same results.

The study likely captured exhaust from nonport trucks as well. The state air board has extended the deadline to replace or retrofit trucks that haul cargo outside California's ports, but has so far refused to extend the compliance deadlines for port trucks because of health concerns for port neighbors, workers and truck drivers.

"The ports were deliberately an early action item because there is such a heavy disproportionate impact," Harley said.

"The bottom line is that diesel trucks are a big source of air pollution in California. The state is committed that there is going to be a serious effort to clean it up."

About 250 more trucks with model year 2004 were banned from the Port of Oakland on Jan. 1, and 1,700 trucks with engine years 1994-2003 will be banned on Jan. 1, 2014.

Harley said he plans to return to the overpass next year to conduct more emissions studies as older trucks are taken off the roads and replaced with cleaner-burning models.

Cynthia Marvin, assistant division chief for the state air board's stationary source division, said the study confirms that the new air quality laws are having an impact, and port trucks are getting cleaner, but there is still work to be done.

"While we recognize this progress, we must also acknowledge that we're not done yet," Marvin wrote in an email. "The remaining requirements in the regulation need to be fully implemented to ensure that the truck fleet serving the Port of Oakland meets the Air Board's targets for pollution reductions to protect West Oakland and nearby communities."

West Oakland is surrounded by freeways, and ships, trains and construction equipment are also contributors to its poor air quality.

Margaret Gordon, a West Oakland resident and environmental activist, said Harley's truck measurements paint a rosy picture, but don't include cumulative impacts from other sources. Somebody, she said, needs to study the health risks from indoor air pollution.

"What's outside originally gets trapped indoors, so we need to have a balance of both indoor and outdoor testing," Gordon said. "You can't just consider one source. It's a matter of public health."

Contact Cecily Burt at 510-208-6441. Follow her on Twitter.com/csburt.