Dozens of ships backed up off the Los Angeles and Long Beach ports in recent days, unable to unload cargo because of a protracted labor dispute. Work resumed at the ports Tuesday, but the slowdown in shipping traffic raised concerns that emissions from waiting vessels would degrade Southern California air quality.

Here’s what you need to know about how congestion at the nation’s busiest port complex affects air quality.

How has the port slowdown affected air emissions?

The backup, largely a result of stalled negotiations between the dockworkers union and employers, boosted emissions from cargo ships. Normally, the vessels would be docked and plugged into shore power. Instead, more than 30 ships at a time were anchored off the ports, burning diesel fuel and releasing exhaust.


But because the cargo wasn’t getting off the ships, the onshore activity of cargo handling equipment, trucks and trains also slowed down and may have reduced pollution from land-based sources, Port of Los Angeles officials said. That could cancel out the increased emissions from ships offshore, said Sam Atwood, a spokesman for the South Coast Air Quality Management District.

Have the waiting ships worsened pollution in harbor communities?

No, port officials said. Air quality monitoring stations in the Port of Los Angeles and neighboring Wilmington and San Pedro have measured pollution levels similar to or lower than they were at the same time last year.

“We can’t hide the fact that having those emissions out there is more than we had anticipated,” said Chris Cannon, environment director for the Port of Los Angeles. “It just hasn’t come ashore, and we’re very relieved about that.”


What about the rest of the region?

Emissions from theships are still contributing to smog across the region, air quality experts said. The impact could become more evident this week as a sea breeze that blows pollution inland replaces last week’s Santa Ana winds, which had swept pollution toward to the ocean.

“We’re starting to see some of that haze redevelop,” said Ed Avol, a professor of preventive medicine at USC’s Keck School of Medicine, an expert on health effects from air pollution. “That’s not wholly because of the ships that are there, but they certainly aren’t helping.”

How big a pollution source are the ports?


The San Pedro Bay ports are the largest single source of air pollution in Southern California, generating about 10% of the region’s smog-forming emissions, according to the South Coast air district.

The seaports are major hubs of freight activity, attracting thousands of ships, trucks and locomotives that transport goods but also pollute the air. Container ships, with their enormous diesel engines, are the largest air pollution source at the complex, port officials say.

What areas are most affected by pollution from the ports?

Diesel emissions from the ports have the greatest health consequences for harbor-area neighborhoods like San Pedro, Wilmington and West Long Beach, where studies have shown that residents have higher rates of asthma and face the region’s highest cancer risk from air pollution.


The ports also contribute to dirty air across Southern California. Ships, trucks and trains that carry goods through the port, across the country and overseas spew pollutants that blow inland and drive up basin-wide levels of smog and soot.

Are the ports getting cleaner?

Yes. The ports of Los Angeles and Long Beach have slashed emissions since adopting their 2006 Clean Air Action Plan. The rules include a ban on old, dirty diesel trucks and requirements that docked vessels to turn off their engines and plug into the electrical grid. Near the shore, ocean vessels are also required to burn low-sulfur fuel that reduces the amount of pollution they release.

tony.barboza@latimes.com


Twitter: @tonybarboza