SEA air is generally regarded as healthy, but it may be polluted with dangerous chemicals from ships, say scientists.
Dirty smoke pouring out of the funnels of ships at sea or in port is having a major impact on the air quality of coastal cities, a study has found.
Researchers used a chemical fingerprinting technique to identify "primary sulphate" in ship emissions.
This consists of tiny sulphur particles, less than 1.5 microns across, which can be carried long distances on the wind.
If inhaled, they lodge deep inside the lungs and pose a serious health hazard.
It is estimated that ship pollution may be responsible for as many as 60,000 deaths a year worldwide.
The scientists, from the University of California at San Diego, found that ships contributed far more of the sulphate in the atmosphere than was previously realised.
Their analysis separated primary sulphate from ship smoke and other sources, such as vehicle exhaust emissions.
On some days ship sulphate accounted for almost a half of the fine particles in the air. Ships burning high sulphur fuel in the ports of Los Angeles, Long Beach and San Diego were largely to blame, the scientists discovered.
The particles are believed to be especially harmful to human health because of their small size.
From July next year, all tankers, cargo and cruise ships sailing into Californian ports will have to switch to more expensive, cleaner fuels when they come within 24 miles of the coast. Similar international rules are due to take effect in 2015.
The research was published in the journal Proceedings of the National Academy of Sciences.