"For a long time there's been this perception that ship emissions are out there in the ocean and they don't really affect anyone on land and I think this study shows that this is clearly false," said David Marshall, senior counsel at the Boston-based Clean Air Task Force, which co-commissioned the study.

"They do matter and they do need to be controlled."

Scientists said the fact that shipping takes place on the high seas -- away from populations who can readily see impacts of emissions -- was part of the reason the industry's fuel standards lagged those of the auto industry.

But sulphur emissions from international shipping represent about 8 percent of sulphur emissions from all fossil fuels, said James Corbett, one of the authors of the study.

Most ships run on bunker fuel, which is cheaper than distillate, but also more polluting. Corbett said it was also getting dirtier over time as distillate fuels become cleaner, since the sulphur driven out of distillates ends up in the residuals used by ships.

"The international treaty process at the IMO (International Maritime Organisation) has been a slow process by which consensus is reached, rather than a process by which a regulatory authority can set standards that an industry must agree to," he said.

"So this study, we think, is important to help policy-makers determine the appropriate path forward as they consider new regulations for shipping," said Corbett, who is at the University of Delaware's College of Marine and Earth Studies.

The number of premature deaths from ship emissions could rise by 40 percent in the next five years because of increases in shipping activity, Corbett said, adding the number did not account for additional health impacts such as bronchitis and asthma.

Switching to distillates would mitigate the mortality rate from ship emissions, but would also likely come at huge cost to the industry.

Other options include cleaning up exhaust gases before they are released using scrubbers, which act as a filter in smokestacks that captures particulates, Corbett said.

COSCO Group, China's largest shipping conglomerate, said there were other measures that could be taken to reduce emissions.

"At COSCO we do our best to reduce emissions, especially we are reducing them in newbuilds," said COSCO president Wei Jiafu.