As European regulators impose controls on industry, motor traffic and refineries, too little attention is being paid to pollution from ships at sea, the study by the National Environmental Assessment Agency found.

Cleaner fuels, newer engines and quayside electricity points for berthed ships could significantly reduce pollution-related deaths, said the study, released Monday. Enforcing controls on shipping will be the most cost-effective way to cut air pollution over the next dozen years, it said.

"The message is that if you want to improve air quality on land, you will have a larger effect from spending one euro on the sea than you will from one euro on land," Pieter Hammingh, a co-author of the report, said Tuesday.

The report said chemicals emitted from ships' diesel engines reach far inland, dirtying the air over nearly the entire country, not just over coastal areas.

Rotterdam, Europe's largest port, hosts 35,000 vessels a year. A major North Sea shipping lane runs 40 kilometers (25 miles) from the Dutch coast -- no barrier for some air particles that easily can travel up to 1,000 kilometers (600 miles).

Other studies have found all North Sea countries to be similarly vulnerable to shipping pollution, Hammingh said.

Emissions from ships contribute 20 percent of sulfur dioxide pollution, 20 percent of nitrogen oxide and 10 percent of particulate matter, a mix of small particles or droplets of dust, metals, acids and chemicals.

But the addition of more ships and controls on land-based emissions will mean that shipping's contribution as a percentage of all air pollution will steadily grow.

Controls on the sulfur content of shipping fuel that go into effect in November will reduce emissions 8 percent by 2020. But without further action, nitrogen oxide will increase by 45 percent, and air particles will go up by 35 percent, the report said.

One of the most effective steps would be to provide docked ships with electrical power from the shore so they need not run their diesel engines, the report said.