Polluted air from America could be damaging the health of people in Britain, experts fear.
Fumes from traffic and industry harm our health
Airborne chemicals from 8,000km away are being dumped in the UK and western Europe and may be to blame for a rise in lung disease, say UK scientists.
Dr Alastair Lewis, from York University and the Intercontinental Transport of Ozone and Precursors programme, is running a series of tests.
His team will track and sample the air stream across the Atlantic.
About 50 scientists from seven UK universities are to join hundreds of other researchers in the Azores in the middle of the Atlantic Ocean.
They will look at how a cocktail of chemicals, emitted from vehicle exhausts and power stations in the US, react together in the air on their way to Europe.
Colleagues in the US will test the air as it leaves the eastern coast of America.
Dr Lewis' team will test the plume of air as it travels over the Atlantic Ocean and colleagues in France and Germany will run checks as it arrives in Europe.
The scientists believe pollutants from fossil fuels may react with nitrogen in the air to form ozone and sooty particles.
In the upper atmosphere, ozone protects the Earth from the Sun's harmful ultraviolet rays. But when inhaled at ground level, it can damage the lungs.
Sooty particles that collect in the lungs are also harmful.
Dr Lewis said: "It's highly likely that air leaving the States contains a cocktail of nitrogen oxides and hydrocarbons, which are emitted from vehicle exhausts and power stations.
"We want to know how these will react together on the way to Europe and notably whether they will form ozone and particles, both of which can be harmful in humans."
During last year's heatwave, during which there were 800 extra deaths across Britain, levels of ozone and particles soared.
Dr Lewis said: "Extreme events like that demonstrate the ozone and particles are quite potent and have a significant impact on health.
"Although we know that some of this pollution was produced locally in the UK, we still don't know what the contribution was from other countries.
"The more you look at ozone the more you understand that it is a global pollutant," he said.
Professor Malcolm Green from the British Lung Foundation said: "Unfortunately we know that pollution is an international problem. We know in the US air pollution is a significant problem.
"The US consumes 25% of the world's fossil fuels - mostly oil - and it's predominantly the consumption of fossil fuels that creates pollution.
"It's perfectly plausible that they are exporting air pollution to us. The predominant wind is from the south west.
Dr John Harvey from the British Thoracic Society said: "Ozone is good when it's right up in the atmosphere but down on the ground it can irritate the lungs.
"If you have asthma it can make it worse. It's bad for the lungs.
"We are very concerned about local levels of ozone, particularly in cities where there is a lot of exhaust pollution.
"People with lung disease, particularly asthmatics should keep an eye on ozone forecasts on Ceefax and in the papers so they can double their inhalers if the levels are likely to be high," he said.