Air pollution is behind one in three cases of asthma hospitalisation, a study has estimated.

The research from George Washington University finds that up to 33 million visits to emergency departments for asthma could have been caused by pollutants entering the lungs.

It is the first study to estimate the impact of air pollution on asthma cases across the globe.

Susan Anenberg, lead author and associate professor at the university's Milken Institute School of Public Health, said policies to clean up the air could reduce the burden of asthma and improve respiratory health.

In particular, Dr Anenberg said targeting emissions from cars in big cities would not only aid people with asthma and other respiratory diseases but would help everyone breathe a little easier.

She added: "Millions of people worldwide have to go to emergency rooms for asthma attacks every year because they are breathing dirty air.

"Our findings suggest that policies aimed at cleaning up the air can reduce the global burden of asthma and improve respiratory health around the world."

Asthma is the most prevalent chronic respiratory disease worldwide, affecting about 358 million people.

The study, published in the journal Environmental Health Perspectives, suggests car emissions and other types of pollution may be a significant source of serious asthma attacks.

Dr Anenberg and her team first looked at emergency department visits for asthma in 54 countries and Hong Kong, and then combined that information with global pollution levels.