Air pollution may be to blame for up to 33 million emergency asthma attack visits to hospital every year, with half of the visits estimated to occur in South and East Asian countries, notably India and China, a global study has found.

Asthma is the most prevalent chronic respiratory disease worldwide, affecting about 358 million people, said researchers, including those from the University of York in the UK.

Countries like India and China may be harder hit by the asthma burden because they have large populations and tend to have fewer restrictions on factories belching smoke and other sources of pollution, they said.

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

"This is the first global study of the potential impacts of air pollution on serious asthma attacks that cause people to visit emergency rooms in hospitals around the world," said Johan Kuylenstierna, Policy Director or the Stockholm Environment Institute (SEI) based at York.

The study found that nine to 23 million annual asthma emergency room (ER) visits globally (8 to 20 per cent of total global asthma ER visits) may be triggered by ozone, a pollutant generated when car, power plant and other types of emissions interact with sunlight.

Five to 10 million asthma emergency room visits every year (4 to 9 per cent of total global asthma ER visits) were linked to fine particulate matter, small particles of pollution that can lodge deep in the lung's airway tubes.

About half of the asthma emergency room visits attributed to dirty air were estimated to occur in South and East Asian countries, notably India and China, researchers said.

Although the air in the US is relatively clean compared to South and East Asian countries, ozone and particulate matter were estimated to contribute 8 to 21 per cent and 3 to 11 per cent of asthma ER visits in the US, respectively, they said.

To estimate the global levels of pollution, the researchers turned to atmospheric models, ground monitors and satellites equipped with remote-sensing devices.

The researchers, including those from the University of Colorado Boulder and NASA in the US, said one way to reduce pollutants quickly would be to target emissions from cars, especially in big cities.

Such policies would not only help people with asthma and other respiratory diseases, but it would help everyone breathe a little easier, they said.