Adverse respiratory effects were observed in all cities, at levels below current USA air quality standards

Report, Harvard School of Public Health

They found that rises in each pollutant were associated with an increased incidence of breathing problems in the morning.

Rises in nitrogen dioxide could lead to almost a 50% increase in morning symptoms.

Rises in ozone were linked to declines in "peak flow readings" from asthmatic children - which measure lung function.

And rises in sulphur dioxide increased the incidence of asthma symptoms by a third.

The researchers wrote: "Summertime air pollution is associated with increased asthma morbidity and decreased pulmonary function among inner-city children with asthma.

"Adverse respiratory effects were observed in all cities, at levels below current USA air quality standards."

The study comes only weeks after another study suggested a similar link between pollution and asthma symptoms.

Coughing at night

The German research, also published in the European Respiratory Journal, looked at children aged two years or less, living in Munich.

Parents were asked to report if they suffered from coughs without having a chest infection.

The researchers found that as particulate air pollution increased, the chances of these dry coughs increased by approximately a third.

The researchers said that while this showed a link between traffic air pollution and cough, it was not enough to show that pollutants might actually be the root cause of asthma in children.