The most polluted urban areas in the UK had less hospital visits for asthma attacks when there were lots of trees in their neighbourhood, according to a University of Exeter study.

Respiratory health could be improved by planting more trees in the most polluted parts of the country, the researchers said.

Planting just three extra trees in the equivalent space of one football pitch could result in 50 fewer emergency asthma cases per 100,000 residents, the study revealed.

There is a greater risk to asthma patients in areas of high air pollution, as the pollutants – from traffic, smoke or dust – can irritate and inflame airways.

“We wanted to clarify how urban vegetation may be related to respiratory health,” said lead author of the research, Dr Ian Alcock.

“We know that trees remove the air pollutants which can bring on asthma attacks, but in some situations they can also cause localised build-ups of particulates by preventing their dispersion by wind.

“We found that on balance, urban vegetation appears to do significantly more good than harm.”

But, green spaces and gardens didn’t cause less asthma attacks in patients, the study revealed.

It was possible that when combined with air pollutants, grass becomes more allergenic, and could be more harmful to patients.

“In contrast, trees can effectively remove pollutants from the air, and this may explain why they appear to be most beneficial where concentrations are high,” said Alcock.

The best combination for asthma patients living in towns was plenty of trees, with little grassy spaces.

Future urban planning policies should take into account the research, the scientists said.

Almost 5.5 million people receive treatment for asthma symptoms in the UK.

About one in five adults have reported asthma within the past 12 months, while around a quarter of 13-14 year olds claimed to have symptoms.

More than 1,000 people die every year form the respiratory condition.

Symptoms of asthma include breathlessness, wheezing and feeling tight-chested.