Asthma hospital admissions could be cut dramatically by lining streets with trees, the largest ever study has found. Researchers from the University of Exeter's medical school studied the impact of urban greenery on the respiratory condition and say the results suggest planting trees could help reduce the dangerous effects of traffic fumes.

However large areas of grassed gardens or parkland could make asthma worse, the findings suggest, because grass pollen merges with pollution - triggering a condition known as 'greyfever.'

Asthma affects about five million people in Britain, costs the NHS £1 billion a year and causes around 1,000 deaths annually.

Pollution is known to exacerbate asthma, but researchers found that even in the most-polluted areas, a high density of trees led to fewer A&E admissions than in less-polluted neighbourhoods with fewer trees.

The findings hold true even though tree pollen can often trigger asthma, suggesting the pollution-absorbing effect is greater than the allergenic impact, particularly when pollution is high.

The researchers concluded, on balance, trees did "significantly more good than harm," with every extra 300 trees per square kilometre associated with around 50 fewer emergency asthma cases per 100,000 residents over a 15-year period.