Researchers from the University of Exeter’s media school studied the impact of urban greenery on the potentially life-threatening respiratory condition.
Their findings suggest planting trees could help reduce the effects of traffic pollution on Britons’ respiratory health.
Asthma affects about five million in the country – including a quarter of 13 to 14-year-olds – at an annual cost of £1 billion to the NHS.
The condition causes around 1,000 deaths per year.
The study, published in the Environmental International journal, explored more than 650,000 serious asthma attacks over a 15-year period in England.
By comparing 26,000 urban neighbourhoods, the researchers found a link between areas highly populated by trees and lower rates of emergency visits to hospital for asthma.
The association was even stronger in highly polluted areas.
In the urban areas with the most pollution, a high density of trees was more strongly linked with low rates of people being taken to A&E than relatively unpolluted neighbourhoods.
Study leader Dr Ian Alcock said trees proved both a defence mechanism and an irritant for asthma sufferers.
He explained that while trees can remove the air pollutants that trigger asthma attacks, they can also spread allergenic pollen. Foliage can retain build-ups of the irritating pollutants that would otherwise have been dispersed by wind.
Yet the study showed an extra 300 trees per square kilometre was associated with around 50 fewer emergency asthma cases per 100,000 residents over a 15-year period.
The researchers found that, on balance, trees did “significantly more good than harm”, but the benefits were not equal everywhere, Dr Alcock said.
“Greenspace and gardens were associated with reductions in asthma hospitalisation at lower pollutant levels, but not in the most polluted urban areas. With trees it was the other way round.
“It may be that grass pollens become more allergenic when combined with air pollutants so that the benefits of greenspace diminish as pollution increases.
“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.”
Met Office senior climate impacts scientist and study co-author Dr Rachel McInnes said the research showing different effects depending on the type of vegetation was important for public health and urban planning policies.
“We also know that the interaction between pollen and air pollution, and the effect on health and asthma is highly complex and this study confirms that more research is required in this area,” she said.
The study emphasised that the researchers had not found a causal link, because while tree-cover was associated with benefiting asthma sufferers in highly polluted areas, the findings suggested sufferers in low-pollution areas may benefit more from the expansions of gardens and greenspaces.
The study also noted that asthma was strongly linked to socio-economic deprivation, with the researchers controlling for that factor.
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