New UK research suggests that the risk of asthma attacks in polluted urban areas can be reduced if the area also includes plenty of trees.
Led by Dr. Ian Alcock, research fellow at the University of Exeter's Medical School, the study set out to look at how trees and vegetation may impact respiratory health.
Many recent studies have highlighted how the worryingly high levels of air pollution around the world can negatively affect health, with Dr. Alcock explaining that the new study set out, "to clarify how urban vegetation may be related to respiratory health."
"We know that trees remove the air pollutants which can bring on asthma attacks, but in some situations they can also cause localized build-ups of particulates by preventing their dispersion by wind. And vegetation can also produce allergenic pollen which exacerbates asthma."
For the research, Dr. Alcock looked at more than 650,000 serious asthma attacks over a 15-year period, comparing emergency hospitalizations across 26,455 urban neighbourhoods in England.
He found that for those who lived in the most polluted urban areas, having trees in the neighbourhood was strongly linked with fewer emergency hospital admissions for asthma cases.
The results showed that for an area with a high level of background air pollution, an extra 300 trees per square kilometre was associated with around 50 fewer emergency asthma cases per 100,000 residents over the 15-year study period.
However, in areas which were relatively unpolluted, urban trees did not have the same impact.
Dr. Alcock explained: "We found that on balance, urban vegetation appears to do significantly more good than harm. However, effects were not equal everywhere. Green space and gardens were associated with reductions in asthma hospitalization 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."
The findings could now have important implications for planning and public health policy and suggest that planting more trees could be one way of combatting the negative health effects of air pollution.
Over 5.4 million people receive treatment for asthma in the U.K. with, 18 percent of adults and a quarter of 13-14 year olds reporting asthma symptoms in the last 12 months.
The findings can be found published online in the journal Environment International.