Trees and green space are associated with a reduction in asthma-related hospitalisations, according to new research out of England.

The findings are based on a study of 650,000 serious asthma attacks over a 15-year period across 26,000 urban neighbourhoods.

The study, published in Environment International, said the effects of urban vegetation were not equal everywhere, however. Green space and gardens decreased asthma hospitalisations in less polluted areas, but not in more polluted areas. For trees it was the reverse.

Dr Ian Alcock, study leader and research fellow at the University of Exeter’s medical school, said grass pollens may become more allergenic when combined with air pollutants so that the benefits of green space diminish as pollution increases.

In contrast to grass, trees can effectively remove pollutants from the air, which could explain why trees appeared to be more beneficial in areas where pollution concentrations were high, but did not have the same impact in relatively unpolluted urban neighbourhoods, Dr Alcock said.

Trees could also cause localised build-up of particulates by preventing their dispersion by wind, but Dr Alcock said the research “found that on balance, urban vegetation appears to do significantly more good than harm”.

The study said in a typical urban area with high levels of air pollution – for example with a nitrogen dioxide concentration of about 33 micrograms per cubic metre – an extra 300 trees a square kilometre was associated with about 50 fewer emergency asthma cases per 100,000 residents over the 15-year study period.

The findings could prove important for city developers and planners, as well as public health policy practitioners, because they suggest tree planting can play a role in reducing air pollution from cars.

Co-author Dr Rachel McInnes said large collaborative research projects were “a very effective way to carry out this type of cross-disciplinary work.”

“This finding that the effects of different types of vegetation – green space and gardens, and tree cover – differ at both very high and very low air pollution levels is particularly relevant for public health and urban planning policies,” Ms McInnes said.

“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.”