Asthma can have many triggers, including pollen from some tree species. Nevertheless, a new study has found that the benefits of living in a leafy area outweigh the harms, and not just aesthetically.

The team used a database of 650,000 serious asthma attacks across the UK between 1997 and 2012, which included more than 26,000 urban residential areas. This was compared to the National Tree Map, which is apparently a thing.

Dr Ian Alcock of the University of Exeter found that tree density was strongly related to reduced asthma attacks. An urban area with 300 more trees per square kilometer (770 per square mile) could have 50 fewer emergency asthma cases reported per 100,000 residents over the period.

The connection, according to Alcock, is in trees’ capacity to soak up pollution. The relationship only held in places with dangerously high levels of car exhaust products, such as fine particulate matter and nitrogen dioxide.

For many asthma sufferers, the condition is a mere annoyance, easily fixed with a puff on an inhaler, but for others it is a very different matter. More than 1,000 deaths a year in the UK are triggered by the condition and the cost of treatment is more than a billion pounds, in addition to the time off work.

Alcock’s work might seem obvious, but there are complexities to the results he reports in Environmental International. Green space and gardens, Alcock found, help reduce asthma rates when pollution levels are middling, but they don’t help in highly polluted areas. Trees helped in the reverse.

Co-author Dr Rachel McInnes of the Met Office said in a statement: “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.”

For modestly polluted areas, planners can focus on creating more parks, but where things are really bad, tree density becomes essential. You can even give them email addresses.

The explanation for the differing patterns remains unclear, but Alcock presents a theory that grass pollens attract pollutants, and when these combine there is a synergistic effect that makes asthma worse. If so, this effect is not large enough to make grassy areas a hazard under high-pollution conditions, but is sufficient to balance any positive effects.

“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,” Alcock said

Of course, asthma is just one of the ways air pollution affects us. With 5.5 million deaths a year from air pollution, actually cutting back on the sources matters as well.

Read more: http://www.iflscience.com/environment/tree-lined-suburbs-induce-fewer-asthma-attacks/