Health.com
November 27, 2017

MONDAY, Nov. 27, 2017 (HealthDay News) -- Urban air pollution can trigger asthma. But lots of trees in cities might lower the odds of a flare-up, a new British study finds.

"We wanted to clarify how urban vegetation may be related to respiratory health," said study leader Ian Alcock of the University of Exeter Medical School.

"We know that trees remove the air pollutants which can bring on asthma attacks, but in some situations they can also cause localized buildups of particulates by preventing their dispersion by wind. And vegetation can also produce allergenic pollen which exacerbates asthma," added Alcock, a research fellow.

His team analyzed data on more than 650,000 asthma-related hospitalizations that occurred among urban residents in England over 15 years.

In neighborhoods with the highest levels of air pollution, an extra 300 trees per square kilometer (0.4 square mile) was associated with about 50 fewer emergency asthma cases per 100,000 residents during the study period.

Trees did not have the same beneficial effect in relatively unpolluted areas, according to the study.

The findings suggest tree planting could help reduce the harms of air pollution and guide planning and public health policy, the researchers concluded.

"We found that on balance, urban vegetation appears to do significantly more good than harm," Alcock said in a university news release.

But effects were not equal everywhere, he noted.

"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," Alcock said.

Perhaps, he theorized, grass pollens become more allergenic when combined with air pollutants so that the benefits of green space 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," Alcock said.

The results were published in the December issue of the journal Environment International.

More information

The California Air Resources Board has more on air pollution and asthma

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