Planting trees can ease asthma attacks in people who live in highly polluted urban areas.

So says a team of researchers from the University of Exeter in England. They found that an abundance of trees led to fewer asthma-related hospital admissions in those areas.

Their findings are published in the journal Environment International.

The data included more than 650,000 serious asthma attacks recorded over 15 years. The team compared asthma-related emergency hospital admissions in 26,000 urban neighborhoods in England. The study focused on areas with high levels of background air pollution.

In areas with the highest pollution levels, the researchers noted, trees had the most beneficial effects. Where there were 300 trees per square kilometer, there were 50 fewer asthma attacks reported per 100,000 residents.

Trees remove pollutants from the air that can trigger asthma attacks, the researchers explained, but vegetation can also produce pollen that may aggravate the effects of asthma. Still, urban greenery is beneficial overall, they said, although its effects depend on the level of air pollution nearby.

People living in less-polluted areas benefited most from other kinds of grassy space and gardens. The team surmised that grass pollen, when mixed with high concentrations of air pollutants, can lead to an increase in asthma attacks.

More research is needed to understand the complex relationship between pollutants and pollen. The researchers hope their findings can help in determining how planting trees in a neighborhood can offset the effects of pollution from car emissions and other sources.