MedWire News: Children who live in urban areas with tree-lined streets have lower asthma rates than those from urban areas with few trees, results of a US study show.

"In the USA, a disproportionate part of the recent increase in the prevalence of asthma has been observed in the inner cities," explain Dr Gina Lovasi, from Columbia University in New York, and colleagues.

They speculate: "Street trees may explain geographic variation in the prevalence of asthma within urban environments. Trees may help prevent asthma, either by encouraging outdoor play or through an effect on local air quality."

To investigate, the team studied data on asthma rates among children aged 4-5 years and hospitalisations due to asthma among those aged up to 15 years from 42 health service catchment areas in New York City.

They then compared this data with information on tree numbers in each area, provided by New York City Department of Parks and Recreation.

After accounting for factors such as pollution levels, population density and ethnicity, the researchers found that children who lived in areas with a lot of street trees were significantly less likely to have asthma than those who lived in neighbourhoods with few street trees.

Indeed, each increase of 343 trees per km2 was associated with more than a 25% reduction in asthma prevalence among children aged 4-5 years.

However, the team found no significant evidence to suggest that tree density had an impact on asthma hospitalisations among children aged up to 15 years.

Writing in the Journal of Epidemiology and Community Health, Dr Lovasi and team conclude: "Our findings, while not conclusive, suggest that street trees may play a role in preventing early childhood asthma.

They add: "Future work to evaluate a major New York City tree planting [scheme] will allow stronger inference as to the effects of nearby trees on the prevalence of childhood asthma."