ASTHMA
May 19, 2008

Greening Cities May Reduce Childhood Asthma

Childhood asthma is rising at an alarming rate in urban areas. Now it appears that a lack of trees may be what's behind it...

Why are asthma rates dramatically higher in some urban neighborhoods than in others? Researchers have searched for years for the answer to this question. Now, a new study has found that youngsters who live in city neighborhoods with tree−lined streets are less likely to develop asthma.

In fact, the more trees there are, the lower the asthma rates.

In fact, the more trees there are, the lower the asthma rates. Asthma rates in preschoolers fall by 24% to 29% for every additional 343 trees per square kilometer, estimates Gina Lovasi, Ph.D., of Columbia University, and colleagues in their report, which appears online in the Journal of Epidemiology and Community Health.

However, living with more trees did not have any effect on hospitalizations for asthma among older children. It may not be the trees themselves that are directly responsible for the protective effect, said Dr. Lovasi. "For example, trees could be more abundant in areas that are well maintained in other ways," she said, "leading to lower exposure to allergens like mold or cockroaches that we know are important triggers for asthma attacks." In any case, these findings are too preliminary and correlational to provide guidance for concerned parents or clinicians, Dr. Lovasi said.

"What we know about preventing asthma is that getting physical activity and being healthy in other ways — keeping to a healthy weight — are important things that one can control," she said. Poor urban areas have been hard hit by skyrocketing asthma rates. In New York City, early childhood asthma rates were three times higher in East Harlem than in the physically adjacent but ethnically and socioeconomically distinct Upper East Side.

As part of the study, researchers compared health department data for 42 health service areas within New York City with tree counts from the department of parks and recreation.

Areas with more trees had a lower prevalence of early childhood asthma and asthma hospitalizations. After the data were adjusted for sociodemographics, population density, and proximity to pollution sources, tree density remained significantly linked to asthma prevalence at ages four and five.

The researchers cautioned that their cross−sectional, ecological study could not establish a causal relationship between tree density and childhood asthma, particularly at the individual level. However, their group plans to study asthma rates as New York proceeds with plans to plant one million trees in the city by the year 2017, Dr. Lovasi said.

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