Lower rates of asthma are found in children who live on tree-lined streets, according to an article released on May 1, 2008 in the Journal of Epidemiology and Community Health, a BMJ Specialist journal. Asthma is a disease of the respiratory tract in humans in which the airway sometimes constricts and is lined with excess mucus, making respiration difficult. It is caused by a combination of many factors which are still being investigated. In the United States, rates of asthma grew at an enormous rate of 50% between 1980 and 2000, with especially high rates of diagnosis in poor urban communities. Asthma is the most common reason for which children in New York City are admitted to the hospital.

The researchers involved in this study examined the asthma rates among 4 to 5 year olds as well as the hospital admissions information for this disease in children up to age 15 in 42 health service districts in New York City. Then, these medical data were examined alongside city data on the number of trees growing, sources of pollution, racial and ethnic make-up, and population density of the area.

There were an average of 613 trees on streets per square kilometer in the City. In total, 9% of young children had asthma. For every standard deviation increase in tree density, 343 trees per square kilometer, asthma rates in this age group fell by almost a quarter. This pattern held fast, even once corrections were made for sources of pollution, levels of affluence, and population density -- all of which would be considered likely to influence the results. However, once children were older, there was no correlation between tree distribution and hospital admission for asthma after taking other factors into account.

The authors note that trees may help curb asthma rates by encouraging children to play outdoors more, or by improving the quality of the air. However, they caution that their findings do not mean that the number of trees in any city is directly related to asthma rates among individuals. Finally, they note that New York City plans to plant 1 million trees before 2017, and this could potentially create an opportunity to truly investigate the impact of tree density on asthma.