A new study suggests that children who live in tree-lined streets are less likely to develop asthma.
The study, based in New York City, was carried out by researchers at Columbia University and the results are published in the Journal of Epidemiology and Community Health.
Between 1980 and 2000, rates of asthma in the US have increased by a staggering 50%, with children in poor urban communities suffering most. In New York City, asthma is the leading cause of admission to hospital among children under 15.
Lead researcher Dr Gina Lovasi and colleagues found that there were an average of 613 street trees per square kilometre in the city and that the prevalence rate of young children with asthma was 9%.
By studying asthma rates in various areas of the city and correlating these with the numbers of trees lining residential streets the researchers found that among children aged four and five, asthma rates fell by 25% for every extra 343 trees per square kilometre.
The reduced risk held true even after other factors such as proximity to sources of pollution, levels of affluence and population density were taken into account. This suggests that tree density is an independent predictor of asthma risk in young children.
However, although higher numbers of trees reduced the overal incidence of asthma, it did not reduce the number of children whose asthma was so severe that they required hospital treatment.
Exactly how trees might reduce the risk for asthma is unclear but there are a number of possible explanations.
Several studies have linked asthma to high levels of pollution, particularly particulate matter which is produced by factories, trucks, and other major sources of combustion. Research published last year by researchers at the University of Southern California which examined the health records of 3000 children found that living close to busy roads raised asthma risk.
Trees may help to counteract these effects by absorbing pollution. Several studies carried out in urban areas in the US during the 1990s clearly demonstrated the ability of trees to remove both gaseous and particulate pollutants from the air. One such study in Philidelphia found that trees had removed 1000 tons of pollution from the city's air during 1994. It has been found that the presence of trees improves overall air quality by up to 2%.
Another theory is that asthma rates are reduced in areas with more trees because it encourages kids to play outdoors where they are exposed to bugs in the dirt. It has been suggested that rates of asthma and allergies are on the rise because kids spend so much time indoors and are not exposed to dirt and micro-organisms. It is thought that such exposure is required for the immune system to develop properly. This is known as the 'Hygiene Hypothesis'.
Dr. Lovasi admits however that the reasons why asthma rates appear to be lower in areas with more trees is far from clear.
She said: "There may be something else healthful about the areas that had more trees."
"For example, trees could be more abundant in areas that are well maintained in other ways."
Further studies which account for a wider range of other influences on asthma incidence should help to provide a clearer picture of the benefits of increasing tree numbers in urban areas. Regardless, New York City is planning to plant and extra 1 million trees by 2017.
Source: Lovasi GS Quinn JW Neckerman KM Perzanowski MS Rundle A (2008) Children living in areas with more street trees have lower prevalence of asthma Journal of Epidemiology and Community Health 62(7):647-9