A new study by researchers at the University of Exeter has found that
people living in polluted urban areas are less likely to be hospitalized with
asthma when there are many trees in their neighborhood. A study on the impact
of urban greenery on asthma suggests that improvements in respiratory health
can be achieved by expanding green space in highly polluted urban areas.
The study identified more than six hundred thousand and fifty severe asthma
attacks over a fifteen year period. Emergency hospitalizations were compared
across 26,000 city blocks in England. In the most polluted urban areas, trees
were particularly associated with fewer asthma exacerbations. In relatively
unpolluted urban areas, trees had no such impact. In a typical urban area with
high levels of background air pollution, about fifteen microprograms of fine
particles (PM2.5) per cubic meter or a nitrogen dioxide concentration of about
thirty-three microprograms per cubic meter, an additional three hundred trees
per square kilometer were associated with a reduction in asthma exacerbations
of about fifty cases per hundred thousand inhabitants over a 15-year study
period. The findings could have important public health implications, as tree
planting can play a role in reducing exposure to air pollution. Over 5.4
million people receive treatment for asthma in the UK, with an annual medical
cost of around £1 billion. 18% of adults report asthma within the previous
twelve months, and a quarter of 13-14 year olds report symptoms. Asthma causes
over a thousand deaths a year. The study found that trees and green spaces were
associated with fewer people hospitalized for asthma. Trees are known to remove
air pollutants that can trigger asthma attacks, but in some situations they can
also cause localized accumulations of particles, preventing them from being
dispersed by wind. Vegetation can also produce allergenic pollen, which
aggravates asthma. However, urban vegetation does far more good than harm, the
authors say. However, the effects were not the same everywhere. Green spaces
and gardens have been associated with reduced asthma hospitalizations at lower
pollutant levels, but not in the most polluted urban areas. The opposite was
true for trees. It is possible that grass pollen becomes more allergenic when
combined with air pollutants, so that the benefits of green space decrease as
pollution increases. In contrast, trees can effectively remove pollutants from
the air, and this may explain why they seem to be most beneficial when airborne
concentrations of harmful substances are high.
The scientists conclude that the impacts of various types of vegetation – green
spaces, gardens and thickets of trees – are characterized by both very high and
very low levels of air pollution, and are especially important for public
health policy and urban planning. It is also known that the interaction between
pollen and air pollution, as well as the effects on health and asthma, is very
complex, and this study confirms that more research is needed in this area.