Do Trees in Cities Help or Harm Our Health?

Air pollution: outdoor air quality and health (National Institute for Health and Care Excellence, Dec.1, 2016)

Also discussed here: Trees could make urban pollution even worse (quartz, Dec.6, 2016)

And here: Neighborhood greenspace and health in a large urban center (Nature, Scientific Reports, Jul. 9, 2015)

Today we review a guide about urban air pollution that looks into the role that street trees play with respect to reducing air pollution. The overall conclusion was that trees are unlikely to reduce air pollution and could add to it, especially if the trees reduce ventilation of air currents. This is true also of the more recent use of green walls. It is also acknowledged [in a Toronto study]that urban trees can improve health – as much as a $10,000 raise or feeling 7 years younger. Pine trees are singled out as a particular contributer to urban pollution through their emissions of volatile organic compounds (VOC) which combine with the NO2 in car emissions to produce low level ozone, one of a handful of pollutants harmful to health.

   

Key Quotes:

“Street trees were unlikely to reduce air pollution in most street designs and could worsen it in some cases,” “Leaves and branches slow air currents, causing pollutants to settle out.”

“Urban trees and plants can improve mental and physical health - an average of 10 extra trees per block made people living there feel like they would be much healthier—as much as a $10,000 raise or being seven years younger would”

“not all trees are equal. The pungency of a cedar, eucalyptus, or pine woodland, to name a few examples, comes from a blend of volatile organic compounds (VOC) in these species. When these VOCs interact with the nitrogen oxide that is in car emissions, in the presence of sunlight, they produce ozone—at ground level, the pollutant can be harmful enough to cause heart disease.”

“having 10 more trees in a city block, on average, improves health perception in ways comparable to an increase in annual personal income of $10,000 and moving to a neighborhood with $10,000 higher median income or being 7 years younger. … having 11 more trees in a city block, on average, decreases cardio-metabolic conditions in ways comparable to an increase in annual personal income of $20,000 and moving to a neighborhood with $20,000 higher median income or being 1.4 years younger.”

“In 2010 the total mortality burden of human-produced PM2.5 in London was 9 52,630 life-years lost and of long-term exposure to NO2 was up to 88,113 life-years 10 lost …The health impact of PM2.5 pollution from human activities in the UK is estimated to 15 cost between £8.5 billion and £18.6 billion a year “

“Where solid barriers are planned alongside major roads (sometimes 10 used to protect local people from noise) consider whether action is 11 needed to mitigate any adverse effects on air quality…Take into account the effect that trees can have on street ventilation, based on where they are planted and how they are maintained, to avoid creating areas of poorer air quality.”

 “Evidence showed that street trees and green walls or roofs have a mixed effect on 20 street air quality – in some cases they restrict street ventilation causing poorer air 21 quality, in others they improve it “

“Leaves and branches slow air currents, causing pollutants to settle out. They may also act as 'sinks' for particulates and chemicals that may have direct or indirect effects on air quality (in particular, volatile organic compounds [VOCs]). The extent to which this is the case depends on factors such as species, time of year and growing conditions.”

 “air quality might deteriorate at street level near vehicle sources if ventilation were restricted, while improving near first floor windows above the canopy. Although it is important to avoid the possible negative”