Particulate Matters and Urban Forestry

The World Health Organization (WHO) released a report last week stating that more than 1 million deaths from pneumonia, asthma, lung cancer and other respiratory diseases could be prevented worldwide each year if countries took measures to improve air quality.  This is the global body’s first large-scale survey of outdoor air pollution from around the world.

While U.S. air pollution does not compare to that found in such nations as Iran, India, and Pakistan, there is little to celebrate when looking at the statistics for California.


The survey relies on country-reported data over the past several years, and measures the levels of airborne particles smaller than 10 micrometers — so-called PM10s — for almost 1,100 cities.  WHO also released a shorter table comparing levels of even finer dust particles, known as PM2.5s.


WHO recommends an upper limit of 20 micrograms per cubic meter for PM10s (described as the “annual mean” in the WHO report), which can cause serious respiratory problems in humans.   More than 10 micrograms per cubic meter of PM2.5s is considered harmful to humans.


Topping the list of worst cities in the nation for increased exposure to both classifications of particle matter was Bakersfield, which receives an annual mean of 38ug/m3 for PM10s, and 22.5ug/m3 for PM2.5s.  Fresno isn’t far behind, taking 2nd place nationwide, with Riverside/San Bernardino claiming the 3rd spot on the U.S. list.  Overall , California cities claimed 11 of the top 20 worst offenders in both categories, all of which exceed the WHO safety threshold.


“We can prevent those deaths,” said Dr. Maria Neira, director of WHO’s department of public health and the environment, who notes investments for lower pollution levels quickly pay off due to lower disease rates and, therefore, lower healthcare costs.


For years, researchers worldwide have been linking reduced particulate matter levels to healthy urban forests. Studies conducted by the Natural Environments Research Council in 2007 suggest that PM10 reductions of 7%-20% could be achieved if a high number of trees were planted, depending on the availability of suitable planting areas.  In the United States, the Center for Urban Forestry Research published a paper in 2006 that notes Sacramento’s six million trees filter 748 tons of PM10 annually.

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