BREATHE CALIFORNIA VEGETATION STUDY

Study: Some Trees Effective as Air Filters, Can Cut Health Risk From Vehicle Emission Particles

SACRAMENTO, Calif., May 2, 2008

Trees not only replenish our oxygen supply, but can also act as air filters by protecting us against inhaling tiny, toxic particles from vehicle emissions.
This concept is explored in a recent study conducted for Breathe California of Sacramento-Emigrant Trails by professor Tom Cahill and the DELTA Group at the University of California, Davis.
The discovery that certain tree species can filter out toxic particles is important because it gives people an effective strategy for protecting their health from the effects of air pollution.
Particle pollution - bits of toxic matter floating in vehicle exhaust - threatens our health because invisible specks can evade the lung's defenses to enter the bloodstream, raising the risk of cardiovascular disease and damage to the brain, as well as lung disease. These particles are so small that 100,000 could fit on a 1-inch line.
"We are all exposed to particulate matter every day and need to find ways to protect ourselves. Trees may be able to provide that protection, and planting more trees is something nearly everyone can do," said Kori Titus, a spokeswoman for Breathe California in Sacramento.
The study found that redwood and deodar cedar, two types of evergreen, had potential for being particularly effective.
"I was surprised at how well the trees absorbed the pollutants equivalent to diesel exhaust, not just by dilution but by capture and removal. The study is valuable because it allows options for attractive and effective mitigation of the most dangerous pollutants emitted by cars and trucks. The fact that this pollution mitigation also absorbs CO2 is an added bonus that could be important if trees are planted along roadways throughout California," Cahill said.
The study helped establish this in tests where fresh tree cuttings were placed into a 60-foot wind tunnel designed for studies of meteorology and air flow. Researchers tested redwood, live oak, and then deodar. Lit highway safety flares generated pollution similar to that in diesel exhaust. Instruments called DRUM samplers recorded particle levels before and after dirtied air passed through the cuttings.
In a breeze below 3 mph, all three tree types removed 65 percent to 85 percent of very-fine particles. Redwood and deodar were superior because the evergreens acted like bottle brushes, their needled branches providing more surface area with which to catch particles than the flat, comparatively broad leaves of live oak. The scientists used gentle winds because strong winds naturally freshen highway air. It's when air barely moves that exposure to traffic particle pollution is greatest.
The researchers also ran experiments in a chamber with stagnant air, this time adding oleander, a flowering shrub planted abundantly along California roadways. Unfortunately, oleander's flat leaves made a poor particle filter; redwood, again, performed best.
Planting trees doesn't by itself solve the problem of particle pollution. Roadway design is important, too. For instance, separating freeways and thoroughfares from homes, businesses and schools with manmade sound walls as well as vegetative barriers creates a chimney effect, pushing polluted air up, away from where people breathe. Median strips planted with evergreens enhance the upward air movement. Existing buildings close to busy roads also should be outfitted with air filters to protect the occupants.
However, the best way to minimize the health threat of traffic pollution remains the same: Drive the cleanest possible vehicles and drive as little as possible.
Breathe California of Sacramento-Emigrant Trails promotes clean air, healthy lungs and a tobacco-free tomorrow. Originally founded in 1917, the organization that is today Breathe California of Sacramento-Emigrant Trails is an independent non-profit that spends 100 percent of every dollar raised or donated in its home community to improve the health of our region's residents. For more information, call (916) 444-5900, or see more info online.