How can you improve your neighborhood if a highway runs through it? One challenge is dealing with air pollution immediately next to the highway. Homes and schools located within 500 feet of freeways can expose people to elevated levels of fine particulate matter (PM) and excess cancer risk from toxic air contaminants (TAC). The first defense is to locate new schools and homes beyond the recommended distance. Additional mitigation can also include locating or re-orienting outdoor play areas as far from the freeway as possible. (You could also tunnel or otherwise get rid of the highway, but let’s focus on some more short-term mitigations.)
For existing and planned developments within 500 feet of a freeway, planners an also consider tiered plantings (off-set rows) of trees such as redwood, deodar cedar, live oak and oleander to reduce toxic air contaminant and particulate matter exposure.
Oleander flowers, a recommended mitigation for poor air quality
This recommendation is based on a laboratory study that measured the removal rates of PM passing through leaves and needles of vegetation. Particles were generated in a wind tunnel and a static
chamber and passed through vegetative layers at low wind velocities.
Redwood, deodar cedar, live oak, and oleander were tested. The results
indicate that all forms of vegetation were able to remove 65-85 percent
of very fine particles at wind velocities below 1.5 meters per second
(approximately 3 miles per hour [mph]) with redwood and deodar cedar
being the most effective. Even greater removal rates were predicted for
ultra-fine PM (i.e., aerodynamic resistance diameter of 0.1 micrometer
or less). However, many urban locations lack the land area needed for
this mitigation.
Additional mitigation measures include:
* Where appropriate, install passive (drop-in) electrostatic
filtering systems, especially those with low air velocities (i.e., 1
mph).
* Locate air intakes and design windows to reduce PM exposure
(e.g., windows nearest to the freeway do not open).
* Install indoor air quality monitoring units in buildings.
* Require rerouting of nearby heavy-duty truck routes.
* Enforce illegal parking and/or idling of heavy-duty trucks in
vicinity.
Special thanks to the Bay Area Air Quality Management District for this information.
You may also find additional information and links on the CARE Program
web page.
