Durable Growth, Environment

Freeways are polluted, but not impossibly so


by cookieevans5, on Flickr

It’s common sense that living near a freeway isn’t healthy. The pollution from the cars and grit from the roadway make for what most would term a wholly unpleasant experience. Unfortunately, the only places for infill development, not to mention quite a few BART and coming SMART stations, are near freeways. As the Bay Area reorients to transit-oriented development, guidelines need to be enacted to mitigate these negative health effects, or we’ll simply be building health problems for the future.

Roadway pollution is almost entirely from tailpipe emissions, and most of the health effects are from particulate material, that brown smoke most recognizably seen coming out of large truck exhaust pipes.  It’s nasty stuff (PDF), not only because the shape of the particles increases the risk of asthma and lung cancer but because they carry heavy metals, which can contribute to diminished brain formation in children.  Gases, such as carbon monoxide, are less hazardous to the health of nearby residents.

These particulates, at least when they come from a freeway, are concentrated within 200 feet of the road, though they are measurable up to half a mile away during the day and 1.5 miles away during the early morning hours.  That means a huge portion of the region lives within a freeway’s pollution plume, especially in the East Bay. Even far from a freeway, arterial roads generate their own problems.

But it is within that 200 foot buffer that we find the most danger, but there are ways to mitigate the problem. Solid barriers, such as sound walls, send the pollution upward, dispersing it. This still leaves high concentrations near the freeway but less than without a barrier.  Plant barriers (PDF) also send a plume upward, but plants collect particulates on leaves and act as natural filters, meaning much less pollution reaches the areas near the freeway. Combing solid and plant yields better results than using only one.

Practically, this means that, wherever pollution is a concern, local government and Caltrans should try to plant trees and build walls to contain and filter out the pollutants.

Since most people spend most of their time indoors, building design must be included in pollution mitigation as well, as the problem is ultimately much pollution gets into apartments or offices. Most obviously, the exterior of buildings – the walls and the rooftops – can be integrated into the mitigating plant and solid barriers of the freeway. Green roofs can capture pollutants pushed upward by freeway barriers, cutting emissions.

More radically, green walls can be an especially effective means of capturing air pollutants by filtering the updrafts themselves. If a line of buildings creates an urban canyon, like along San Francisco’s Market Street, winds vertically mix the air, running up one building side, over the roadway, then down the other side. An urban canyon with green walls will filter the air on the updraft and downdraft alike.

Inside buildings, air filters should be required. Most particulates can be filtered with specialized HVAC systems that, though they run upwards of $700 per apartment unit per year to operate, yield an estimated $2,100 in health care savings annually.  These systems are required in San Francisco for developments near freeways and are a logical step for other counties to take. An extra step would be to subsidize the filters to make housing development more viable in what are otherwise marginal lands.

Unfortunately, even these specialized HVAC systems don’t filter out ultrafine particles, which constitute most of the particulates in freeway pollution. Laboratory-quality HEPA filters are even more expensive than San Francisco’s standard, but not much more, and could be encouraged through subsidy or required by law.

Exposure could be further limited by encouraging office development closest to the freeway. The buildings, along with rooftop gardens, would act as a pollution wall for residences further back. Children, as a sensitive group, would be best protected under this pattern.

In short, while air pollution is a major concern for building new residences along the freeway, it should not be a show-stopper. Building higher up the valleys or sprawling outward in other parts of the region will only make traffic and pollution worse. The Bay Area’s governments need to make mitigation part of their building codes before any more major developments are built if they want to get ahead of the curve. It will save them money in the long-run and will make their new communities far more livable than they would be otherwise.

Written by David Edmondson

David Edmondson

David is a native Marinite working in Washington, DC. He writes about how to apply smart-growth and urbanist thinking to the low-density towns of his home.

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