Freeway Pollution Can and Should Be Filtered

It’s 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 SMART stations, are near Highway 101. Before any post-SMART buildings are built, communities in Marin and Sonoma need to take measures 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. In Marin, that means a huge portion of the county lives with 101’s pollution: all of San Rafael, most of Novato, Greenbrae, Mill Valley, Corte Madera and Larkspur, Marin City, and Sausalito lives within the freeway’s pollution plume. Only Ross Valley and West Marin don’t need to deal with the problem, though arterial roads generate their own plumes.

Within that 200 foot buffer, though, is the most danger, and the most opportunity to cut pollution. Solid barriers, such as sound walls, send the pollution upward, dispersing it but still leaving high concentrations near the freeway. Plant barriers (PDF) also send a plume upward, but much less pollution reaches the areas near the freeway. Instead, they collect the particulates on leaves and act as natural filters. Using both solid barriers and plant barriers, of course, yields better results than using only one.

In "urban canyon" environments, as along Fourth Street in San Rafael, green walls can have a huge positive effect on air pollution, cutting out nearly 60% of the particulate matter in diesel pollution. This is because the canyon creates wind patterns that pass air through the folliage multiple times, allowing the air to be filtered again and again. The plans for the Downtown Station Area Plan call for extended the Fourth Street style of development to the areas immediately adjacent to the freeway, making that area a superb opportunity for green walls.

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 while encouraging green walls.

Another tool in our air pollution mitigation toolbox is building design. Most people spend most of their time inside. When discussing pollutants, it’s ultimately about how the pollution gets into apartments or offices. Most obviously, plants can be grown on rooftops and on the sides of buildings to filter pollutants in concert with whatever is next to the freeway. Inside the building, the county can require air filters.

Air filters for freeway pollution are effective. Most particulates can be filtered with specialized HVAC systems that, though they run upwards of $700 per apartment unit per year to operate, they 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 Marin to take. The county might go the extra step to subsidize the filters for affordable units included in market-rate developments. However, these 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. Though office workers will be exposed, sealed windows and strategically placed entrances should keep much of the pollution out of the work area.

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 North Bay’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 safer and more livable.

A version of this post was originally posted in two parts on The Greater Marin. Follow The Greater Marin on Twitter and Facebook.