There’s more to roadway emissions than what comes out of the tailpipe

By Bill Holloway

While the push to reduce vehicle emissions has focused on cleaner fuels, more efficient engines, and other technologies that can reduce or eliminate tailpipe emissions, non-tailpipe emissions have remained largely under the radar. Non-tailpipe emissions include dust generated from brake pad and tire wear, as well as salt and other material kicked up from the roadway by passing vehicles. Studies have shown that along roadways, tailpipe emissions and non-exhaust sources are often responsible for roughly equal amounts of airborne particulate matter (PM), with non-exhaust sources sometimes accounting for the lion’s share.

While non-tailpipe sources do not contribute to global warming, they can substantially impact urban air quality. In addition, the health effect of breathing these emissions is uncertain, particularly for road users and on those living and working in close proximity to roadways. This uncertainty is due to several factors. First, populations exposed to high levels of these non-tailpipe emissions are often also exposed to high levels of tailpipe emissions, making it hard to separate the effects. Second, non-tailpipe PM emissions are made up of many different components, and the relative proportions of these components differ by location and time.

Unlike other single-molecule air pollutants regulated by the EPA, such as carbon monoxide or nitrogen dioxide, PM10 and PM2.5—particulate matter between 10 and 2.5 micrometers in diameter and below 2.5 micrometers, respectively—are blanket categories that cover a large variety of solid and liquid particulate matter. While exposure to PM10 and PM2.5 is associated with a variety of health problems, the specific health impacts of different components of these particulates are not well understood, and certain types of matter are likely to be more harmful than others. For example roadway PM may be the primary source of human exposure to many elements, including some metals that have been associated with negative health effects, as noted in this 2006 report.

As we continue to reduce tailpipe emissions with increasingly efficient engines, electric vehicles, and other technologies, we should begin considering how to reduce these non-tailpipe sources of roadway air pollution as well.

Dr. James Schauer, Professor of Civil and Environmental Engineering at the University of Wisconsin-Madison, recently presented the lecture Air Pollution and Sustainable Cities. It covers pollution from all sources, provides an excellent overview of the impacts of air pollution on cities, and sheds light on the complexities and common misconceptions surrounding it.

Bill Holloway is a Transportation Policy Analyst at SSTI.