If you want to improve air quality, end the sprawl

interchange los angeles

The I-10/I-110 Interchange in Los Angeles (courtesy of Politico).

For centuries, people have fled the supposed squalor of cities in pursuit of the fresh air that is so vital for our health and well-being. Before Louis Pasteur’s development of germ theory, most scientists and physicians subscribed to the belief that miasmas – essentially the foul smells associated with rotting organic matter – were the source of major diseases. The cure for illness, they argued, was for people to escape cities to get fresh country air.

Doctors prescribed fresh air as a treatment for various illnesses into the 20th century. American physicians encouraged their patients suffering from tuberculosis to head West in pursuit of the restorative benefits of the clean air. This movement helped foster the growth of many prominent Western cities, including Denver and Phoenix.

The clean air premium

Today, we tend to refer to the deleterious emissions that plague many cities by a different term: air pollution. But that same underlying precept – that we need to leave cities behind in the pursuit of fresh air and better health – remains.

Multiple studies demonstrate that people view air quality as an amenity and are willing to pay for it. According to economists Kennethy Chay and Michael Greenstone, reductions in particulate matter during the 1970s were “associated with a $45 billion aggregate increase in housing values,” while a separate study found that Americans were willing to pay $149–$185 for a one unit reduction in particulate matter levels.

Clearly, we place a premium on the concept of “fresh air.” But could our pursuit of this good actually be making air quality worse for others?

How might sprawl affect air quality?

The link between our sprawl-based development patterns and air pollution seems pretty obvious on the surface. The more we spread out, the more we have to drive to reach workplaces, schools, stores, entertainment venues, etc. All of this adds to vehicle miles traveled (VMT), and each additional mile we drive increases our mobile emissions.

But perhaps it’s not as simple as it seems. Moving people away from the worst polluters was clearly beneficial for public health. And pollution levels are typically higher in denser areas with high traffic volumes.

Additionally, one of the main targets of the landmark 1970 Clean Air Act Amendments (CAAA) was vehicle emissions. As a result, cars purchased today run more than 90% cleaner than those manufactured 46 years ago. This trend has helped to offset the rise in VMT. Thanks to these regulations, emissions of carbon monoxide and ground-level ozone have fallen by 85% and 33%, respectively, since 1980, while particulate matter emissions have declined by more than one-third since 1990. These improvements have occurred even as VMT has nearly trebled from 1.12 trillion when the 1970 CAAA was signed to just under 3.14 trillion last year.

annual vmt

Rolling 12-month change in vehicle miles traveled in the US (courtesy of St. Louis Federal Reserve).

With all of this in mind, I decided to comb through the literature to see what the best available science says on the relationship between sprawl and air pollution, and what I found may surprise you.

Just kidding, no it won’t.

Sprawl and air quality: The evidence

One of the first scholars to explore this relationship, in depth, was Brian Stone, Jr., who published his findings back in 2008. He probed the relationship between the number of days that ground-level ozone concentrations exceeded the EPA’s National Ambient Air Quality Standard (NAAQS) and sprawl index scores for 45 large cities from 1990 to 2002.

Stone, Jr. argued there are three major reasons that urban form could affect ozone levels. First, it can influence emissions of nitrogen oxides (NOx) and volatile organic compounds (VOCs), the two pollutants that combine to form ground-level ozone. Secondly, sprawling cities are more prone to the urban heat island effect, which can increase the ambient temperatures in urban areas by more than 20°F; higher temperatures facilitate the formation of ozone. Thirdly, large, sprawling cities are more likely to have a broad ozone monitoring network, which may increase the odds that high ozone levels are observed.

The study found a strong, statistically significant link between urban form and air quality, even when controlling for weather conditions. According to Stone, Jr.’s results, a one standard deviation increase a city’s sprawl index score produced 5.6 more ozone exceedance days per year. In turn, a one standard deviation increase in population density – one of the four components of the sprawl index – was associated with 8 fewer exceedance days. Based on his findings, he notes that “urban form is significantly associated with both ozone precursor emissions and ozone exceedances…Overall, the most sprawling cities were found to experience over 60% more high ozone days than the most compact cities.”

To put that into perspective, Cleveland has a composite sprawl score of 85.62, meaning it is just over 14 units less compact than the average metro area. If it was as dense as Madison, Wisconsin (136.69) or Detroit (137.17), we would have had 11.2 fewer ozone exceedance days per year through 2002. Given that we averaged 20.5 exceedances per year from 1997-2002, this would represent a 55% reduction.

Strengthening the connection

Multiple subsequent studies support these findings. A 2013 article from Bradley Bereitschaft and Keith Debbage examined the connections between ozone and fine particulate matter (PM2.5) emissions and five separate sprawl indices. Each of the indices computed sprawl in a slightly different way, accounting for various measures of urban form. The authors folded these into two key measures – urban continuity (the degree to which the urban landscape is fragmented) and urban complexity (the degree of the “jaggedness” of the urban boundary).

The authors note that each of the sprawl indices produced a significant connection between sprawl and pollution levels. A one standard deviation increase in the most prominent sprawl index score was associated with 3.4% and 7.8% increases in ozone and PM2.5, respectively. The density of residential properties appears to be a particularly important correlate for air quality. Increasing residential density by one standard deviation lowers ozone and PM2.5 levels by 8% and 16%, respectively. This change in density can also lower on-road CO2 emissions by an average of 1.9 million tons. Using a social cost of carbon of $37 per ton, this produces social benefits of $70.3 million.

Bereitschaft and Debbage argue that “an increase in residential density might improve air quality and contribute to a reduction in per capita CO2 emissions at the metropolitan scale primarily by decreasing automotive dependency and tailpipe emissions.”

Additionally, the authors examined the connection between urban form and the direct emissions of NOx, VOCs, and PM2.5. Their results are striking. A one standard deviation rise in urban shape complexity increases PM2.5 emissions by 3,055 tons (12.4%) per year. Using EPA damage factors for on-road emissions, this additional pollution would cause 128 to 287 premature deaths and carry public health costs of up to $2.47 billion per year.

If more sprawl leads to worse air quality, will reversing that trend in Rust Belt cities lead to cleaner air? Stone, Jr. looked at this issue in a 2007 piece for the Joumal of the American Planning Association. He studied how shifting from a business as usual scenario to a more compact growth approach modeled on Portland could affect VMT and air quality in 11 Midwestern cities through 2050. By shifting from our current sprawl-heavy trajectory, Cleveland could lower household VMT by 9% and reduce emissions of CO, NOx, VOCs, and PM2.5 by anywhere from 7-9.2% each.

All the evidence points to one conclusion – sprawl is exacerbating air pollution. In our haste to find fresh air, we’ve simply made things worse for those who do not have the means to keep moving farther and farther out.

As Bereitschaft and Debbage put it, “Planning for density therefore becomes an issue of environmental justice, particularly at the metropolitan level. Simulations suggest that by relocating to peripheral suburban areas, residents might reduce their exposure to certain air pollutants…[while] simultaneously contributing to a decline in regional air quality by increasing the total volume of automotive traffic.”

I’ve argued before that, despite our poor air quality, almost no one in Greater Cleveland seems interested in talking about the issue and how we might solve it. Perhaps that’s because, unlike in the past, the real source of the problem isn’t just some large coal-fired power plant or steel mill. No, the problem is our individual driving habits. We’ve met the enemy, and it is us. Unless we face up to that fact, we aren’t going to change things for the better.

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