New research from UNC-Chapel Hill challenges the new orthodoxy that dense urban development is better for the health of people living there.
By Gabe Rivin
“Smart growth” may not be so smart after all.
According to a recent study, North Carolina’s Triangle, if developed more densely today, would be home to spikes in harmful air pollution that would endanger the region’s residents.
The study, published in December in the peer-reviewed journal Risk Analysis, came to a counterintuitive finding: Denser development would slightly reduce the Triangle’s air pollution on a regional level, but at a more local level it would expose a greater number of citizens to “hotspots” of particulate matter, a harmful pollutant.
Dense development is an overarching goal for many city planners, who see it as a solution to sprawling urban landscapes like Raleigh. Sprawling development, they say, is largely responsible for car dependency, which reduces physical activity and increases air pollution, including the greenhouse gas carbon dioxide.
Yet a cross-disciplinary team from UNC-Chapel Hill and NC State University is challenging this orthodoxy.
“Our suggestion is not necessarily that density is wrong,” said Daniel Rodriguez, a professor at UNC’s department of city and regional planning and a coauthor of the study, “but that in itself, in isolation, it’s probably not going to be beneficial for people.”
According to the study, a denser Triangle in 2010 would have meant more citizens living near car traffic-heavy corridors. And that would have meant more citizens breathing in high levels of particulate matter.
Particulate matter is a complex mixture of small particles and liquid droplets, made up of acids, chemicals and metals, among other matter, according to the U.S. Environmental Protection Agency. Smaller particles develop when cars and other pollution sources emit gases into the air. The EPA says particles whose diameters are 10 micrometers or less – a human hair is about 50 micrometers – can deeply penetrate human lungs, potentially causing heart attacks or lung disease, among other problems.
Some urban planners say that dense development can limit air pollution since it encourages walking and biking. But the UNC researchers reached a different conclusion.
They argue that if the Triangle had been more densely developed in 2010, 65 people would have died from breathing particulate matter. In comparison, a model of pollution in the Triangle found 47 people likely died as a result of breathing particulates in that year.
A very sprawled development, on the other hand, would slightly increase regional levels of particulate matter. But because the pollution would be more dispersed over a wider area, only an estimated 31 people would die from breathing pollutants, compared to the model of the actual Triangle in 2010, the study found.
A novel trio of models
To reach their conclusion, the researchers relied on a system of modeling that they believe is the first of its kind.
First, the researchers modeled three scenarios: the density of the actual Triangle in 2010; a more compact, dense version of that “base” scenario; and a sprawling version of the base scenario.
The researchers then modeled likely traffic patterns for each, which they used to predict air pollution. Finally, the researchers funneled this information into a health model, which estimated the resulting human health effects.
This linking together of public health and urban planning is relatively rare, despite the two fields’ shared history, Rodriguez said.
“Urban planning emerged from urban health issues,” he said. “Back in the late 1800s, an understanding of access to water as a source of potential cholera in London led to the first sort of GIS urban maps and zoning.”
But while the two fields were intertwined, that began to change in the 1930s and ’40s, as public health research focused more on individuals’ health and city planners focused more on landscape architecture and urban design. Only since the 1990s have the two begun to overlap again, Rodriguez said.
Still, planners tend mainly to consider the environmental effects of a development – say, the effect a shopping mall will have on nearby wetlands. They often don’t consider the effects their projects will have on human health using the concept of formal health impact assessments, Rodriguez said.
Yet as the two fields – public health and urban planning – increasingly overlap, that may need to change, according to the researchers.
The need for a ‘menu of policies’
Theodore Mansfield, the study’s lead author and a Ph.D. student at UNC, said there’s an important discussion to be had about the costs and benefits of city living.
“There are a lot of great things that cities do,” he said. “But at the same time, the concentration of all those activities in a small space can have some negative health impacts.”
Density offers many benefits, like an increased ability to provide mass transit, Rodriguez added. But it isn’t necessarily a boon for public health.
“We need to have a menu of policies that are complementary and synergistic,” he said.
Those might include car-free zones in urban centers, a popular feature in Scandinavian cities, and electrified public transit, he said.
The study authors add that urban planning and transportation planning – much of which takes place in government agencies – should rely on formal health impact assessments in order to understand a large-scale project’s effects on the public’s health.
In North Carolina’s state government, that’s still a goal for the future.
Steve Abbott, a spokesman for the state’s Department of Transportation, said that conducting a health assessment for a project “is a consideration” at the department, but that “to say it’s done all the time would not be accurate.”
“However,” he added, “as we move into the future and continue to develop our planning procedures, it will become a more standard consideration and is definitely something we will better integrate into our plans.”
The limits of modeled data
Mansfield admitted that the study’s findings need to be considered with caution.
In addition to the limitations inherent to modeled data, the study’s models produced a broad range of results, he said.
The study’s models may have predicted 65 deaths for a dense Triangle. But those numbers could have varied dramatically, the models also found. In fact, deaths could have been as few as only six or as many as 220, a range large enough to call into question the study’s conclusions.
The authors also admit that their study did not consider some health benefits that accrue from dense development, such as walking.
Those benefits can be big, according to Julian Marshall, a professor of environmental engineering at the University of Minnesota and an expert on the topic.
“When you’re looking at health impacts of the built environment, air pollution and physical activity both matter,” he said. In fact, the benefits of increased exercise could outweigh the drawbacks of increased air pollution, he added.
But that also requires some qualification, he said. A neighborhood may be densely developed, but that doesn’t necessarily mean it’s walkable. The latter characteristic can result from an area that’s safe and has good sidewalks, for example.
“Are there stores to walk to?” he said. “If there’s no grocery store, then you can’t walk to the grocery store by definition.”