Compact development reduces regional air pollution, may harm local health, risk study finds

In a first-of-its-kind study, Theodore J. Mansfield, Daniel A. Rodriguez and Jacqueline MacDonald Gibson from University of North Carolina at Chapel Hill and Joseph Huegy from North Carolina State University linked a transportation demand model used by regional planners to health risk models. Their paper, "The Effects of Urban Form on Ambient Air Pollution and Public Health Risk: A Case Study in Raleigh, North Carolina," was recently posted electronically in the journal Risk Analysis, published by the Society for Risk Analysis.

The researchers examined three alternative land development scenarios for North Carolina's Raleigh-Durham-Chapel Hill area. Using linked models, they analyzed the annual average ultrafine PM concentrations in the region's air under each scenario and the related health effects. The first scenario was a Base Case that used 2010 data for the region, with several dense urban core areas but also generally dispersed development. The second was Compact Development, based on possible land use distribution in the region if authorities had implemented growth management and land conservation policies, such as density incentives and urban growth boundaries. The third was Increased Sprawl, based on typical post-1950's urban development policies in the United States that supported decentralization and suburbs.

Their approach uses automobile traffic patterns linked to a land use regression (LUR) model that predicts ultrafine PM concentrations in air. They then linked the LUR model to a health risk-assessment model. This modeling approach, the paper notes, "may be applicable to urban planners, environmental regulators, and public health practitioners in other regions considering land-use and transportation policies as means to improve health outcomes."

For all three scenarios, the analysis found that the local areas with the highest predicted ultrafine PM-related death rates were close to major regional transportation corridors and had a high population density. The consistent occurrence of such hotspots in all three scenarios, despite significant land-use variations, "underscores the need to address transportation issues holistically when considering health outcomes," the researchers state. Such local hotspots are likely the result of the "hierarchal nature of transportation systems, which tend to have limited redundancy and funnel large proportions of regional traffic through specific network links." Because of those characteristics, transportation infrastructure may play a significant role in linking land use, transportation behavior, air quality and health. Furthermore, existing infrastructure may "constrain the ability of certain policy instruments to positively affect public health outcomes."

Although the analysis does not consider equity and environmental justice issues, compact housing with specific income or racial demographics could raise such concerns. The researchers comment that besides using dense development to improve regional air quality, planners might need to adopt complementary incentives to protect against local adverse health-effects hotspots. For example, planners could adopt incentives to shorten the length of trips, encourage the use of public and non-motorized transit, and increase the adoption of lower emitting vehicle technologies. The researchers also note that public health specialists have advocated using formal health impact assessment (HIA) as a means to help transportation and city planners to consider their decisions' health impacts. They urge the integration of quantitative HIA into long-range scenario planning to gauge what health outcomes might result from land use visions. They also did not consider the health benefits associated with urban residents who use transit and walk more, and therefore may get more exercise than their suburban counterparts.

The authors note that their findings supporting "decentralization" to curb health impacts from air pollution must be "strongly qualified." Suburban development may present barriers to physical activity, leading to negative health impacts that counterbalance health gains associated with lower exposure to PM. In addition, decentralized development may increase emissions that contribute to global climate change, reduce ecosystem services such as clean water, and limit land available for parks and recreation.