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Research: Air pollution & public health

My research is at the intersection of air pollution and public health: understanding how much pollution people breathe, and how to reduce those exposures. My specific areas of focus are

(1)  Mechanistic and empirical modeling of air pollution, to understand how concentrations vary in space and time, and how concentrations and health impacts would change in response to changes in emissions.

(2)  Measuring and modeling air pollution exposures in developing countries (at present, mainly India), including how exposures change in response to interventions.

(3)  Environmental justice: understanding who is more exposed or less exposed to air pollution, how those exposures correlate with demographic attributes such as race and income, and how exposure disparities might shift if emissions from specific sources were to increase or decrease.

Three main focus areas, and specific projects for each area, are listed below. Our research is supported by the National Science Foundation, the Environmental Protection Agency, and the State of Minnesota.


The World Health Organization estimates that outdoor urban air pollution is responsible for 81% of environmental health risks in high-income countries such as the U.S., and 17% of environmental health risks in low- and middle-income countries. Urban air pollution is one of the top 15 causes of death globally, and one of the top 10 causes in high- and middle-income countries. Overall, urban air pollution is responsible for ~ 1.7% of deaths annual (2.1% in high-income countries, 1.7% in low- and middle-income countries).1 Comparison of costs and benefits of federal regulation in the United States2 conclude that monetized benefits are greater for the Clean Air Act than for all other federal regulations combined; those health benefits are mainly attributable to reduced mortality from regulating fine particles, starting in 2007.

My group studies how much pollution people breathe from specific sources and what would be the health benefits from reducing those emissions.

(1) Air pollution impacts of urban form

In approximately 2007, for the first time in history, the global population shifted from being a majority rural to a majority urban. In coming decades, urban populations are expected to double (to ~ 6 billion), while rural populations (~3 billion) hold constant or decline slightly. The majority of this urban growth will occur in developing countries. I am interested in how the size, shape, and layout of cities impact their environmental footprint can influence air pollution emissions, concentrations, and exposures. As one example, in a more sprawling city residents may need to drive greater distances each day, thereby increasing emissions. However, a sprawling city, which has low population density, may also spread those (potentially larger) emissions over a larger area, which reduces the emissions density and increases dilution of emissions, thereby potentially improving concentrations. A priori, it is unclear whether sprawl will improve or deteriorate urban air pollution.

Our research approach employs empirical evidence, including satellite-derived measurements of air pollution and urban form, to evaluate cities in the US and globally.

(2) Air pollution and health impacts of transportation energy consumption

Transportation is the second-largest end use of energy in the US (behind industrial uses, and ahead of residential and commercial uses)3. My research aims to understand the exposure aspects of transportation emissions, including exploring issues such as switching from conventional vehicles to alternative-energy vehicles (biofuels; electric vehicles) or to active travel (walking; bicycling).

I am interested in intake fraction for motor vehicles - i.e., estimating the fraction of vehicle emissions that are inhaled by people. That metric is useful for understanding how the exposure impacts of transportation may vary among cities, or among vehicle-types within a single city; the metric is also useful for understanding how the exposure impacts of emission controls would differ for vehicles versus other sources (e.g., for power plants).

To understand the air pollution impacts of biofuels, I am collaborating with Professor Jason Hill, an expert on the life cycle of producing alternative fuels. That aspect (fuel life cycle) of the research question is important because emissions do not only occur at the tailpipe; for biofuels a significant portion of the emission occur upstream - during crop production and fuel refining.

Our involvement in active travel (walking; biking) derives from the concern that active travel provides health benefits (physical activity) yet also potentially increases exposure and inhalation of air pollution. My group has shown how spatial patterns for risk from air pollution and from physical inactivity differ, depending on where a person lives within a city; for example, downtown areas typically have more walking than do outer suburbs, but for many pollutants concentrations are higher downtown.

You can watch three brief air pollution "movies" by clicking here: diesel movie; benzene movie; nitrogen dioxide movie. [After clicking the link to open the file with Powerpoint, view as slide show (button F5). The movie loops through 24 hours of concentrations in 1-hour time steps. To pause or un-pause, press the {left-arrow} or {right-arrow}; to exit, hit {Esc}. Credits: Abby Hoats and I made the diesel and benzene movies based on output from ENVIRON's CAMx model for the South Coast Air Basin (map). The nitrogen dioxide movie is based on output from a national land use regression by Eric Novotny and Matt Bechle.]

(3) In situ measurement of fine particles in developing countries

Concentrations of fine particles can be an order of magnitude higher in developing country contexts than in the US. For example, typical concentrations (units: micrograms per cubic meter) in US cities are 5 - 20; concentrations in Minneapolis - St Paul are 8 - 11; the US EPA standard (which provides the enormous monetized benefits mentioned above) is 15 and may be reduced to 12 or 13 in the coming years. In contrast, long-term average concentrations measured in urban areas of low-income countries are regularly 50 - 100 or greater; in-vehicle concentrations may be 100 - 200 or greater; average concentrations indoors for households cooking with solid fuels are 100 - 1000 or greater. Our current research in India involves measurements to understand (1) spatial and temporal variability in outdoor fine particle concentrations along an urban-to-rural gradient, and (2) exposure and health impacts of indoor air pollution from traditional versus more-efficient cook-stoves.

1 CD Mathers, G Stevens, M Masarenhas. "Global Health Risks: Mortality and Burden of Disease Attributable to Selected Major Risks". World Health Organization, Geneva, Switzerland, 2009

2 Reports availalbe here. See, for example, page 15 of the 2012 draft report.

3 US Department of Energy, Annual Energy Review.