September 2004
Modeling
How Population Growth Affects Exposure to Vehicle Emissions
Urban planners, air quality managers, and public health officials take
note. Doctoral student Julian Marshall of UC Berkeley's Energy and Resources
Group is studying ways to reduce exposure to automobile emissions, and
he's finding that, contrary to the popular premise, increasing the number
of people in one place is not necessarily enough.
Traditionally, air quality
has been measured in terms of pollutants in the air, but the direct
impact of these pollutants on human health needs to be understood in
terms of the amount a person inhales. Putting people in a high-density
urban area may make it easier for them to get access to the services
they need without using their cars, which should reduce the emissions
to which they are exposed, but teasing enough of them away from their
cars takes careful planning, Marshall says.
Marshall, who is working
on the health effects of vehicle emissions with COEH faculty members
Tom McKone and William Nazaroff, has developed mathematical models to
examine how population changes affect the extent to which people inhale
pollutants from passenger cars. He is examining per capita inhalation
of emissions such as benzene and carbon monoxide for three scenarios:
-
increasing
the number of people in an area ("infill")
increasing
the number of people and increasing the area in which they live
increasing
the land area without increasing the population ("sprawl")
His models show that increasing
population density alone does not necessarily decrease exposure to harmful
vehicle emissions, since exposure depends on the distribution of pollutants
as well as the overall quantity of pollutants in the air. For example,
in a sprawling suburb, where people use cars to get everywhere and distances
between destinations are great, overall emissions are relatively high.
But, since the population is spread out, the concentrations of pollutants
to which people are exposed may be fairly low. Conversely, in dense
urban areas, although driving distances are reduced and more people
use public transit and walk, bike, or carpool, the cars—and therefore
the emissions—and the people are in closer proximity. Therefore,
the total amount of pollution that people inhale can be higher. To see
a net benefit from the decreased vehicle emissions that can come with
increased population density, Marshall argues, urban development must
be designed in ways that reduce driving enough to lower individual exposures.
"In our market-based
society," Marshall says, "urban planning has a negative connotation
for some people. They will ask why we shouldn't leave the way a city
grows to the free market. One of the justifications for urban planning
is air pollution. Growth has implications for air quality, which affects
human health. This study investigates what will happen if we grow in
certain ways. We can't merely increase population density and expect
that everything else will take care of itself. We also need to introduce
transportation and land-use planning measures that give people transportation
choices other than private vehicles."
Such measures, he says, could
include putting taller buildings on transit corridors so that more people
have easy access to public transportation, reducing the percentage of
urban land devoted to roads and parking, providing more and better public
transit, offering financial incentives to forgo private automobile use,
and creating urban environments that are safe, pleasant, and convenient
for pedestrians and bicycle riders.
