Air pollution tends
to be worst in large cities and their urban cores. As a result, it is urban air
pollution that makes the headlines when the media report on pollution and its
effects (see, for example, this, this and this). Since it is mainly an urban problem, air
pollution exposure is shaped by urban planning and policy. In particular,
it can be affected by population density, the defining feature of urbanization
that distinguishes cities from smaller towns and villages.
In a recent paper
we study how air pollution - as measured by fine particulate matter (PM2.5)
concentration - is shaped by cities’ population density in the United States. In
particular, we want to find out whether residents in dense urban areas are
exposed to worse air quality. For this purpose, we use new data on
satellite-derived measures of PM2.5 concentration at a fine spatial scale and
demographic information from the US census.
Intuitively, our analysis is
conducted by comparing cities of different densities and their average
concentration of PM2.5. The resulting scatter plot is provided in Figure 1 and
serves to illustrate our main result: denser cities tend to have worse air
quality. But this naïve approach cannot give us a definitive (causal) answer on
whether population density affects air pollution. Why? Because many confounding
factors can bias the associated estimates. For example, people decide on where
to locate based on various factors including local amenities and
employment opportunities. Given that many productive activities (e.g.
factories) generate pollution; if people move into areas close to these
activities, a naïve estimation which ignores these confounders will overstate
the true effect of density on pollution.
Figure
1 – PM2.5 Concentration and Population Density
Note: The vertical
axis represents PM2.5 average residential exposure (in μg /m3),
as obtained from the satellite-derived measures. The
horizontal axis represents the natural logarithm of population density. The
points represent 933 CBSAs (metro and micropolitan areas). The black line is
estimated by OLS using the underlying data.
To deal with this and other
endogeneity issues, we use data on the geological characteristics at and around
US cities as instrumental variables for density. Intuitively, we use these
variables to generate variation in density that is not shaped by pollution or
other confounders.[1]
Our instrumental variable estimates confirm the message in Figure 1: denser
locations are associated with higher concentrations of PM2.5. How much higher?
Quite a bit. According to our estimates, doubling density – say, increasing the
density in Houston to match that of Chicago – increases PM2.5 concentrations by
0.73 μg/m3, which is roughly 10% of the average pollution across cities. Using
well-established dose-response functions that map pollution concentration to
mortality rates, in conjunction with official mortality costs estimates from
the US Environmental Protection Agency, we find that doubling densities would
lead to annual mortality costs of as much as USD 630 per capita.
So
denser cities lead to higher pollutant concentration. But was it not the case
that denser cities are also greener? A prolific strand of research has
emphasized the environmental advantages of denser cities (see Kahn and Walsh
2015 for a review). In dense cities, households enjoy shorter commutes when
driving to work (Duranton and Turner 2017) and may even switch to other
transport modes when these are available (Cervero and Guerra 2011). In a world
in which a significant amount of emissions is generated by transport –
especially driving – this observation has led to the conclusion that denser cities
are also greener (see Glaeser and Kahn 2010). But our results indicate that this should not be interpreted as meaning
that high density leads to better air quality. Yes, denser cities are
associated with lower emissions, and this is important for reducing global
greenhouse gas concentrations. However, if we are concerned about local air
quality, having lower emissions does not suffice. Even if emissions are low in
denser cities, it is the concentration of pollutants that determines local
pollution levels. Our paper shows that pollution exposure is higher in denser
cities, making the environmental quality in these cities lower than in other
locations.[2]
What
should be done?
For decades now,
the compact city urban planning approach has been promoting urban densification
as a way to contain sprawl, reduce car use and promote some of the beneficial
agglomeration forces normally associated with density. There are many things to
enjoy about compact cities, including shorter commutes and better access to
commercial and recreational activities. Yet our results indicate that the
purposed environmental advantages of compactness may be limited to reductions
in global pollutants. When it comes to our lungs and hearts, denser cities
create a more polluted, harmful environment. Urban planners should take note of
these trade-offs when designing the cities of the future.
References:
Cervero, R., & Guerra, E. (2011). Urban densities and transit: A
multi-dimensional perspective. Institute of Transportation Studies, University
of California, Berkeley.
Combes, P. P., Duranton, G., Gobillon, L.,
& Roux, S. (2010). Estimating
Agglomeration Economies with History, Geology, and Worker Effects.
Agglomeration Economics, 15.
Combes, P.
P., & Gobillon, L. (2015). The empirics of agglomeration economies. In Handbook of regional and urban
economics (Vol. 5, pp. 247-348). Elsevier.
Duranton,
G., & Turner, M. A. (2018). Urban form and driving: Evidence from US
cities. Journal of Urban Economics, 108, 170-191.
Glaeser, E.
L., & Kahn, M. E. (2010). The greenness of cities: carbon dioxide emissions
and urban development. Journal of Urban Economics, 67(3), 404-418.
Kahn, M. E., & Walsh, R. (2015). Cities and the
Environment. In Handbook of regional and urban economics (Vol. 5, pp.
405-465). Elsevier.
[1] This strategy was initially
developed in the agglomeration literature trying to estimate the productive
advantages of cities (see Combes et al. 2010, Combes and Gobillon 2015). In our
case, we use it to study one of the congestion forces that constrain city
growth.
[2] Complementary results in the
paper show that the observed increase in PM2.5 is not driven by a different
sectoral composition of production in larger cities or by differences in total
city population. See details in the paper.
