The environmental movement is, rightfully, focused almost entirely on greenhouse gas emissions right now. That is almost certainly strategically correct, given the stakes. It’s important to remember, though, that there lots of kinds of pollution out there that aren’t GHG emissions that we also need to be keeping an eye on. For example, a recent study found that prenatal exposure to urban air pollution significantly decreases children’s IQ. I’m not qualified to comment on the science of the study, nor do I want to give too much credence to IQ tests in general, but it’s an important reminder of the importance of local emissions.

It also complicates what is normally the easiest solution to environmental issues, simply increasing density in our urban areas. It’s nearly always the case that the reduced carbon emissions from decreased commuter VMT, from efficiencies in heating and cooling, and from more centralized economic activity mean that density leads to fewer emissions, per capita. There’s a wonderful New Yorker article from a few years back that goes through this all in the (sui generis) case of Manhattan.

Per capita, though, isn’t always the best way of looking at environmental problems.

Per capita numbers show efficiency; they tell us how much a certain technology or lifestyle pollutes. It’s usually the right way of evaluating a given choice as more or less green. Per capita does not, however, look at effects. The atmosphere, for example, does not care about per capita emissions. It cares about total emissions.

In the case of the particulate pollution that the IQ article talks about, the relevant number is the total but roughly the total per square mile. The suburbs might have much higher particulate emissions per capita—those trucks have to drive many more miles to get to the suburban supermarket—but by diffusing it over a larger area the health and ecosystem effects of that pollution are mitigated. It’s precisely this kind of pollution that makes people feel like suburbs are clean and green rather than dirty.

This isn’t a call for us all to move out to the suburbs, fleeing air pollution. It is a reminder that it is not enough to simply call for more density. D.C. already has some of the worst air pollution in the country. Increasing density must happen, but it is non-negotiable that increasing density be paired with mechanisms to improve local air quality.

Luckily, there are many such programs. D.C. has an anti-idling law that I’m quite sure could be better enforced. Anything that increases the speed and efficiency of our bus fleet would decrease local air pollution, even beyond the fuel improvements that WMATA has made. More broadly, efforts to get people on transit, on bicycles, and walking need to be strengthened.

The District is the kind of place that is doing very well on emissions, per capita. We need not to be have our environmental efficiency punished with high densities of local pollution.