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Levels of pollution exposure can vary dramatically within a city.

Los Angeles is famous for the layer of smog that's visible, miles away, from an approaching airplane. It looks like an even smear over the city, a low-lying haze that blankets every part of town equally. Smog, for the most part, is a regional phenomenon. But the fresh emissions that contribute to it, like pollution from the tailpipes of passing cars, permeate some neighborhoods much more than others.

Even if you live in a smog-covered city like Los Angeles, in other words, exactly where you live influences your direct exposure to pollution.

Researchers at UCLA and the California Air Resources Board recently illustrated this with a study of four neighborhoods throughout the metro area with varying socioeconomic status and proximity to transportation infrastructure. Neighborhoods hemmed in by highways and full of popular arterial roads had far greater concentrations of ultrafine particle pollutants. The Mar Vista neighborhood, which sits just downwind of the Santa Monica airport, fared the worst.

The results, published in the journal Atmospheric Environment, underscore that pollution is in part a hyper-local phenomenon, with even starker differences than what you may have detected with your own nose.

"The differences are pretty large," says Suzanne Paulson, the study's senior author and a professor in UCLA's Atmospheric and Oceanic Sciences Department. "It's probably larger than I would have expected, certainly in the case of the airport. Those levels are really astonishingly high."

The differences stem from the local infrastructure – whether large numbers of diesel delivery trucks happen to pass through a community, whether it's veined with major roads (or saddled with a nearby airport). But socioeconomics matter, too. Wealthier communities, Paulson says, tend to own newer cars, which produce less pollution. "That seems to be a really big factor," she says.

Stop signs also matter, because cars that are cruising along produce less pollution than vehicles accelerating from a stop. Counterintuitively, this means that stop signs may contribute to poorer air quality even as they make streets safer in other ways.

The researchers measured these differences in pollution levels by driving a heavily instrumented electric car (producing no exhaust of its own) through these neighborhoods. Their results are now part of one broader puzzle trying to understand the link between transportation pollution and childhood asthma or heart health among the people who live most intimately with it.

"Unfortunately, science is not at the point to be able to say, this level [of pollution] will produce this much increase in this particular health outcome," Paulson says. "So it’s really hard to get down to the level of basically risk assessment."

These stark neighborhood-level differences in exposure, though, are not likely unique to L.A. The smog there may be particularly bad.

"A lot of other places don’t have that," Paulson says. "But everybody has freeways and arterial roads and dirty trucks driving around. In fact, the vehicle fleet in California is probably cleaner than most vehicle fleets. In general, most places will experience this problem to one degree or another."

Top image: spirit of america/Shutterstock.com

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