Now, however, researchers at the University of Minnesota, writing in the journal PLOS ONE, have created a sweeping picture of unequal exposure to one key pollutant -- nitrogen dioxide, produced by cars, construction equipment and industrial sources -- that's been linked to higher risks of asthma and heart attack. They've found, all over the country, in even the most rural states and the cleanest cities, that minorities are exposed to more of the pollution than whites.

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"The biggest finding is that we have this national picture of environmental injustice and how it varies by state and by city," says Julian Marshall, a professor of environmental engineering at the University of Minnesota and one of the authors of the study along with Lara P. Clark and Dylan B. Millet. "The levels of disparity that we see here are large and likely have health implications."

Specifically, they found that minorities are on average exposed to 38 percent higher levels of outdoor NO2 than whites in the communities where they live, based on demographic data from the 2000 census. That gap varies across the country, though, and it's substantially wider in the biggest cities. Nationwide, the difference in exposure is akin to approximately 7,000 deaths a year from heart disease.

"It’s a shockingly large number," Marshall says. "You’re taking what's a major killer of people [in heart disease] and increasing it slightly, by a few percent. But that’s a lot."

Regionally, the disparities are largest in the upper Midwest and the Northeast, but the model Marshall and his coauthors developed can drill down from there. These maps show the differences in average exposure to NO2 between low-income nonwhites and high-income whites:

And at an even more local scale:

In that bottom map, the New York/Newark metropolitan area ranks as having the widest disparity in average exposure between lower-income minority census block groups and upper-income white ones across the entire metro area. New York is followed by Philadelphia; Bridgeport/Stamford, Conn.; Boston; Providence, R.I.; Detroit; Los Angeles; and New Haven, Conn.

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The demographic data contained in this analysis is the simple piece ("whites," by the way, refer to non-Hispanic whites). The much more complicated part came in modeling NO2 exposure in such fine-grained detail across the country. Researchers can't realistically deploy pollution monitors everywhere. But, based on a methodology developed in previous work with other researchers, Marshall and his co-authors used a model that predicts NO2 concentrations at the census block level based on satellite and ground measurements alongside land-use data on roads, population density, built-up environments and tree cover.

Trees are a key part of the model not because they mitigate NO2, but because they're a proxy for the absence of cars or other sources of pollution. "It’s because those trees are not cars," Marshall says. Land that looks like a park in satellite data probably doesn't contain a power plant. NO2 tends to be high, on the other hand, around highways and busy central business districts.

The maps above compare low-income minorities to upper-income whites, an admittedly stark way of illustrating the problem. But even after controlling for income, large disparities remain between white and non-white communities. On average, within individual urban areas, the disparities in exposure by race (after controlling for income) were more than two times as large as the disparities by income (after controlling for race).

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"If we look at race only, without accounting for income, then we see a larger gap," Marshall says. "If you account for income, you still get a gap."

Race becomes an even larger factor the bigger the city. This means that the environmental injustice measured here is not purely a disparity between the poor and the wealthy. Something is going on here in the kinds of communities where minorities of all incomes live. That finding is consistent with other data suggesting that even upper-income blacks tend to live in neighborhoods with worse outcomes and higher poverty levels than lower-income whites.

This new research doesn't delve into the underlying questions of why these patterns exist. "Fundamentally," Marshall says, "these outcomes are the result of spatial patterns in air pollution and spatial patterns in demographic groups, and the correlations between them."

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