National patterns in environmental injustice and inequality: outdoor NO2 air pollution in the United States

PLoS One. 2014 Apr 15;9(4):e94431. doi: 10.1371/journal.pone.0094431. eCollection 2014.


We describe spatial patterns in environmental injustice and inequality for residential outdoor nitrogen dioxide (NO2) concentrations in the contiguous United States. Our approach employs Census demographic data and a recently published high-resolution dataset of outdoor NO2 concentrations. Nationally, population-weighted mean NO2 concentrations are 4.6 ppb (38%, p<0.01) higher for nonwhites than for whites. The environmental health implications of that concentration disparity are compelling. For example, we estimate that reducing nonwhites' NO2 concentrations to levels experienced by whites would reduce Ischemic Heart Disease (IHD) mortality by ∼7,000 deaths per year, which is equivalent to 16 million people increasing their physical activity level from inactive (0 hours/week of physical activity) to sufficiently active (>2.5 hours/week of physical activity). Inequality for NO2 concentration is greater than inequality for income (Atkinson Index: 0.11 versus 0.08). Low-income nonwhite young children and elderly people are disproportionately exposed to residential outdoor NO2. Our findings establish a national context for previous work that has documented air pollution environmental injustice and inequality within individual US metropolitan areas and regions. Results given here can aid policy-makers in identifying locations with high environmental injustice and inequality. For example, states with both high injustice and high inequality (top quintile) for outdoor residential NO2 include New York, Michigan, and Wisconsin.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adolescent
  • Adult
  • Aged
  • Air Pollutants / analysis*
  • Air Pollution / statistics & numerical data*
  • Child
  • Child, Preschool
  • Demography
  • Environment*
  • Humans
  • Income / statistics & numerical data
  • Middle Aged
  • Nitrogen Dioxide / analysis*
  • Socioeconomic Factors*
  • United States / ethnology
  • Young Adult


  • Air Pollutants
  • Nitrogen Dioxide

Grant support

This material is based upon work partially supported by University of Minnesota and by the National Science Foundation under Grant No. 0853467. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the funders. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.