In Reply: In response to our study, Drs González-Santiago and Balderas-Renteria highlight the importance of air pollution as an explanatory factor for social inequalities in health. Similarly, Dr Gross raises the possibility of psychological distress as a factor. We agree with these possibilities; indeed, there are multiple pathways linking socioeconomic position to health.
The Black Report, commissioned by the Department of Health in the United Kingdom in 1977, was the first comprehensive review of evidence in this domain.1 It identified 4 theoretical explanations for social inequalities: artifactual, natural or social selection, materialist/structural, and cultural/behavioral. Subsequent research has attempted to examine (and promote) the relative importance of one pathway over another, with little consensus.2 In the last decade or so, there has been a surge of interest in neighborhood studies, with increasing recognition of the methodological challenges and the need to identify the causal chain linking exposure to outcome.3 Exposure to environmental toxins and air pollution is likely to contribute to health disparities. Unfortunately, we have no data on air pollution.