Hypothesis
Since traffic-related air pollution has been linked to worse cognition, long-term traffic exposure and short-term rises in pollutants would also be associated with depressive symptoms in older adults.
What failed
Among 732 Boston-area adults aged 65 and over, neither distance to a major road nor two-week averages of PM2.5, sulfates, black carbon, ultrafine particles or gaseous pollutants showed a positive association with a CESD-R score of 16 or above; for PM2.5 the odds ratio per IQR was 0.67 (95% CI 0.46-0.98).
Broken assumption
Pollution exposures harmful to cognition at these levels also raise depressive symptoms.
Why
The abstract offers no mechanism; the authors note the study area met current US air quality standards.
From the published paper: Wang et al. Environmental Health Perspectives, 2014. ↗Public domain