Fine particulate matter (PM2.5) concentrations (C) in air are often considered to contribute causally to increased human mortality rates (R). In collaboration with Cox Associates, we tested this causal hypothesis in 100 United States cities, using the large, publicly available, NMMAPS database and US Census data. Conditional independence tests for potential causation, non-parametric classification tree analysis, Bayesian Model Averaging (BMA), and Granger-Sims causality testing, show no evidence that PM2.5 concentrations have any causal impact on increasing mortality rates. This apparent absence of a causal C-R relation has potentially important policy and economic implications.
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