Confounding in air pollution epidemiology: the broader context.

Recent research appropriately suggests that caution is warranted when interpreting long-term trends of improving air quality and trends in reduced mortality rates as causal because of concerns about confounding. The use of long-term time trends as the primary source of exposure variability, however, has been avoided in most air pollution epidemiology studies. Multiple studies, using different study designs that exploit various temporal and spatial dimensions of exposure variability, have observed reasonably consistent and coherent associations between particulate air pollution and cardiopulmonary health. It's possible that these air pollution studies have experienced "epidemiological bad luck" with multiple confounders that are inadequately controlled for and coincidentally cause spurious correlations that are coherent across the different study designs. To be consistent with the overall epidemiologic results, these confounders must correlate with pollution across various dimensions of time and space and be stronger risk factors for cardiopulmonary disease than for other disease. The more likely explanation of recent air pollution epidemiology is that air pollution, especially fine particulate and related pollution, has measurable effects on cardiopulmonary health.