Author Affiliations: Institute for Healthy Aging, Rush University Medical Center, Chicago, Illinois.
We appreciate the insight offered by Dr Grant and agree that the health impacts of polycyclic aromatic hydrocarbons (PAHs) and ambient elemental carbon (of which black carbon is a surrogate measure) are an important area of current and future public health research, particularly research concerning cognitive aging.
Emerging findings have implicated exposure to PAHs, particularly benzo[a]pryrene, as a potential deleterious influence on children's cognitive and behavioral development.1 To our knowledge, no study has evaluated exposure to PAHs in relation to cognitive outcomes in adulthood, but an adverse link is plausible, supported in part by accumulating evidence that exposure to tobacco smoke is associated with cognitive decline and dementia.2- 4
As demonstrated by Perera et al1 and others, high-quality measurements of PAH exposures require the use of participant-level monitors and/or biomarkers. We did not estimate our participants' exposures to PAHs specifically, although particle-phase PAHs are a component of the PM2.5 (particulate matter <2.5 μm in diameter) mass measurements routinely made at ambient air-quality monitoring sites. By contrast, our study capitalized on spatially dense pollutant measurement data available across the contiguous 48 states over long periods. Because these monitors do not measure PAHs specifically—such measurements would be prohibitively difficult and costly to perform—our approach was not suitable for estimating participants' PAH exposures. Moreover, in our judgment, our data did not support the examination of geographic variation in effect size of PM2.5 exposures. Owing to the limited sample size in some areas, it would be difficult to distinguish variation because of statistical uncertainty from real geographic variation in effect. Importantly, however, several PAH compounds are likely enriched near roads, and our PM models do account for the distance to the nearest road of 3 of the largest road classes delineated by the US Census. Thus, while it is plausible that PAH exposure is responsible at least in part for the cognitive associations we observed, we were not able to evaluate this association directly.
The evidence on black carbon and cognitive aging is more direct. In a recent study, older men with higher exposure to black carbon had significantly worse global cognition, even after researchers accounted for other factors such as smoking and several socioeconomic indices.5 We are currently completing a study of black carbon exposure and cognitive decline, and we share Dr Grant's enthusiasm in learning what the data will reveal.
Correspondence: Dr Weuve, Institute for Healthy Aging, Rush University Medical Center, 1645 W Jackson Blvd, Ste 675, Chicago, IL 60612 (email@example.com).
Financial Disclosure: None reported.
Weuve J, Yanosky JD. Polycyclic Aromatic Hydrocarbons, Particulate Air Pollution, and Cognitive Decline—Reply. Arch Intern Med. 2012;172(13):1045-1046. doi:10.1001/archinternmed.2012.2159