Response to Letter Regarding Article, “Residential Exposure to Traffic Is Associated With Coronary Atherosclerosis”
We thank Dr Brook for his insightful comments and agree with his opinion that short-term effects are important mechanisms of particulate matter (PM) –related health effects. However, we do not agree that our study results represent a “minor” proatherosclerotic effect.1 The 7% higher coronary calcification score is related to a reduction in the distance between the residence and a major road by half. Therefore, it is not comparable to a “PM estimate” because we did not measure air pollutant concentrations in relation to proximity to roadways. Furthermore, a comparatively small exposure contrast of 3.91 μg/m3 PM2.5 is associated with an, albeit nonsignificant, 17% higher coronary calcification score (Table 2). This corresponds to ≈1 year of higher vascular age in our study population. Moreover, nondifferential exposure misclassification is larger in long-term than in short-term studies. This leads to a more pronounced underestimation of the true effect size in long-term studies than in short-term studies. The observed association is therefore a conservative estimate of the true association.
If we assume a causal relationship, we can calculate the number of attributable cases to estimate the public health relevance of our results. The odds for being above the 75th percentile of the age- and gender-specific coronary calcification distribution, the clinically used cut point for risk stratification that is highly predictive of coronary events,2 was elevated by 1.64, 1.34, and 1.08 for living within 50, 51 to 100, and 101 to 200 m of a major road, respectively, compared with >200 m away. If we consider that ≈500 000 inhabitants 45 to 74 years of age in our study region, of which ≈15% live ≤200 m of a major road, and assume a mean odds ratio (OR) of 1.2, the population attributable risk (PAR), calculated as PAR=P(OR−1)/[1+P(OR−1)], where P is the prevalence of the exposed, is 3%. In our study region, where 125 000 inhabitants are in the top quarter of the coronary calcification distribution, this amounts to 3750 cases of high coronary calcification resulting from high residential traffic exposure.
Hoffmann B, Moebus S, Möhlenkamp S, Stang A, Lehmann N, Dragano N, Schmermund A, Memmesheimer M, Mann K, Erbel R, Jöckel KH, for the Heinz Nixdorf Recall Study Investigative Group. Residential exposure to traffic is associated with coronary atherosclerosis. Circulation. 2007; 116: 489–496.
Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults. Executive Summary of the third report of the National Cholesterol Education Program (NCEP) Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults (Adult Treatment Panel III). JAMA. 2001; 285: 2486–2497.