socioeconomic status is possible. Additionally, we did not measure factors such as psychologi- cal stress, lack of physical exercise, and envi- ronmental tobacco smoke (ETS), all suspected to be associated with shorter telomere length and potential confounders if they are associ- ated with exposure to traffic particles in the NAS cohort (Cherkas et al. 2008). Schikowski et al. (2008) found that distance to major roads and ETS exposure were both associ- ated with lower socioeconomic status in the Ruhr area of Germany, which suggests that confounding by ETS is an important concern for ambient BC studies in general. However, we assume that ETS exposure would be fairly low among the population of elderly, male never-smokers included in our analyses and that any such bias would be reduced by the adjustment for socioeconomic variables in our models. Moreover, among the overall NAS cohort under follow-up during the same study period (605 subjects), we found no association between active smoking and near-residence annual average BC concentration (0.01 gμ/m 3 higher BC among active smokers; p = 0.531), which suggests that ETS, presumably having a similar geographic distribution as active smok- ing, is unlikely to be an important confounder (data not shown). In addition to ETS, there are other impor- tant indoor sources of BC, such as cooking and candle burning (LaRosa et al. 2002), that we did not take into account in our ambient BC predictions. If we were investigating the biological effects of total BC exposure, this would result in substantial measurement error and would be an important limitation of our study. However, we used ambient BC as a surrogate for traffic-related particles, in which case exclusion of BC of indoor origin actually reduces measurement error. Nevertheless, BC from indoor sources presents a possible source of confounding if ambient BC and indoor- generated BC are correlated. An ongoing exposure assessment study including indoor BC and sulfate-based penetration ratios shows that the correlation between BC of outdoor origin and BC of indoor origin is low in NAS homes (r = –0.13; data not shown). Therefore, confounding of the association between ambi- ent BC and telomere length by BC of indoor origin is unlikely to be an important source of bias in our study. Our findings suggest that higher annual average exposure to traffic-related air pollution is associated with shorter leukocyte telomeres among the elderly. Telomere attrition is a measure of biological aging that explains some interindividual variation in risk of athero- sclerosis and coronary artery disease and may play an important role in the chronic health effects of airborne particles, particularly those rich in BC, which are primarily related to automobile traffic. c ectinor Steve Melly was omitted from the list of authors in the manuscript originally pub- lished online. His name has been added here. Ref Rencs Allsopp RC, Vaziri H, Patterson C, Goldstein S, Younglai EV, Futcher AB, et al. 1992. Telomere length predicts replica- tive capacity of human fibroblasts. Proc Natl Acad Sci USA 89(21):10114–10118. Aviv A. 2002. Chronology versus biology: telomeres, essen- tial hypertension, and vascular aging. Hypertension 40(3):229–232. Aviv A, Chen W, Gardner JP, Kimura M, Brimacombe M, Cao X, et al. 2009. Leukocyte telomere dynamics: longitudinal findings among young adults in the Bogalusa Heart Study. Am J Epidemiol 169(3):323–329. Baccarelli A, Cassano PA, Litonjua A, Park SK, Suh H, Sparrow D, et al. 2008a. Cardiac autonomic dysfunction: effects from particulate air pollution and protection by dietary methyl nutrients and metabolic polymorphisms. Circulation 117(14):1802–1809. Baccarelli A, Martinelli I, Zanobetti A, Grillo P, Hou LF, Bertazzi PA, et al. 2008b. Exposure to particulate air pol- lution and risk of deep vein thrombosis. Arch Intern Med 168(9):920–927. Baccarelli A, Wright RO, Bollati V, Tarantini L, Litonjua AA, Suh HH, et al. 2009. Rapid DNA methylation changes after exposure to traffic particles. Am J Respir Crit Care Med 179(7):572–578. Baccarelli A, Zanobetti A, Martinelli I, Grillo P, Hou L, Giacomini S, et al. 2007. Effects of exposure to air pollution on blood coagulation. J Thromb Haemost 5(2):252–260. Benetos A, Gardner JP, Zureik M, Labat C, Xiaobin L, Adamopoulos C, et al. 2004. Short telomeres are associ- ated with increased carotid atherosclerosis in hyperten- sive subjects. Hypertension 43(2):182–185. Blackburn EH. 2001. Switching and signaling at the telomere. Cell 106(6):661–673. Brook RD, Brook JR, Urch B, Vincent R, Rajagopalan S, Silverman F. 2002. Inhalation of fine particulate air pollu- tion and ozone causes acute arterial vasoconstriction in healthy adults. Circulation 105(13):1534–1536. Brook RD, Franklin B, Cascio W, Hong Y, Howard G, Lipsett M, et al. 2004. Air pollution and cardiovascular disease: a statement for healthcare professionals from the Expert Panel on Population and Prevention Science of the American Heart Association. Circulation 109(21):2655–2671. Brouilette SW, Moore JS, McMahon AD, Thompson JR, Ford I, Shepherd J, et al. 2007. Telomere length, risk of coro- nary heart disease, and statin treatment in the West of Scotland Primary Prevention Study: a nested case-control study. Lancet 369(9556):107–114. Cawthon RM. 2002. Telomere measurement by quantitative PCR. Nucleic Acids Res 30(10):e47. Chahine T, Baccarelli A, Litonjua A, Wright RO, Suh H, Gold DR, et al. 2007. Particulate air pollution, oxidative stress genes, and heart rate variability in an elderly cohort. Environ Health Perspect 115:1617–1622. Chen JC, Schwartz J. 2008. Metabolic syndrome and inflam- matory responses to long-term particulate air pollutants. Environ Health Perspect 116:612–617. Cherkas LF, Hunkin JL, Kato BS, Richards JB, Gardner JP, Surdulescu GL, et al. 2008. The association between physi- cal activity in leisure time and leukocyte telomere length. Arch Intern Med 168(2):154–158. Donaldson K, Tran CL. 2002. Inflammation caused by particles and fibers. Inhal Toxicol 14(1):5–27. Ehrlenbach S, Willeit P, Kiechl S, Willeit J, Reindl M, Schanda K, et al. 2009. Influences on the reduction of rela- tive telomere length over 10 years in the population-based Bruneck Study: introduction of a well-controlled high- throughput assay. Int J Epidemiol 38(6):1725–1734. Farzaneh-Far R, Cawthon RM, Na B, Browner WS, Schiller NB, Whooley MA. 2008. Prognostic value of leukocyte telom- ere length in patients with stable coronary artery disease: data from the Heart and Soul Study. Arterioscler Thromb Vasc Biol 28(7):1379–1384. Environmental Health Perspectives • volume 118 | number 11 | November 2010 Traffic particles may shorten telomeres Fitzpatrick AL, Kronmal RA, Gardner JP, Psaty BM, Jenny NS, Tracy RP, et al. 2007. Leukocyte telomere length and car- diovascular disease in the cardiovascular health study. Am J Epidemiol 165(1):14–21. Gonzalez-Flecha B. 2004. Oxidant mechanisms in response to ambient air particles. Mol Aspects Med 25(1–2):169–182. Gryparis A, Coull B, Schwartz J, Suh H. 2007. Semiparametric latent variable regression models for spatiotemporal mod- eling of mobile source particles in the greater Boston area. Appl Stat 56(2):183–209. Gryparis A, Paciorek CJ, Zeka A, Schwartz J, Coull BA. 2009. Measurement error caused by spatial misalignment in environmental epidemiology. Biostatistics 10(2):258–274. Honda S, Hjelmeland LM, Handa JT. 2001. Oxidative stress- induced single-strand breaks in chromosomal telomeres of human retinal pigment epithelial cells in vitro. Invest Ophthalmol Vis Sci 42(9):2139–2144. Hou L, Savage SA, Blaser MJ, Perez-Perez G, Hoxha M, Dioni L, et al. 2009. Telomere length in peripheral leukocyte DNA and gastric cancer risk. Cancer Epidemiol Biomarkers Prev 18(11):3103–3109. Hoxha M, Dioni L, Bonzini M, Pesatori AC, Fustinoni S, Cavallo D, et al. 2009. Association between leukocyte telomere shortening and exposure to traffic pollution: a cross-sectional study on traffic officers and indoor office workers. Environ Health 8:41. Kim JJ, Smorodinsky S, Lipsett M, Singer BC, Hodgson AT, Ostro B. 2004. Traffic-related air pollution near busy roads: the East Bay Children’s Respiratory Health Study. Am J Respir Crit Care Med 170(5):520–526. Kim S, Parks CG, DeRoo LA, Chen H, Taylor JA, Cawthon RM, et al. 2009. Obesity and weight gain in adulthood and telomere length. Cancer Epidemiol Biomarkers Prev 18(3):816–820. Künzli N, Jerrett M, Mack WJ, Beckerman B, LaBree L, Gilliland F, et al. 2005. Ambient air pollution and atherosclerosis in Los Angeles. Environ Health Perspect 113:201–206. LaRosa LE, Buckley TJ, Wallace LA. 2002. Real-time indoor and outdoor measurements of black carbon in an occupied house: an examination of sources. J Air Waste Manag Assoc 52(1):41–49. Liu PY, Liu YW, Lin LJ, Chen JH, Liao JK. 2009. Evidence for sta- tin pleiotropy in humans: differential effects of statins and ezetimibe on rho-associated coiled-coil containing protein kinase activity, endothelial function, and inflammation. Circulation 119(1):131–138. Martin-Ruiz CM, Gussekloo J, van Heemst D, von Zglinicki T, Westendorp RG. 2005. Telomere length in white blood cells is not associated with morbidity or mortality in the oldest old: a population-based study. Aging Cell 4(6):287–290. McGrath M, Wong JY, Michaud D, Hunter DJ, De Vivo I. 2007. Telomere length, cigarette smoking, and bladder cancer risk in men and women. Cancer Epidemiol Biomarkers Prev 16(4):815–819. Mills NL, Tornqvist H, Robinson SD, Gonzalez MC, Soderberg S, Sandstrom T, et al. 2007. Air pollution and athero- thrombosis. Inhal Toxicol 19(suppl 1):81–89. Morla M, Busquets X, Pons J, Sauleda J, MacNee W, Agusti AG. 2006. Telomere shortening in smokers with and without COPD. Eur Respir J 27(3):525–528. Mukherjee M, Brouilette S, Stevens S, Shetty KR, Samani NJ. 2009. Association of shorter telomeres with coronary artery disease in Indian subjects. Heart 95(8):669–673. Nettleton JA, Diez-Roux A, Jenny NS, Fitzpatrick AL, Jacobs DR Jr. 2008. Dietary patterns, food groups, and telomere length in the Multi-ethnic Study of Atherosclerosis (MESA). Am J Clin Nutr 88(5):1405–1412. O’Neill MS, Veves A, Sarnat JA, Zanobetti A, Gold DR, Economides PA, et al. 2007. Air pollution and inflammation in type 2 diabetes: a mechanism for susceptibility. Occup Environ Med 64(6):373–379. Patel MM, Chillrud SN, Correa JC, Feinberg M, Hazi Y, KC D, et al. 2009. Spatial and temporal variations in traffic- related particulate matter at New York City high schools. Atmos Environ 43(32):4975–4981. Perez-Rivero G, Ruiz-Torres MP, Rivas-Elena JV, Jerkic M, Diez-Marques ML, Lopez-Novoa JM, et al. 2006. Mice deficient in telomerase activity develop hypertension because of an excess of endothelin production. Circulation 114(4):309–317. Pope CA III, Verrier RL, Lovett EG, Larson AC, Raizenne ME, Kanner RE, et al. 1999. Heart rate variability associated with particulate air pollution. Am Heart J 138(5 pt 1):890–899. Risom L, Moller P, Loft S. 2005. Oxidative stress-induced 1569