McCracken et al. we assigned the earlier observations for each subject a weight of 1. Although the advantage of using a spatio- temporal exposure model is that we can esti- mate BC at a subject’s residence without placing a monitor there, we acknowledge that there is uncertainty about the extent to which the model can be extrapolated across space. For the main analyses, we aimed to balance this concern against that of maintaining an adequate sample size and limited the distance to 40 km. As a sensitivity analysis, we exam- ined the association between BC and LTL among subjects living < 10 km from one of the monitors. Telomere shortening is related to biologi- cal aging (Aviv et al. 2009). Thus, we evalu- ated whether chronological age modifies the effect of annual BC on LTL by including a BC × age interaction term in the model. Table 1. Characteristics of NAS subjects: male never-smokers from eastern Massachusetts, 1999–2006. aNeutrophil and lymphocyte percentages of total leukocytes obtained for 270 visits. Variable No. of measures Age [years (mean ± SD)] Education level [n (%)] < High school High school < 4 years of college 4 years of college > 4 years of college Living in an urban area [n (%)] BMI [kg/m2 (mean ± SD)] Waist circumference [cm (mean ± SD)] Coronary artery disease [n (%)] Fasting blood glucose [mg/dL (mean ± SD)] Diabetes [n (%)] Statin treatment [n (%)] Leukocyte count [cells/cm3 (mean ± SD)] Neutrophils [% (mean ± SD)]a Lymphocytes [% (mean ± SD)]a Telomere length Median Geometric mean (geometric SD) Census-tract median income, $1000s (mean ± SD) 62.3 ± 19.8 All visits 274 74.6 ± 7.1 10 (4) 83 (30) 61 (22) 59 (22) 61 (22) 164 (60) 27.6 ± 3.7 98.6 ± 9.4 82 (30) 105 ± 24 32 (12) 106 (39) 6.3 ± 2.0 62 ± 8 26 ± 7 1.25 1.25 (1.42) First visit 165 73.6 ± 7.1 6 (4) 45 (27) 39 (24) 39 (24) 36 (22) 63.3 ± 20.4 99 (60) 27.7 ± 3.9 98.5 ± 9.0 43 (26) 106 ± 27 17 (10) 46 (28) 6.1 ± 1.6 62 ± 7 26 ± 7 1.27 1.27 (1.34) Central BC monitor BC monitors Second visit 90 75.9 ± 7.0 4 (4) 32 (36) 17 (19) 17 (19) 20 (22) 60.5 ± 19.6 55 (61) 27.4 ± 3.6 99.1 ± 9.9 31 (34) 104 ± 18 12 (13) 48 (53) 6.3 ± 2.0 62 ± 8 26 ± 7 1.23 1.25 (1.54) Third visit 19 76.5 ± 6.6 0 (0) 6 (32) 5 (26) 3 (16) 5 (26) 60.8 ± 13.6 10 (53) 26.8 ± 3.0 103.0 ± 10.3 8 (42) 104 ± 17 3 (16) 12 (63) 7.5 ± 3.8 61 ± 12 28 ± 13 1.12 1.02 (1.41) Because accelerated telomere shortening is associated with inflammation, we also tested for an interaction between BC and treatment with statin medications (then-current use based on self-report), which have been shown to reduce inflammation (Liu et al. 2009). This study protocol was approved by the institutional review boards of all participat- ing institutions. All participants gave written informed consent. Results Although limited to never-smokers and almost entirely white (97%), this subgroup of men from the NAS was fairly heterogeneous accord- ing to BMI, education level, and census-tract median income (Table 1). We obtained two LTL measurements for 90 of the 165 participants (55%) and a third measurement for 19 participants (12%); 17 (10%) of the participants died before a repeat measure could be collected, and the rest were lost to follow-up for various reasons, includ- ing refusal and relocation out of the study area. Over the 274 measures, LTL ranged from a T:S ratio of 0.31 to 2.84 and had a median 1.24 and a geometric mean (geomet- ric SD) 1.25 (1.42). Although there was only a small decrease of 2% in geometric mean LTL between the first and second measures, the LTL was 19% shorter at the third meas- urement occasion compared with the second. Rather than being individuals with typically short telomeres that happened to have three Annual average BC 0.00017–0.21 0.22–0.30 0.31–0.39 0.40–0.51 0.52–1.2 0 20 40 Kilometers 0 20 40 Kilometers Figure 1. Maps of study area in eastern Massachusetts showing (A) locations of BC monitors used in spatiotemporal statistical model and (B) approximate locations of residences of NAS participants, with coding (white, gray, black) to indicate quintiles of annual moving averages of predicted daily BC concentrations, 1999–2006. 1566 ovlume 118 | number 11 | November 2010 • Environmental Health Perspectives