ambient particle measures to estimate expo- sure is likely to be a modest underestimation of pollution effects (Zeger et al. 2000). Our ability to investigate interactions between par- ticipant characteristics such as beta-blocker use and particle effects was limited by the size of the population. Conﬁdence in and generaliz- ability of our estimates for our dichotomous outcome were limited by small numbers of observations. Although our cohort was vul- nerable on the basis of age, previous smoking, or hypertension history, our potential to doc- ument overt ischemic episodes was also lim- ited by the choice of a population, only 18% of whom had diagnosed clinical coronary artery disease. Even in the ULTRA study of a population with doctor-diagnosed coronary artery disease subjected to submaximal exer- cise, sufficient episodes to examine the out- come of ischemia were documented only among Finnish participants and not among participants from the two other countries (Pekkanen et al. 2002). Our primary analyses did include one individual who, on 3 of 12 visits, reported smoking one to three ciga- rettes or cigarillos within the previous 48 hr. His data met the inclusion criteria for exam- ining the dichotomous ST-segment depres- sion ≥ 0.5 mm only during the exercise period; exclusion of this individual from analyses did not inﬂuence our ﬁndings. In our continuous analyses, we included only those whom we considered vulnerable on the basis of ST-segment depression (23 of 28). 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