fourth postexposure years, thereby allowing differentiation between symptom patterns of persistence, delayed onset, resolution, and never symptomatic at year-4 follow-up. It also allowed us to explore the evolution of symptoms within individuals, as opposed to tracking change over time in the larger popu- lation. Multivariate analyses of the cohort data yielded one of our major findings. After adjusting for other variables in the model, we found that each month worked at the WTC site increased the odds of symptoms at fol- low-up by 11% for both LRS and GERD and by 8% for URS. At the maximum dura- tion of 10 months, the odds of symptoms in year 4 were 2.8 times greater for LRS and GERD and 2.2 times greater for URS. In fact, 10 months of work at the site was much more strongly associated with year 4 symptoms than even the earliest arrival time. Most studies have not reported on work dura- tion (Banauch et al. 2005a; Buyantseva et al. 2007; Fireman et al. 2004; Landrigan et al. 2004; Oppenheimer et al. 2007). Of those that did, one found an association with new- onset asthma (Wheeler et al. 2007); another reported associations with respiratory symp- toms at 20 months (Buyantseva et al. 2007), although it did not control for early arrival at the WTC site; and a third found no asso- ciation with post-9/11 symptoms (de la Hoz et al. 2008). Our data strongly suggest that the impact of protracted WTC work exposure was of critical importance and should be con- sidered in future disaster plans. Another important finding of the cohort analyses was that cough resolution did not mean that persons were free of LRS. Of those with resolved cough, half (50.9%) reported persistent or delayed-onset dyspnea, wheeze, or both at follow-up. Furthermore, because cough may arise from either lower or upper respira- tory problems, we looked at URS in those with resolved cough and found that 61.3% of former coughers reported sore throat, rhino- sinusitis symptoms, or both at follow-up. Thus, our data confirm our clinical impression Table 3. Multiple logistic regression models in the cohort of 3,722 firefighters [OR (95% CI)] for LRS at year 4. Variable Any URS, year 1 questionnaire GERD, year 1 questionnaire Cough, year 1 questionnaire Dyspnea, year 1 questionnaire Wheeze, year 1 questionnaire Current smoking Age on 9/11 (years) Arrival group 1a Arrival group 2a Arrival group 3a Months of work at WTC site Days between questionnaires OR (95% CI) 1.45 (1.21–1.73) 1.27 (1.07–1.51) 1.62 (1.38–1.90) 2.34 (1.94–2.82) 1.91 (1.59–2.30) 1.56 (1.26–1.93) 1.03 (1.02–1.04) 1.66 (1.16–2.37) 1.49 (1.08–2.04) 1.13 (0.77–1.64) 1.11 (1.08–1.14) 0.999 (0.998–1.00) that persons with resolved cough should not be considered symptom-free (Buyantseva et al. 2007). Similarly, among persons with URS, those with resolved sore throat had a 36.3% prevalence of rhinosinusitis symptoms, persis- tent or delayed in onset, during year 4. Consistent with initial reports by this group (Banauch et al. 2005a, 2005b; Prezant et al. 2002) and others (de la Hoz et al. 2008; Herbert et al. 2006), we found considerable overlap of LRS, URS, and GERD symptoms. Adding terms for symptoms of the other respi- ratory groups at the initial period improved all multivariate models. Describing cough as an LRS simplifies analyses but may misclassify cough, which can also be due to rhinosinus- itis symptoms (congestion/drip), sore throat, and/or GERD (Irwin et al. 1993). In fact, we found that about 30% of the cohort reported at least one LRS, one URS, and GERD symp- toms during both initial and follow-up times (data not shown). Cigarette smoking is a modifiable risk factor increasing the likelihood of LRS and GERD by approximately 50% at follow-up. This is but one of many reasons for support- ing aggressive efforts to promote nonpunitive tobacco cessation programs and counseling services, available without cost at FDNY since 9/11 (Bars et al. 2006). This study’s primary limitation was its lack of access to information about treatment, which precluded estimating the effects of treat- ment on reported symptoms over time. We also acknowledge that both arrival group and months of work are only crude measures of exposure to the WTC site, which would better be measured by knowing specific hours and days an individual worked on-site. However, our arrival group measure was indepen- dently corroborated by others who described the intensity of outdoor exposures based on days post-9/11 (Lioy et al. 2002). Symptom reporting could have been biased because of issues related to workers’ compensation, retire- ment disability, and civil litigation; it could also have been affected by recall bias or by the wording of our questions, which asked about symptoms since 9/11. However, in a previous Table 4. Multiple logistic regression models in the cohort of 3,722 firefighters [OR (95% CI)] for URS at year 4. Variable Any LRS, year 1 questionnaire Rhinosinusitis, year 1 questionnaire Sore throat, year 1 questionnaire Arrival group 1a Arrival group 2a Arrival group 3a Age on 9/11 (years) Months of work at WTC site OR (95% CI) 1.53 (1.29–1.80) 2.06 (1.77–2.39) 1.73 (1.48–2.04) 1.63 (1.17–2.28) 1.40 (1.04–1.88) 1.09 (0.77–1.55) 1.011 (1.00–1.02) 1.08 (1.05–1.11) aReference is arrival group 4 (arrival at WTC site 3–14 days post-9/11). aReference is arrival group 4 (arrival at WTC site 3–14 days post-9/11). Environmental Health Perspectives • volume 117 | number 6 | June 2009 Trends in post-9/11 respiratory symptoms study (Banauch et al. 2006) we reported that objective spirometric measurements correlated with clinical complaints, and each additional respiratory symptom was associated with an additional decrement in forced expiratory vol- ume in 1 sec. Despite potential limitations, we found significant exposure–response gradients for symptoms at all time points. We believe the strengths of this study outweigh its limi- tations. First, we examined secular trends in > 10,000 firefighters and were able to study symptom progression in the cohort of 3,722 based on their questionnaire responses at both years 1 and 4. Compared with other pub- lished studies, this represents a larger group of highly exposed individuals, all with similar firefighting job tasks, and the most years of follow-up. Second, we asked about the preva- lence of symptoms pre-9/11, which enabled us to compare pre- and post-9/11 symptoms in individuals whom we knew to be extremely healthy pre-9/11. Finally, we explored the data in several ways. For example, we initially selected the cohort based on persons with two or more visits at least 1 year apart to increase the number of participants, but ultimately selected only those with first- and fourth-year questionnaires to allow the maximum time for symptoms to resolve, either spontaneously or through treatment. Using either cohort did not change findings. Conclusion We found that cough and sore throat were the most sensitive indicators of initial respi- ratory insult early after 9/11, but that the other symptoms (wheeze, dyspnea, rhinosi- nusitis, and GERD) were more sensitive indi- cators at follow-up. In year 4, even cough, the symptom with the greatest decline rate, was still reported at four times the pre-9/11 level, and the other symptoms were reported at levels from 8.2 to 28.6 times their pre- 9/11 rates. We found a significant exposure– response gradient based on arrival time for all symptoms. In the cohort we found that each month worked at the site conferred a substantial and highly significant increase in the odds of symptoms at follow-up. For LRS, Table 5. Multiple logistic regression models in the cohort of 3,722 firefighters [OR (95% CI)] for GERD at year 4. Variable Any LRS, year 1 questionnaire Rhinosinusitis, year 1 questionnaire GERD, year 1 questionnaire Current smoking Arrival group 1a Arrival group 2a Arrival group 3a Age on 9/11 (years) Months of work at WTC site OR (95% CI) 1.71 (1.45–2.03) 1.20 (1.03–1.39) 3.48 (2.98–4.05) 1.49 (1.22–1.83) 1.49 (1.05–2.11) 1.15 (0.84–1.58) 1.03 (0.71–1.49) 1.02 (1.01–1.03) 1.11 (1.08–1.14) aReference is arrival group 4 (arrival at WTC site 3–14 days post-9/11). 979