≤ 2.5 µm in aerodynamic diameter (PM2.5) in Lincoln County exceeded the annual stan- dard for PM2.5 (standard for PM2.5 was an annual mean of 15.0 µg/m3; annual mean in Lincoln county was 17.1 µg/m3 in 2000 and 16.2 µg/m3 in 2001) (U.S. Environmental Protection Agency 2009). However, there is no reason to think that exposure to ambient particulate matter confounds the association seen between vermiculite exposure and respi- ratory outcomes. No previous studies have examined expo- sure to asbestos-containing vermiculite and respiratory symptoms among this age group. There are numerous case reports for individu- als with asbestos exposure during childhood and the development of mesothelioma early in adulthood (Arul and Holt 1977; Cazzadori et al. 1992; Inase et al. 1991; Schneider et al. 1996), but only two studies (Andrion et al. 1994; Lieben and Pistawka 1967) have reported on individuals diagnosed with meso- thelioma before age 18, which presumably resulted from childhood exposure to asbes- tos. Our study population had no cases of mesothelioma, but respiratory symptoms were apparent and showed an association with exposure to asbestos-containing vermiculite. Our study is unique in that it explored respi- ratory symptoms among a group of individu- als between the ages of 10 and 29 years. The results are in line with what other researchers have reported for the same study population. As reported by Peipins et al. (2003), there was a positive association between asbestos-con- taining vermiculite exposure and radiographic abnormalities among adults (≥ 18 years of age). Although Peipins et al. did not exam- ine self-reported respiratory symptoms, they found that respiratory morbidity (i.e., radio- graphic abnormalities in the form of pleural plaques) was associated with exposure to the vermiculite in Libby. One of the strengths of this study is that it included a large number of individuals, many of whom had only community-based exposure to vermiculite. Although some individuals had additional exposure through household contact with a W.R. Grace employee, we did not detect any effect measure modification or confounding by this variable. Also, there were very few missing data on the exposures and outcomes of interest. One limitation of the study is that we were not able to determine if the elevated levels of cough were due to asbes- tos-contaminated vermiculite exposure or to some other differential exposure present while these individuals were children in Libby. Although possible, it is not clear how another exposure, such as particulate matter, could be highly correlated with frequently engaging in activities resulting in exposure to vermiculite. Second, Libby residents are well informed about the contamination in their town and the potential health effects. This may have resulted in recall bias or exaggerated report- ing of respiratory symptoms or of exposure frequency. However, we found no association between self-reported physician-diagnosed lung disease and activities related to vermicu- lite exposure. If individuals were biased in their reporting, we would have expected to see associations between activities with ver- miculite exposure and all self-reported respi- ratory outcomes, although lung disease may be different from the other self-reported out- comes because it is less subjective and possibly less susceptible to recall bias. Exposure assess- ment involving questions about vermiculite insulation are also subject to recall. Typically, individuals may not be knowledgeable about whether there was vermiculite insulation in any of their childhood residences. In Libby, individuals are acutely aware of vermiculite contamination of their town and households, so it is more likely that they would know what type of insulation was present in their house. However, “don’t know” was a response option and was chosen in similar proportions across all age groups. We performed a sensi- tivity analysis that included these individuals; the results were essentially unchanged in the sensitivity analysis, demonstrating that the missing data were unlikely to have biased the results. Another limitation is that the par- ticipants volunteered to take part in the study and were not a random sample of the popula- tion. Also, bloody phlegm is reported as a respiratory outcome in this study, but this variable may not represent health outcomes that are as serious as are usually indicated by this symptom. The most common causes of bloody phlegm in children are not associ- ated with asbestos-related diseases (Pianosi and Al-Sadoon 1996). On the other hand, among adults exposed to asbestos, bloody phlegm would lead to a suspicion of lung can- cer, although lung cancer is unlikely in this age group. Finally, the accuracy of the data is based on the reporting by 10- to 29-year-olds, who may not be able to recall with complete confidence all of the exposures and outcomes in question. The data were not linked to any medical records for verification. However, it is unlikely that general inaccuracies in report- ing would produce the consistent pattern seen among the prevalence rates for cough and the associations with adverse respiratory symp- toms reported here. Conclusions This study was the first to examine the associa- tion between asbestos-contaminated vermicu- lite exposure and the prevalence of respiratory symptoms among individuals no older than 18 years of age while the mine was active and who were continually exposed through lin- gering contamination in Libby. We found Environmental Health Perspectives • volume 118 | number 7 | July 2010 Respiratory symptoms in Libby, MT positive associations between some activities likely resulting in asbestos-contaminated ver- miculite exposure and certain self-reported respiratory symptoms among Libby residents who were children when the mine closed. The number/frequency of activities was positively associated with self-report of usually having a cough. Frequently handling vermiculite insu- lation was associated with usually having a cough, having shortness of breath when walk- ing up a hill or hurrying on level ground, and coughing up bloody phlegm. However, the presence of vermiculite insulation in the house was not associated with these outcomes. Other activities associated with self-reported respiratory symptoms were frequently partici- pating in recreational activities along Rainy Creek Road, playing at the ball fields near the expansion plant, and heating the vermiculite to make it expand/pop. 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