© 2007 American Thoracic Society
How Much Smoke Do We Need in Order to Assume That There Is a Fire?From the Authors:We are grateful for the opportunity to clarify a few issues related to our recent article (1). We do not imply that questionnaire data of fire smoke exposure are better than objective measurements of PM10. However, personal or home outdoor measurements of markers of fire smoke intensity are extremely hard to get and unlikely to be available in fire emergencies. It is for that reason that we had to use questionnaire data. To grade the intensity of exposure, we asked about the number of days fire was smelled in the home. It is biologically plausible that occurrence of symptoms increases not only with the concentration level but with the duration of the exposure to thick fire smoke. While the "dose response" between (reported) duration of fire smell and symptoms is rather convincing, we are well aware—and we extensively discussed—the potential of reporting bias. This concern also originates from our previous research on the association between pollutant levels and reported annoyance. In contrast to the statement in Dr. Paredi's letter, this association was rather poor on the individual level. It was only on the aggregate group level that annoyance and pollution did correlate, while reported annoyance was determined by a range of individual factors, including sex and health status. Objective measures of PM were only available on the community level. As shown in the online supplement to our article and by Wu and coworkers (Reference 1 of our article), fire smoke concentrations can strongly differ between locations even within the same community. These contrasts in exposure cannot be captured with a single monitor measuring PM10; thus, the objective measurements of pollution come with inherent limitations. Moreover, the mean concentrations across the 5 days with the maximum PM10 would not be expected to correlate with duration of smoke exposure as captured in the questionnaire. Despite the conceptual differences in the two main exposure metrics, namely, reported duration of fire smoke smell and ambient PM10 at the fixed site monitor, associations between symptoms and the two exposure terms were in most, but not all, cases (i.e., not for asthma symptoms or doctor's visits) rather comparable. While one may thus infer that asthma symptoms and doctor's visits are more strongly related to the duration of high smoke exposure rather than the absolute level, we believe that this would be an overinterpretation of our data where both exposure metrics had their strength and limitations. While a study can never prove an hypothesis, the null hypothesis of no association between wildfire smoke and a range of symptoms in children is refuted by our results. We have not yet answered the question about major susceptibility factors for these strong effects but will do so in future analyses.
Keck School of Medicine, University of Southern California, Los Angeles, California and ICREA and Institut Municipal de Investigacio Medica, Barcelona, Spain
Keck School of Medicine, University of Southern California, Los Angeles, California FOOTNOTES Conflict of Interest Statement: None of the authors has a financial relationship with a commercial entity that has an interest in the subject of this manuscript. REFERENCES
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