Our goal in this study was to assess whether the WTC plume was associated with increased respiratory symptoms among residents of New York City living beyond Lower Manhattan. This study is the first we know of to have assessed the possible respiratory effects of the plume beyond Lower Manhattan and to have used a continuous measure of relative plume intensity, rather than binary residential location information, to infer exposures. Within the limitations of our data, we saw no associations of plume intensity with respiratory symptoms beyond Lower Manhattan.
Among residents who reported no previous diagnosis of asthma, there were no statistically significant associations between estimated plume intensities and new-onset or persistent cough/wheeze, using a plume intensity cutpoint of the 75th percentile () or higher or lower cutpoints of the 50th, 60th, 70th, 80th, or 90th and 95th percentiles (see and ). Among residents who reported a previous diagnosis of asthma, there was no statistically significant association between estimated plume intensities and self-reported worsening of asthma symptoms or nonroutine medical care for asthma. There were too few asthmatics in the sample to conduct additional reliable sensitivity analyses with higher exposure cutpoints.
Previous studies using binary exposure metrics based principally on residential location found associations between living near Ground Zero and higher risk of respiratory symptoms. In 2 studies (12
), investigators compared exposed subjects who resided in apartment buildings within 1.5 km of Ground Zero with unexposed subjects living in apartments at least 9 km from Ground Zero and north of the site. Among previously asymptomatic persons, Reibman et al. (13
) found consistent and statistically significant elevations in levels of respiratory symptoms, with greater persistence, and increased use of respiratory medications among persons living in the exposed areas as compared with those living in unexposed areas, but no differences in spirometry test results. For example, they reported elevated crude incidence ratios for a number of new-onset respiratory symptoms among previously asymptomatic residents, including “any cough without cold” (incidence ratio
3.36, 95% CI: 2.38, 4.74) and wheeze (incidence ratio
4.32, 95% CI: 2.68, 6.98). Among these subjects, Lin et al. (12
) found elevated levels of new-onset upper respiratory symptoms, unplanned medical visits for respiratory problems, and respiratory medication use in exposed persons (12
). Limitations of these studies included response rates less than 25%, the use of geographic location as the determinant of exposure, and possible reporting bias.
To assess the consistency of our results with those from previous studies, we compared responses among residents of Manhattan living at or below 14th Street (about 3 km north of Ground Zero), which includes persons living in the area engulfed by the more intense event-related cloud of dust and debris created by the WTC collapse, with responses among persons living above 14th Street (). We found that among nonasthmatics, residents living at or below 14th Street had statistically significantly increased crude and adjusted odds of reporting new-onset cough/wheeze and persistent new-onset cough/wheeze compared with residents living above 14th Street, which is similar to results reported in the other studies. Our smaller risk estimate may be due to smaller differences in exposure between Manhattan populations living above and below 14th Street, as compared with differences in exposure between the exposed and comparison locations studied by Reibman et al. (13
). This is supported by our additional sensitivity analyses, which showed statistically significant results in our Manhattan-only analyses only using higher exposure cutpoints (90th and 95th percentiles).
In contrast to other studies of asthmatics (10
), among asthmatics we found no statistically significant increase in self-reported worsening or nonroutine asthma care when comparing residents of Lower and Upper Manhattan (). Because of the nature of the asthma severity survey question, which assessed post-9/11 asthma severity in the 4 weeks prior to the survey (several months after 9/11), we could not identify subjects who may have had acute exacerbations of asthma that were resolved prior to the question's time frame. Finally, asthmatics who were most affected by their exposure may have been underrepresented among participants because of the large numbers of Lower Manhattan residents who relocated and had not returned as of June 2002, when the survey was conducted (16
Strengths and limitations
A strength of our study is that we assigned relative plume intensities to individuals based on the results of sophisticated atmospheric dispersion modeling, which we believe was more accurate than assigning exposures based on residence in broad geographic areas or distance from Ground Zero. Observations and models indicated that the cumulative plume intensities were neither evenly distributed within geographic areas nor closely correlated with distance from the WTC. Furthermore, modeled plume intensities were less likely than proximity to the WTC to be associated with self-identification as exposed or not exposed, thereby reducing the likelihood of biased recall of symptoms after 9/11.
In general, analysis of health outcomes in communities exposed to air contaminants related to the WTC collapse has been hampered by the lack of “real-time” exposure data and limited outcome data. We based our estimates of relative ground-level plume intensities on the dispersion properties of a generic fine particulate aerosol. We calibrated these estimates using satellite images and actual air monitoring data collected at 4 stations close to the WTC. Despite these efforts, possible explanations for the lack of association with respiratory symptoms include misclassification of exposure. The relative estimates of exposure did not account for changes in chemical and physical characteristics of air contaminants, which occurred over time following the building collapse. We limited our analysis to the 5-day period immediately following the collapse, because modeled concentrations after that time period made a negligible contribution to cumulative exposure at any location. The location of survey participants was based on residential address. We did not have work location or any type of time-activity data with which to link the locations of estimated particulate matter concentrations with the locations of individual subjects over time. In addition, large numbers of residents in the vicinity of the WTC were displaced and therefore not captured by the survey. Some of these displaced residents, who may have had high initial exposures to the plume, were likely to be residing at other locations in the survey area at the time of the survey. Finally, cumulative exposure may be less biologically relevant than peak exposure for the respiratory outcomes in this study, and we did not evaluate sensitive subpopulations, including children with asthma and the elderly.
We found no strong associations between estimated 5-day cumulative residential WTC plume intensity beyond the immediate vicinity of the WTC and new-onset respiratory symptoms in nonasthmatics, or worsening asthma in asthmatics, on the basis of community survey data. The estimated residential plume intensities based on our dispersion modeling were the best available exposure estimates, but differential or nondifferential exposure misclassification might have biased the results towards our null findings. This study highlights the need for a rapid epidemiologic and environmental response capability to immediately follow up after other such events (terrorist-driven or not), to improve our understanding of the immediate and longer-term exposure hazards of these events and their possible health effects.