ResearchAn integrated model of environmental factors in adult asthma lung function and disease severity : a cross-sectional study

Background: Diverse environmental exposures, studied separately, have been linked to health outcomes in adult asthma, but integrated multi-factorial effects have not been modeled. We sought to evaluate the contribution of combined social and physical environmental exposures to adult asthma lung function and disease severity. Methods: Data on 176 subjects with asthma and/or rhinitis were collected via telephone interviews for sociodemographic factors and asthma severity (scored on a 0-28 point range). Dust, indoor air quality, antigen-specific IgE antibodies, and lung function (percent predicted FEV1) were assessed through home visits. Neighborhood socioeconomic status, proximity to traffic, land use, and ambient air quality data were linked to the individual-level data via residential geocoding. Multiple linear regression separately tested the explanatory power of five groups of environmental factors for the outcomes, percent predicted FEV1 and asthma severity. Final models retained all variables statistically associated (p < 0.20) with each of the two outcomes. Results: Mean FEV1 was 85.0 ± 18.6%; mean asthma severity score was 6.9 ± 5.6. Of 29 variables screened, 13 were retained in the final model of FEV1 (R2 = 0.30; p < 0.001) and 15 for severity (R2 = 0.16; p < 0.001), including factors from each of the five groups. Adding FEV1 as an independent variable to the severity model further increased its explanatory power (R2 = 0.25). Conclusions: Multivariate models covering a range of individual and environmental factors explained nearly a third of FEV1 variability and, taking into account lung function, one quarter of variability in asthma severity. These data support an integrated approach to modeling adult asthma outcomes, including both the physical and the social environment. Background The potential relationship of environmental factors to morbidity in asthma is highly complex and difficult to study. Environmental risk factors for developing asthma (such as ambient pollution, antigens, and indoor air quality) have been given considerable research attention, particularly in the onset of childhood disease. In contrast, the role of the environment in asthma severity and disease-related quality of life for those who have established asthma has been less well studied, particularly among adults. As importantly, studies of environmental factors in relation to asthma morbidity frequently focus on only a subset of exposures. Certain studies of indoor environmental exposures have emphasized specific allergens, such as from house dust mites, pets, cockroach, and rodents [1-5] or inflammatory agents such as endotoxin or glucans [6]. Others have focused on other indoor factors, such as secondhand smoke [7,8], dampness [9], or combustion heating and cooking sources [10]. Outdoor physical environmental factors have also been studied in relation to asthma severity or morbidity, including ambient air pollution (both fine and coarse fraction particulate matter, NO2, and ozone), or, on the other hand, residential proximity to roadways and traffic density, although * Correspondence: laura.trupin@ucsf.edu 1 Department of Medicine, University of California San Francisco, San Francisco, CA USA Full list of author information is available at the end of the article © 2010 Trupin et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Trupin et al. Environmental Health 2010, 9:24 http://www.ehjournal. net/content/9/1/24 Page 2 of 14 typically both subsets of exposures are not included in the same analysis [11-18]. Several comprehensive literature reviews have summarized the evidence for these environmental factors, but cannot address the interplay among the various kinds of exposures because of the limitations of the analyses noted above [19-21]. Beyond these topics, occupational factors in relation to asthma, such as workplace exposure to vapors, gas, dust, or fumes (VGDF), represent an entirely separate avenue of investigation [2226]. Finally, social environmental factors, in particular socioeconomic status (SES) at both the individual and neighborhood level [27-30] have been implicated in asthma severity, but have been analyzed almost entirely as exposures distinct from physical environmental factors. Thus, while the body of environmental research on this subject has demonstrated that asthma indeed can be affected by multiple factors, integrated approaches have not been used to simultaneously examine the combined effects of a range of physical and social environmental exposures acting together. The present analysis takes advantage of a well-characterized cohort of adults with asthma over a spectrum of disease severity for whom a broad range of physical and social environmental exposures have been assessed. We hypothesized that exposure-response modeling, taking into account an integrated and wide-ranging group of environmental factors, would demonstrate substantial explanatory power for lung function and asthma severity associated with these variables. Moreover, we anticipated that, combined together, these effects would be substantially greater than those manifested by any isolated subset of environmental factors. Underlying these hypotheses is a conceptual model in which heterogeneous environmental exposures can act individually, collectively in clustered groups and, potentially, interactively to affect asthma-related outcomes. Methods Overview We incorporated data on a wide array of potential environmental risk factors for adverse outcomes collected in an ongoing cohort study of adults with asthma and/or chronic rhinitis. Data derive from three sources: structured telephone interviews, home visits with direct sampling of key measures, and secondary sources of data on air quality, land use, traffic, and the socioeconomic status of the immediate neighborhood linked via geocoding to each subject's residential address. The two principal outcomes were the percent predicted forced expiratory volume in one second (FEV1) and asthma severity, defined using a validated scoring metric that correlates with, but is independent of, lung function. In addition, asthma-specific quality of life was included as a secondary outcome. In order to examine a wide range of potential exposures while minimizing the number of items tested, we prioritized for inclusion candidate risk factors based on previous analyses of this cohort. We also divided the exposures into categories that provided a hierarchical structure for an initial screening step executed prior to final model specification. These five categories are: sociodemographic and behavioral characteristics; household biologics, including selected antigens (and subject antibody status); indoor air quality; metal concentrations in house dust; and the ambient physical and surrounding socioeconomic environments. The study was approved by the University of California, San Francisco Committee on Human Research; all subjects provided informed consent prior to participation. Subject Recruitment and Follow-up Subjects with asthma were recruited initially through a random sample of pulmonary and allergy specialty practices in northern California beginning in 1992, followed by an additional sample of family medicine specialty practices in 1996. Eligibility was based on clinical diagnostic criteria consistent with American Thoracic Society and American College of Chest Physician definitions. Patients with concomitant diagnoses of chronic bronchitis or emphysema were ineligible; there were no other exclusions based on disease factors such as duration, age of onset, or presence or absence of atopy. In 1999, a third group of subjects was recruited by random digit dialing in the community at large within the same northern California catchment area. For this sample, eligibility was based on respondent report of a physician's diagnosis of asthma, or (to ascertain rhinitis) a physician's diagnosis of allergic rhinitis, chronic sinusitis, hay fever, or chronic post nasal drip. Respondents reporting a diagnosis of COPD or emphysema were ineligible. Subjects in all three samples were between age 18 and 50 at enrollment. Interviews were conducted by telephone at enrollment and at 18 to 24 month intervals thereafter. Since the last recruitment, an average of 74% of eligible participants was reinterviewed in each wave of multiple follow-up interviews. Details about the enrollment and retention of cohort members have been reported previously [22,3133]. Sources of Data The present analysis includes data from the telephone interviews and home visits conducted in 2002-2003. The interviews include information on asthma and rhinitis symptoms, medication use, measures of health status and asthma-specific quality of life, smoking status, sociodemographic characteristics, and employment. Details of the home visit have been previously reported [33]. In brief, a study nurse and a research assistant conducted a Trupin et al. Environmental Health 2010, 9:24 http://www.ehjournal. net/content/9/1/24 Page 3 of 14 comprehensive survey of the home environment, performed spirometry testing on each subject, and collected additional medical and psychosocial data. Environmental data gathered during the home visit included measures of indoor air quality and dust samples from the bedroom and living room. A blood sample was collected for antibody testing to selected allergens. Spirometry was performed with the EasyOneTM (ndd Medical Technologies, Chelmsford, MA) which meets ATS 1994 diagnostic standards for spirometry measurements [34]. Spirometry measurements, including FEV1, were obtained using a standard protocol conforming to ATS guidelines [35]. In addition to the primary data collection, th