Academic journal article Environmental Health Perspectives

Exposure Assessment in Cohort Studies of Childhood Asthma

Academic journal article Environmental Health Perspectives

Exposure Assessment in Cohort Studies of Childhood Asthma

Article excerpt

BACKGROUND: The environment is suspected to play an important role in the development of childhood asthma. Cohort studies are a powerful observational design for studying exposure-response relationships, but their power depends in part upon the accuracy of the exposure assessment.

OBJECTIVE: The purpose of this paper is to summarize and discuss issues that make accurate exposure assessment a challenge and to suggest strategies for improving exposure assessment in longitudinal cohort studies of childhood asthma and allergies.

DATA SYNTHESIS: Exposures of interest need to be prioritized, because a single study cannot measure ail potentially relevant exposures. Hypotheses need to be based on proposed mechanisms, critical time windows for effects, prior knowledge of physical, physiologic, and immunologic development, as well as genetic pathways potentially influenced by the exposures. Modifiable exposures are most important from me public health perspective. Given the interest in evaluating gene--environment interactions, large cohort sizes are required, and planning for data pooling across independent studies is critical. Collection of additional samples, possibly through subject participation, will permit secondary analyses. Models combining air quality, environmental, and dose data provide exposure estimates across large cohorts but can still be improved.

CONCLUSIONS: Exposure is best characterized through a combination of information sources. Improving exposure assessment is critical for reducing measurement error and increasing power, which increase confidence in characterization of children at risk, leading to improved health outcomes.

KEY WORDS: childhood asthma, cohort studies, exposure assessment. Environ Health Perspect 119:591-597(2011). doi:10.1289/ehp.1002267 [Online 16 November 2010]

There is a growing interest in advancing our understanding of environmental exposures that influence the development of disease. Asthma is a complex and heterogeneous syndrome, with a variable phenotype characterized by chronic airway inflammation, reversible airflow limitation, airway hyperreactivity, and excess mucous secretion (Bosse and Hudson 2007; von Mutius 2008). Multiple genes have been associated with the development of asthma (Ober and Hoffjan 2006), as have numerous exposures (Miller and Ho 2008) [see Supplemental Material, Table 1 (doi:10.1289/ehp.1002267)].

The environment clearly influences the development of allergies and asthma, because having a particular gene or combination of genes does not guarantee the development of these conditions. Occupational asthma likewise points toward the role of environment in disease development (Bernstein et al. 2006), as do the changes in asthma prevalence in the past 30-40 years (Platts-Mills et al. 2005; von Mutius 1998).

Gene and environment interactions are also suspected in asthma and allergies (Hunter 2005), as are epigenetic mechanisms (Baccarelli et al. 2009; Nawrot and Adcock 2009). However, knowledge about how environmental exposures participate in epigenetic mechanisms is in its infancy (Baccarelli et al. 2009; Miller and Ho 2008; Nawrot and Adcock 2009).

Longitudinal cohort studies are the most powerful observational study design for studying exposure--response relationships pertaining to disease development. Because of the relatively long observation period for cohort studies, it is also possible to undertake intervention studies to investigate selective avoidance and disease outcomes (Clayton and McKeigue 2001). Examples of this approach can be found in the Prevention and Incidence of Asthma and Mite Allergy study (PIAMA) (Brunekreef et al. 2002) as well as the Canadian Allergy Primary Prevention Study (CAPPS) (Becker et al. 2004).

Cohort studies have drawbacks: They are expensive and time consuming, and investigators must carefully plan the exposure assessment component (Rothman and Greenland 1998). …

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