Academic journal article Environmental Health Perspectives

A Brief Targeted Review of Susceptibility Factors, Environmental Exposures, Asthma Incidence, and Recommendations for Future Asthma Incidence Research

Academic journal article Environmental Health Perspectives

A Brief Targeted Review of Susceptibility Factors, Environmental Exposures, Asthma Incidence, and Recommendations for Future Asthma Incidence Research

Article excerpt

Relative to research on effects of environmental exposures on exacerbation of existing asthma, little research on incident asthma and environmental exposures has been conducted. However, this research is needed to better devise strategies for the prevention of asthma. The U.S. Environmental Protection Agency (EPA) and National Institute of Environmental Health Sciences held a conference in October 2004 to collaboratively discuss a future research agenda in this area. The first three articles in this mini-monograph summarize the discussion on potential putative environmental exposure; they include an overview of asthma and conclusions of the workshop participants with respect to public health actions that could currently be applied to the problem and research needs to better understand and control the induction and incidence of asthma, the potential role of indoor/outdoor air pollutants in the induction of asthma), and biologics in the induction of asthma. Susceptibility is a key concept in the U.S. EPA "Asthma Research Strategy" document and is associated with the U.S. EPA framework of protecting vulnerable populations from potentially harmful environmental exposures. Genetics, age, and lifestyle (obesity, diet) are major susceptibility factors in the induction of asthma and can interact with environmental exposures either synergistically or antagonistically. Therefore, in this fourth and last article we consider a number of "susceptibility factors" that potentially influence the asthmatic response to environmental exposures and propose a framework for developing research hypotheses regarding the effects of environmental exposures on asthma incidence and induction. Key words: asthma, epidemiology, genetics, hygiene hypothesis, incidence, obesity, occupational asthma, smoking, susceptibility, windows of exposure and age (in utero, childhood, adult, elderly). doi:10.1289/ehp.8381 available via[Online 26 January 2006]


The documented increase in asthma prevalence over the last 25 years (Mannino et al. 2002) is likely due to changes in our environment or lifestyle because changes in our genetic makeup would take more than several generations to occur. When investigating environmental exposures [e.g., outdoor, environmental tobacco smoke, pollen, viruses (Gilmour et al. 2006; Zeldin et al. 2006)] in relation to asthma incidence and induction, we need to consider "susceptibility" factors in order to ascertain the relative contribution of an environmental effect and potential interactions. In this article, we define "susceptibility factors" broadly to include populations at risk (e.g., the very young, elderly, or genetically at risk), known risk factors, and known protective factors ("farm" exposure in utero or in infancy). From an epidemiologic framework, we have included variables that are sometimes considered confounders or effect modifiers, such as smoking or obesity (Figure 1).


Most epidemiologic research on asthma and environmental risk factors has focused on prevalence because prevalent asthma is easier to measure than incident asthma. Prevalence (P) is the product of the incidence (I) of the disease times the duration (D): P = I x D. Compared with prevalence, much less is known about factors associated with incident asthma. Thus, we include in this article factors associated with prevalent asthma, acknowledging that factors that influence the prevalence of the disease may not necessarily affect the incidence of the disease in the same way and that additional factors may affect the remission of the disease.

The following "susceptibility factors," discussed at the U.S. Environmental Protection Agency (EPA) workshop "Environmental Influences on the Induction and Incidence of Asthma" held 19-20 October 2004, are considered in this article by experts in their fields: genetics, "window of exposure" (age: fetus, infant, childhood, adult, elderly), occupational asthma (OA), and lifestyle factors (diet, obesity). …

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