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Air Pollution: Commentary

Advancing Industrial Livestock Production

Health Effects Research and Sustainability

Merchant, James A.

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doi: 10.1097/EDE.0b013e318209d3a4
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In this issue of Epidemiology, Schinasi et al1 make an important addition to the growing literature regarding adverse health effects of community exposures to emissions from animal feeding operations. Several controlled epidemiologic studies2–5 have reported increased respiratory and other symptoms among residents living close to animal feeding operations, but this is the first study to measure symptomatic and functional responses to specific, well-recognized environmental exposures arising from the industrial production of swine. Even with a relatively small study population of 101, qualitative but repeated measures of odor, H2S (a characteristic emission that comes from animal feeding operations and no other common rural source), PM10, PM2.5, and endotoxin were related to eye and nasal irritation, respiratory symptoms, difficulty breathing, wheezing, chest tightness, nausea, and declines in forced expiratory volume (FEV1). These findings are highly consistent with well-documented adverse health effects among those who were occupationally exposed to animal feeding operations,6 as well as with the community-based studies cited above.

This research team from the University of North Carolina joined with the Concerned Citizens of Tillery to conduct a community-driven, participatory, and longitudinal study. Their methodological approach is worthy of replication for a number of reasons. The authors have previously documented the reliability and objectivity of this research project in a population of predominantly black, eastern North Carolina residents in 16 communities and living within 2 miles of 1–16 animal feeding operations.7 Ironically, the poverty and lack of political capital that ties environmentally exposed populations such as these to their homes and communities not only motivates the exposed population to participate in research, but also limits the outward migration of those with exposure-associated health effects (such as asthma), which has been observed in more prosperous rural communities with animal feeding operations.8 This tends to reduce the selection bias typically observed in populations exposed to airway irritants.

The longitudinal study design, with repeated (hundreds) of well-selected measures of biologically relevant acute responses over a 2-week period, provided robust health outcome data. A common misconception is that clinical diseases are the only valid health outcomes, but clinical endpoints such as doctor-diagnosed asthma or chronic changes in lung function are late manifestations of environmental exposures. Furthermore, these endpoints are highly susceptible to selection bias in dose-response studies of airway irritants. It is instructive to note that the US Supreme Court9 upheld the Department of Labor Cotton Dust Standard which concluded that “grade 1/2 byssinosis (occasional Monday chest tightness) and associated pulmonary function decrements are significant health effects in themselves and should be prevented in so far as possible.” Episodic dose-related respiratory symptoms and declines in expiratory flow are clearly valid measures in setting public policy.

Schinasi et al1 also collected thousands of measures of exposures relevant to animal feeding operations, paired with repeated measures of acute responses. This approach overcame some study limitations. One was the study's focus on a single exposure site in each community, which would be expected to misclassify individual home exposures but would tend to bias exposure estimates toward the null. Another limitation addressed by this design is the even smaller number of available lung-function measurements (limited by PM and endotoxin measurements in 12 communities); nevertheless, declines in FEV1 were found to be associated with unit increases in PM2.5. A third exposure-related limitation was the presence of the study's highly visible air-monitoring equipment. The presence of such equipment may well have limited usual waste-management practices (such as spraying of hog waste) during the study period, thereby limiting higher exposures that would likely cause more prevalent acute responses. Nonetheless, substantially significant health effects were observed.

Finally, unlike occupationally exposed populations (who are relatively healthy), community residents typically include more susceptible members of society—children, the elderly, and those with chronic diseases. So as to not bias their study with community residents who may be more susceptible to the effects of swine emissions, the authors chose to study only adult nonsmokers who had a low prevalence of prior respiratory diseases and few exposures to passive smoking. In sum, these investigators avoided common pitfalls of many community-based environmental health studies. In doing so, they have provided both important new dose-response data and a very useful methodological approach to the study of community residents exposed to livestock and other environmental air emissions.

This study has important policy implications. It is not necessarily generalizable to all communities exposed to swine emissions, but it is a broadly-based study of 16 communities in a region of North Carolina that is densely populated with animal feeding operations. These dose-response estimates are therefore a solid starting point for models of exposures and health outcomes, using air-dispersion models now being developed for industrial-livestock exposures. It will be necessary to validate such models with paired health outcome and exposures from various animal feeding operations before more generalizable exposure-based conclusions can be drawn.

While the paper by Schinasi et al1 focuses only on reports of odor, irritant-related symptoms, and changes in lung function, the environmental and societal effects of industrial livestock production are much broader. Streams and estuaries are polluted by pathogenic organisms, pharmaceuticals, and hormones; antimicrobial resistance is promoted by the use of antibiotics; industrial livestock production contributes to greenhouse gases, (especially methane) and to regional increases in ammonia-related PM2.5; animal abuse occurs through a variety of confinement, transportation, and slaughtering practices; and industrial livestock production degrades rural social capital, quality of life, and property values.10–16

Industrial livestock production is a “successful” economic model sustained by global markets and US laws and regulations, and with few constraints. The economic models of industrial livestock production externalize most societal costs while emphasizing the undeniable importance of this industry to farm-state economies.15

What then is the way forward? Put simply, societal costs must be more explicitly documented. The health research community has many tools to quantify and attribute exposures arising from industrial livestock production: increasingly sensitive and specific measures of environmental chemicals, biomarkers specific to animal feeding operations, tools for genotyping microbes, air dispersion, and GIS modeling. Assessment of health outcomes related to animal feeding operations, such as asthma and nontypable methicillin-resistant Staphylococcus aureus (MRSA), can provide more complete estimates of the burden of diseases with use of GIS technology and clinical and health-utilization databases. Further modeling is needed to measure and provide attribution estimates of livestock methane emissions and regional increases in PM2.5.16,17 Robust economic studies are needed to more fully assess the impact of industrial livestock production on independent farmers and rural communities. Multidisciplinary research teams, present in many university settings, are well-qualified to address these interlocking issues, but they will need the support of federal agencies charged with rural development, environmental health research, and protection of the public's health.

The industrial livestock industry enjoys both economic and political advantages that currently contribute to its global growth. There are, nonetheless, several trends that challenge its sustainability. These include climate change, the depletion of our reserves of inexpensive energy, and rapidly decreasing fresh water reserves.14 To these trends may be added better informed and more skeptical consumers of their products—which, when contaminated by antibiotics or infectious agents, can have a marked and lasting impact on the economies of livestock-producing states. Given the rate of change in these trends and the growing body of industry-related health effects research, the sustainability of the current industrial model of livestock production is likely to be measured in decades rather than generations.

Although a good deal more research is needed to advance the national policy agenda regarding livestock production and its global societal costs, there are established emission-reducing strategies, industry guidelines, and state and federal regulations currently available to livestock producers.14,17,18 There is no reason for large livestock producers—including their contract growers—to delay in doing all they can to prevent and reduce environmental and infectious exposures. Such an approach would both advance industry-wide sustainability and protect the health of their employees, their communities, and their customers.


JAMES A. MERCHANT is a Professor in the University of Iowa Department of Occupational and Environmental Health in the College of Public Health and the Department of Pulmonary, Critical Care and Occupational Medicine in the College of Medicine. He previously directed the Institute for Rural and Environmental Health and served as the Founding Dean of the College of Public Health. His research interests include the epidemiology of occupational lung disease and rural health outcomes and policy.


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