The increase in the world population in recent decades has been accompanied by a greater demand for food of animal origin. To meet this demand, intensive farms have emerged. As a consequence of this intensive production, large amounts of animal waste are produced impacting the soil, water, and air near the farms. Along with the increase in the number of intensive farms in several countries, known as Animal Feeding Operations (AFOs), there is also greater concern in the public opinion about the potential negative effects that these farms can have on the health of the people who live nearby AFO areas. In order to obtain evidence on the effects of AFOs on the health of people who live nearby, several studies have been conducted, however the findings are mixed, with studies describing negative effects and others not finding significant associations. The studies developed have been observational in nature making the results susceptible to various sources of systematic error. In addition to the bias present in the primary studies, it is not entirely clear whether the studies developed, despite being classified as cross-sectional or case-control, are capable of providing estimates of the incidence of health events, which is necessary to elucidate causality.The first study identified prevalence studies as part of a systematic review conducted to evaluate in each exposure-outcome pair reported the assumptions needed to provide estimates of comparative incidence. We identified that primary studies have not been discussing epidemiological assumptions necessary to interpret the measure of effect as estimates of comparative incidence. Similarly, we identified that a large percentage of exposure-outcome effect sizes might be interpreted as providing estimates of comparative incidence. The second study identified case-control studies as part of a systematic review to evaluate if the authors discussed the assumptions about the underlying population, the apparent nature of the cases (incident or prevalent), and the methods for sampling cases and controls in order to interpretate the effect size measure reported. We identify that authors have not been discussing the assumptions necessary to interpret the measure of effect as incidence. Similarly, we identity that a large percentage of exposure-outcome effect sizes might be interpreted as providing estimates of incidence. The third study used references that provide estimates of comparative incidence to understand what effect authors reported being of interest and the rationale for the selection and retention of potential confounding variables. Likewise, we used manuscripts where the authors reported a lower-respiratory disease outcome to conduct an analysis based on DAGs on what effect sizes may have been estimated (direct or total causal effect), remaining biasing pathways and sources of bias that might exist associated with control for confounding. We identified that none of the authors reported if they intended to estimate the total or direct effect. Only two studies included the rationale for the set of variables selected as confounders and the rationale for retention as confounders. No paper provided a DAG or causal pathway that supported the adjustment set included in the models. Among the studies addressing lower respiratory tract conditions, no study could estimate either the direct effect or the total effect of residential exposure to AFOs. The final study of this dissertation used references that provide estimates of comparative incidence to evaluate the characteristics of measurement of the exposure and outcomes and the authors approach to discussing consistency and measurement error. This study identifies those measurements of exposure based on AFOs density, measurement of direct emissions, distance from home to AFOs, dispersion models and perceived odor in the home were the measures used by the authors. Outcomes categorized as lower respiratory conditions and gastrointestinal conditions were the most investigated and its main source of information were medical records, questionaries, and mortality records. None of the measures of exposure captured an individual exposure to a metric of AFOs exposure such as personal exposure to ammonia levels. Authors did not discuss the consistency assumption.
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