Pesticide exposure and poor pregnancy outcomes: weaknesses of the evidence

Abstract

recently suggested that prenatal exposure to pes-ticides is a risk factor for poor pregnancy outcomes such as pre-term births and lower Apgar scores. The authors used an ecological (correlational) study to investigate whether there was an association be-tween exposure to pesticides (in general) and poor pregnancy outcomes in micro-regions of states in the South Region of Brazil. Making use of group-lev-el variables and routinely collected data, ecological studies have the advantage of being relatively inex-pensive and quick and easy to conduct. Nonetheless, owing to their inherent methodological limitations, correlational studies are generally regarded as one of the weakest approaches to analytic epidemiol-ogy. Ecological studies are even weaker strategies for testing associations between “exposure” and health outcomes, when group-level measures are used as surrogates for individual measures. The study by Cremonese et al. is a good example of this type of ap-proach. The exposure status (exposure to pesticides in individual pregnant women) was estimated based on the group average (“per capita pesticide expen-diture in the micro-region” referred to as per capita pesticide consumption) i.e., the group-level variable served as a proxy for individual values. Besides be-ing imprecise, using group average data to represent individual exposure may lead to the erroneous infer-ence (“ecologic fallacy”) that specific individuals in the local group share the characteristics of the group. Groups are far from being homogeneous in terms of pesticide exposure and a subgroup may markedly dif-fer from the group mean.In addition to the common drawbacks of an eco-logical epidemiology approach, important flaws re-garding hypothesis formulation and exposure assess-ment can be observed in the aforementioned study.The authors’ hypothesis was that exposure to pes-ticides (in general and not to a particular substance or group of related compounds) is associated with adverse pregnancy outcomes. Pesticides are a highly diverse range of substances with quite different toxi-cological and physical-chemical properties and ac-tions on pests (insecticides, fungicides, herbicides, miticides or acaricides, nematicides, bactericides and so on) and non-target species. It seems, therefore, un-reasonable to estimate collective exposure to these substances and investigate the association of this “general pesticide exposure” with a particular adverse health outcome without considering the dissimi-larities. Per capita pesticide expenditure calculated from local sales (in the local currency, Real R$) is a rather strange and unreliable, not to say meaningless, estimate of exposure. The authors make an uncriti-cal assumption without demonstrating its validity. It should be borne in mind that the active ingredients used in pesticides (insecticides, fungicides and so on) and their sale price and volume of use (and also how they are used) depend on the crops (soybean, rice, beans, tobacco, maize, vegetables and so on) and pre-vailing agricultural practices in the micro-region. It is therefore likely that micro-regions are heterogeneous in terms of the type of pesticide used. For instance, in regions where soybean is a major crop it is likely that expenditure on pesticides will relate predominately to herbicides, whereas in regions with greater empha-sis on vegetable crops insecticide and fungicide use is likely to be more dominant. Calculations of consump-tion based on local per capita pesticide expenditure are a quantitative estimate of qualitatively different types of pesticides. In other words, micro-regions may differ not only regarding the amount of pesticide use but also due to the type of active ingredients used (i.e., micro-regions may differ in terms of degree of exposure and type of exposure). The investigation of an association between exposure to different types of uncharacterized pesticides and the occurrence of ad-verse health outcomes seems meaningless.In their conclusions, the authors are cautious and suggest that further prospective studies should be un-dertaken involving individuals as analytical units rath-er than populations to better evaluate their findings.Ecological studies on adverse health effects of environmental exposure are increasingly common in the literature. Essentially negative studies are rarely found in the literature and most published ecological studies generally report findings suggestive of a posi-tive correlation. A possible explanation for this appar-ent publication bias is the perception that although ecological studies are methodologically weak in dem-