Exposure assessment is the process of measuring the magnitude, frequency, and duration of exposure to a chemical, 11 and occupational health studies typically have excellent assessments of exposure due to the defined geospatial boundaries (ie, the workplace) and the use of predefined exposure definitions by job title. We contend that the relationship between watershed geography and contaminant distribution is critical for certain classes of chemical contaminants, and this article illustrates a methodology for investigating that relationship.įrom an epidemiologic perspective, exposure assessment is more complicated when dealing with environmental health studies than it is in occupational health studies. The central hypothesis of this article is that there is an advantage in conducting geospatial analysis relative to adverse health outcomes using watersheds, rather than anthropogenic census tracts, particularly with respect to agrichemical runoff. Furthermore, when it comes to waterborne contaminant exposure, two individuals who live miles apart, but within the same watershed, may experience similar exposures, whereas two individuals who live close to each other, but in different watersheds, may experience very different exposure profiles.
8 9– 10 States or other geographic regions can easily be divided into specific watersheds, as everyone live within one watershed or another. These waters ultimately flow downstream within well-defined watersheds.Ī watershed is a topographic area within which surface and shallow groundwater drain to a specific point. 8 Rather, these contaminants become mobile when rainstorms induce surface runoff that transports the chemicals from land and deposits them into local waterways. Although this may be appropriate for some environmental exposures, it may not be at all appropriate for waterborne agrichemicals, steroids, and antibiotics because the pathways by which these contaminants travel do not respect anthropogenic geospatial boundaries.
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7 In this case, the chemical source can be agricultural fields, and efforts to map the geospatial organization of the exposure must invariably involve mapping large areas upstream from the communities whose water is affected.Įfforts to map the geospatial distribution of diseases are often conducted by first compartmentalizing the relevant geography into established geographical census units, such as census blocks and block groups, census tracts, zip codes, counties, or states. 6 Pesticides in particular have been detected in human breast milk, which has led to concerns about prenatal exposure and various health effects in children. 3 4– 5 Although the concentrations found are within set safe limits, the true health risk at these levels is not well understood and could be subjected to synergistic effects. Agrichemical residues have been found in food, water, and juices. 2 The agrichemicals can include pharmaceuticals, such as steroids or antibiotics used on livestock, nutrients, such as nitrates and phosphates, as well as herbicides and insecticides. One such example would be the development of cancers or birth defects when individuals are exposed to water contaminated with agrichemicals. Unlike the relationship between lead in drinking water and elevated blood lead levels, there are other examples where the water contamination is diffuse and the latency period for adverse health effects may be years or decades.
1 The geospatial distribution of lead exposure in Flint is relatively easy to map as the source of the exposure (the drinking water) is easy to identify, and the latency period between the initial exposure and elevated blood lead levels is short. In that community, lead contamination is a posttreatment issue, as the corrosive water from the Pontiac River solubilized lead from the distribution system that delivers water to individual households.
A poignant example of this is the contamination of Flint, Michigan, drinking water with lead and the subsequent elevated blood levels in children.
Contaminants in water can cause adverse health effects.
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