3.7 Exposure data
Environmental exposure entails human contact with potentially harmful chemical, physical, or biological agents in the environment. Exposure studies are therefore concerned with the contact between a human population and environmental hazards such as ionizing radiation, electro-magnetic fields, low-level ozone, or pathogenic bacteria in water. Here, we introduce some common difficulties in representing exposure within a GIS.
Natural and man-made exposure: Many of the pathogens or substances to which humans are exposed through the environment may originate from either natural or man-made sources. For example, the bacteria E. coli are sometimes associated with gastro-enteritis. E. coli may be introduced into the environment through sewage and wastewater, but also occur naturally in the faeces of wild animals. In a GIS context, it is often relatively straightforward to spatially represent man-made sources of pollution (e.g. sewage outlet pipes as points), but more difficult to represent naturally occurring sources. Disentangling the relative contributions to ill health of man-made and naturally occurring substances can therefore be complex.
Multiple exposure pathways: Humans are exposed to hazardous substances or pathogens in the environment through several different pathways. It may be difficult to represent all of these sources of exposure spatially within a GIS. For example, humans may ingest E. coli bacteria in drinking water, in food, or through dirty hands. Whilst a GIS may indicate those parts of a city served with microbiologically safe piped water, the exposure to E. coli in food or on hands may be practically impossible to represent spatially.
Exposure at many locations and times: Any individual’s exposure to a given pathogen or substance reflects their pattern of daily activity at multiple locations and not merely their exposure at home. Thus, whilst the type of water supply serving different areas of the city indicates exposure to E. coli in drinking water in the home, individuals are also exposed to E. coli in drinking water elsewhere. For example, children may attend schools that lack a proper water supply and farmhands with safe water supplies at home may work in fields irrigated with wastewater. For an acute disease such as gastro-enteritis, short-term mobility of the population at risk affects exposure. For chronic diseases such as cancer, long-term mobility and migration also affect exposure. In many GIS applications, individuals are represented spatially in terms of a single place of residence and so these additional exposures elsewhere may never be captured.
In practice then, GIS often measures exposure at one place, from a subset of the possible exposure pathways, and may ignore natural sources of an environmental hazard. Because GIS often measures only a proportion of the total exposure, it can be difficult to discern any exposure effects on health.
Activity: using GIS to investigate exposure to uranium
Scenario: A nuclear processing plant has been operating for some time in a rural area of eastern Europe. The local residents surrounding the plant mostly use small-scale village boreholes or family wells to extract groundwater for drinking. Following budgetary cutbacks to the state energy agency, concern has arisen about possible seepage of uranium from the plant’s outflow and holding tanks into groundwater.
You have been hired as a consultant by the local government authority to assess the risks to the local population. Because of the emotive nature of the study, your report is likely to be highly controversial, with the plant’s operators’ sensitivity to accusations of mis-management and the local population’s concerns about their health. As a result, the local authority has advised you to be cautious in analysing the data and drawing any conclusions from it.
Data: The local government authority have made available to you the locations of all homes in the area as a point file, the location of all domestic wells, and the location of the nuclear processing plant as point vector files. For a sample of around one third of domestic wells, you have also been provided with the results of chemical water quality tests, which give the amount of uranium in the water in micrograms per litre. These tests were conducted by the local authority over a six-month period, visiting each well once.
Your task: Post a message to the course web site, briefly describing how you would assess the risk of uranium exposure among the local population using the available data. In view of the sensitive nature of the study, what caveats would you put in your report about any proposed analysis that you might carry out? Post a message to the course web site, briefly describing how you would assess the risk of uranium exposure among the local population using the available data. In view of the sensitive nature of the study, what caveats would you put in your report about any proposed analysis that you might carry out?
References (Essential reading for this learning object indicated by *)
For an overview of the use of GIS in exposure studies, see:
Nuckos, J. R., Ward, M. H., and Jarup, L. (2004) Using Geographic Information Systems for Exposure Assessment in Environmental Epidemiology Studies. Environmental Health Perspectives 112 (9), 1007-1015. Available on line at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1247194/