7.1 Methods in environmental epidemiology

Before assessing the role of GIS in modelling environmental exposure, it is important to understand the methods used to quantify environmental hazards. In such studies, the GIS analyst is typically one member of a multi-disciplinary team and needs to understand epidemiological study design to work effectively with colleagues.

There are two groups of methods by which environmental hazards may be identified: laboratory-based methods and epidemiological methods. GIS is used only in epidemiological studies. Laboratory-based methods include toxicological studies, which examine the response of animals to varying doses of hazardous substances. However, in determining what constitutes a critical dose, it can be difficult to extrapolate results from animals to humans. Controlled clinical trials of human subjects have also taken place, but these are necessarily limited to toxicants that produce only short-term health effects and investigate low levels of exposure only. Aside from these methodological problems, both types of study are ethically controversial.

Epidemiological studies in contrast examine the human response to ‘real world’ exposure to environmental hazards and it is these studies where GIS is widely used. Three types of epidemiological study design are commonly used: cross-sectional, case-control and cohort. Cross-sectional studies take place at a single point in time and measure environmental hazards and their possible health effects simultaneously. Such studies examine the strength of a relationship between exposure (e.g. low-level ozone from traffic) and health outcomes (e.g. asthma and respiratory disease) at this one point in time. Cross-sectional studies focus on either the whole population at risk or a representative sample drawn from it.

 
Cohort studies also recruit a representative sample of the population, but the exposure status and health of these individuals is assessed at several points in time. Exposure status of new disease cases can then be compared to exposure status of those who remain disease free. Case-control studies are especially useful for rare diseases. A case-control study focuses on two groups of individuals: cases, who have been diagnosed with the disease being studied; and controls, who are similar individuals without the disease. Commonly, a case-control study recruits equal numbers of cases and controls so as to maximise the chances of distinguishing between the two groups.

GIS is used with all three types of study as a means of identifying levels of exposure. Typically, this involves geo-coding residential addresses and then estimating exposure at these geo-coded locations from an environmental hazard map.

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