ABSTRACT
Epidemiology is the scientific method of investigating correlations and ultimately causal relationships between human disease or conditions and a substance or agent or activity in the workplace, in the community, at home, or in food, medicines, or water. This chapter is focused on exposures to microbial agents, chemicals, and radioactivity in drinking water. Diseases of interest can range from acute effects, such as temporary gastrointestinal illness from bacterial toxoids, to acute and/or chronic disease stemming from acute high exposure, such as chronic reduction in kidney function from acute cadmium poisoning, to chronic disease stemming from long-term low-level exposures, such as bone cancer due to radium
in drinking water or birth defects associated with disinfection byproducts in drinking water. Epidemiological studies can be conducted by sampling subjects from the general population, specific locations with elevated exposures or occupational settings. Exposure status can be obtained by interview, survey form, job-related exposure assessment, or biomarker levels. Some biomarkers are short-lived, such as volatile organic chemicals, while others may be long-lived, such as organochlorines in adipose tissue or lead in bones (see Section 5.2.8, Estimating Exposure). Disease status or history can be obtained by interviews, survey forms, employment-related medical history, medical records from hospitals or private doctors, or vital records from public health agencies or from study-associated clinical assessments. Epidemiological data have been used by states, the United States Environmental Protection Agency (USEPA) or agencies in other countries as the partial or entire basis of risk assessments for a number of important drinking-water contaminants (Table 5.1). In contrast to laboratory animal toxicology studies, epidemiology provides information about actual human health responses to toxic substances and agents. However, while laboratory experiments can be designed to expose animals to only one substance/agent and compare them to animals equivalent in every other way, humans are exposed to many substance/agents. Epidemiology study design attempts to account for the other risk factors. The most famous early example of drinking-water epidemiology is the work of physician John Snow. During the 1854 cholera outbreak in London, Snow created a geographic map of cases over time in the Soho section and was able to deduce that the outbreak might be related to something in the water. After convincing local leaders of his theory, he removed the pump handle, which resulted in a reduction in the incidence of disease. Later inquiry found that the well was located near an old cesspit. This occurred before the acceptance of germ theory. Modern epidemiologists attempt to understand the effect of various environmental and occupational exposures using methods that have been developed and refined over the past century.
