Level I, II, and III Epidemiological Studies

A level II study gathers data that are not available in public records. The kinds of data depend on whether the study design is case control or cohort. A level II case-control study typically investigates pathways of exposure for the at-risk population. An investigation of exposure includes interviews with individuals in the at-risk population about their length of residence in the presumed impact area, dietary habits, workplace conditions, and places where children play. It may also include qualitative or semiquantitative modeling of the environmental fate and transport of chemicals of concern (Chapter 2). Samples of body fluids and tissues such as blood, urine, hair, or nails may be analyzed for evidence of exposure to bioaccumulative chemicals such as arsenic, mercury, or PCBs. An effort is made to identify and exclude possible confounding factors. Erin Brockovich and A Civil Action are examples of level II case-control studies. 

A level II cohort investigation is based on known exposure and focuses on symptoms of disease. Individuals in the exposed population are interviewed in detail about their medical histories to determine whether their symptoms coincide with toxic effects associated with the chemical of concern. Potential confounding factors are also investigated. Clinical drug trials and investigations of the health effects of workplace exposures to toxic chemicals are examples of level II cohort studies. 

Level III applies only to prospective cohort studies, which involve decades-long medical follow-ups of large study populations. For example, the Nurses Health Studies have been investigating possible health effects of oral contraceptives by following cohorts totaling 238,000 female nurses since 1976. The two cohorts of atomic bomb survivors and their descendants, one at Hiroshima and the other at Nagasaki, Japan, have been followed continuously since shortly after the end of World War II. 

Essentials of Toxic Chemical Risk Science and Society 

What is the null hypothesis How is it used to investigate toxic chemical risk in human populations  

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