Introduction

Epidemiological studies have shown that environmental exposures influence a person's risk of disease. For instance, migration studies show that a person's risk of cancer changes as he/she moves across environments; such changes must be due to changing exposures, as the host genome remains essentially constant (Haenszel, 1982). Studies comparing the lifestyle of persons with cancer (cases) and those without cancer (controls) have identified a number of consistent differences, assumed to be risk factors for that cancer, although the critical aspect of exposure is usually unknown or poorly measured. For instance, case-control studies show that persons with higher levels of smoking are more at risk of lung cancer, or those reporting diets higher in animal fat have a higher risk of bowel cancer but the critical combination and timing of mutagens, carcinogens, tumor promoters etc. is unknown. Instead, exposures are reported as number of packs of cigarettes per year or estimated average number of calories from fat. Chapter 11 discusses some of these issues in greater depth including the definition of the term “environment”.

Genetic studies have shown that, for some persons, susceptibility is determined at least in part by genes (Eeles et al., 2004). Family history studies indicate that for most common cancers, close relatives of cases have a risk of that same cancer which is between twice and four times that of the general population (Goldgar et al., 1994; Hemminki et al., 2004).