Acquired risk factors and susceptibility to environmental toxicants

Susceptibility to illness after exposure to environmental toxicants is determined by the interaction of numerous factors involving both constitutive and acquired traits. Constitutive susceptibility (risk) factors are the intrinsic traits determined by developmental stage, gender, and genetic makeup. Within a population, changes in constitutive risk factors tend to occur slowly, through aging, alterations in the birth or death rate, or by migration in or out of the population. Often overlooked is the effect of acquired susceptibility factors on susceptibility to environmental toxicants. Acquired susceptibility factors, which are related to the effects of living conditions, psychosocial factors, diet, behavior and access to medical care, may modify the effect of constitutive factors. Three examples demonstrate the interaction of acquired susceptibility factors with exposure and constitutive factors. The increased prevalence of asthma in children is suspected of having a strong environmental component but the underlying acquired susceptibility factors, if any, are difficult to identify because of the multifactorial nature of asthma and the use of surrogate risk factors such as parent's education. β-Carotene is a dietary component which may modify acquired susceptibility. While numerous observational studies find that dietary β-carotene reduces the risk of lung cancer in cigarette smokers, intervention studies do not support this role. Hepatitis B is an example of an infectious agent functioning as an acquired susceptibility factor. Hepatitis B synergistically increases the risk of hepatocellular carcinoma when accompanied by exposure to aflatoxin, a relationship that may be modified by constitutive risk factors, such as epoxide hydrolase capabilities. Acquired risk factors have the potential to greatly influence risk and their impact should be included in future studies of the health effects of environmental toxicants.