PRINCIPLES OF TOXICOLOGY - Biology East Borneo

304 CHEMICAL CARCINOGENESISTABLE 13.12 ContinuedDimethyl carbamoyl chloride1,1-DimethylhydrazineDimethyl sulfateDinitrotoluenesDi-sec-octyl phthalateDioxaneEnvironmental tobacco smokeEpichlorohydrinEthyl acrylateEthylene dibromideEthylene dichlorideEthyleneimineEthylene oxideEthylene thioureao-Toluidinep-Toluidine1,1,2-TrichloroethaneTrichloroethylene1,2,3-TrichloropropaneUraniumVinyl bromideVinyl chlorideVinyl cyclohexene dioxideVinylidene chlorideWelding fumesWood dustZince chromatesSource: NIOSH Pocket Guide, 1999.13.9 CANCER AND OUR ENVIRONMENT: FACTORS THAT MODULATE OUR RISKSTO OCCUPATIONAL HAZARDSIncreased awareness of the ubiquity of synthetic, industrial chemicals in our environment has led anumber of scientists to try to determine what role environmental exposures play in cancer causation.The USEPA devotes a great deal of its resources to this question as do other federal, international andprivate agencies such as the Agency for Toxic Substances and Disease Registry (ATSDR) of the Centersfor Disease Control (CDC), the American Cancer Society (ACS), and the World Health Organization’s(WHO) International Agency for Research on Cancer (IARC) (see Table 13.14). While each organizationresearching the impact of our occupations, lifestyles, diets, and environmental exposures oncancer have differing agendas and views as to the predicted cancer risks associated with environmentalexposures or our daily routines, there is widespread agreement that the most substantial risks, and thegreatest causes of cancer, are those factors that are controlled by the individual (e.g., diet, smoking,alcohol intake).The importance of this fact is twofold: (1) it should be recognized that cancer is a phenomenonassociated with normal biologic processes, and is therefore impacted by those factors that may affectour normal biologic processes (e.g., diet); and (2) many environmental risk factors exist, and these, incombination with hereditary risk factors, may frequently provide overwhelming influences inepidemiological studies of occupational hazards. Thus, the risk factors not being studied (and sofrequently not controlled for) may mask or exacerbate the response being studied and so confound anystudy that is not normalized in a manner that removes all potential influences from the associationbeing studied.Estimates of the contribution of various factors to the rate of cancer in humans were perhaps firstput forth by Doll and Peto, who produced the results plotted in Figure 13.10. As can easily be seen inFigure 13.10, the vast majority of the cancers were thought to be related to lifestyle factors; tobaccoand alcohol use, diet, and sexual behavior accounted for 75 percent of all cancers in this initial analysis.Conversely, industrial products, pollution, and occupation were thought to be related to only 7 percentof all cancers. Currently, the contributions of diet, disease, and viral agents are still being researchedas perhaps the most common causes of cancer.In the years following Doll and Peto’s initial assertions, some scientists have questioned whethersuch a large proportion of the cancers in humans had such clearly defined causal associations. However,the most recent evidence accumulated by researchers in this area indicates that less than 1 percent oftoday’s cancers result from exposure to environmental pollution, and diet has since been identified asa key risk factor for cancer in nearly 200 epidemiologic studies. More importantly, the view that there