PRINCIPLES OF TOXICOLOGY

304 CHEMICAL CARCINOGENESIS
TABLE 13.12 Continued
Dimethyl carbamoyl chloride o-Toluidine
1,1-Dimethylhydrazine p-Toluidine
Dimethyl sulfate 1,1,2-Trichloroethane
Dinitrotoluenes Trichloroethylene
Di-sec-octyl phthalate 1,2,3-Trichloropropane
Dioxane Uranium
Environmental tobacco smoke Vinyl bromide
Epichlorohydrin Vinyl chloride
Ethyl acrylate Vinyl cyclohexene dioxide
Ethylene dibromide Vinylidene chloride
Ethylene dichloride Welding fumes
Ethyleneimine Wood dust
Ethylene oxide Zince chromates
Ethylene thiourea
Source: NIOSH Pocket Guide, 1999.
13.9 CANCER AND OUR ENVIRONMENT: FACTORS THAT MODULATE OUR RISKS
TO OCCUPATIONAL HAZARDS
Increased awareness of the ubiquity of synthetic, industrial chemicals in our environment has led a
number 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 and
private agencies such as the Agency for Toxic Substances and Disease Registry (ATSDR) of the Centers
for 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 organization
researching the impact of our occupations, lifestyles, diets, and environmental exposures on
cancer have differing agendas and views as to the predicted cancer risks associated with environmental
exposures or our daily routines, there is widespread agreement that the most substantial risks, and the
greatest 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 phenomenon
associated with normal biologic processes, and is therefore impacted by those factors that may affect
our normal biologic processes (e.g., diet); and (2) many environmental risk factors exist, and these, in
combination with hereditary risk factors, may frequently provide overwhelming influences in
epidemiological studies of occupational hazards. Thus, the risk factors not being studied (and so
frequently not controlled for) may mask or exacerbate the response being studied and so confound any
study that is not normalized in a manner that removes all potential influences from the association
being studied.
Estimates of the contribution of various factors to the rate of cancer in humans were perhaps first
put forth by Doll and Peto, who produced the results plotted in Figure 13.10. As can easily be seen in
Figure 13.10, the vast majority of the cancers were thought to be related to lifestyle factors; tobacco
and 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 percent
of all cancers. Currently, the contributions of diet, disease, and viral agents are still being researched
as perhaps the most common causes of cancer.
In the years following Doll and Peto’s initial assertions, some scientists have questioned whether
such 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 of
today’s cancers result from exposure to environmental pollution, and diet has since been identified as
a key risk factor for cancer in nearly 200 epidemiologic studies. More importantly, the view that there