Cancer represents the second most common cause of death in children older than one.

Environmental factors (both alone and in interaction with genetic susceptibilities) are implicated in the etiology of some 80 to 90 percent of human cancers.

"It's not isolated exposure to these contaminants but rather a lifetime of being exposed to hundreds of chemicals, some of which become more potent when combined, that may heighten one's risk. "
Dr. Nicole Bruinsma, CAPE

Cancer

Cancer is arguably the most emblematic health effect related to environmental factors and is a major cause of mortality in industrialized nations. According to current statistics for Canada, an individual's lifetime probability of developing cancer is just over 40% for men and about 35% for women.1

We think of cancer primarily as a disease of adults, especially of the elderly. Childhood cancer is at once a feared and foreign phenomenon. However, data are suggesting that it is becoming a more familiar affliction as there is a sense that incidence has increased steadily in the last few decades, although this is largely unexplained.

Two things can be said with some certainty regarding childhood cancer. First, although cancer still represents the second most common cause of death in children older than one (following accidents), survival rates from childhood cancer in Canada have improved substantially in the last 40 years.2 Second, the precise causes of childhood cancers are still insufficiently known, although it is believed that exposure to carcinogens in the environment, particularly during the prenatal stage, represents a substantial, largely preventable cause of cancer in the young.

Carcinogenesis

Cancer is not a single disease, but rather represents different diseases of varying etiology and causal mechanisms that share the common characteristic of "uncontrolled, abnormal growth of cells in different parts of the body that can spread to other parts of the body."3 In practical terms, cancer is a non-specific term for normal cell division that has "run amuck."4

Our understanding of the means by which cancer occurs has been dramatically enhanced in recent years. The most commonly held theory suggests that this aberrant process arises mainly as a result of the accumulation of many small alterations, or mutations, to the DNA of normal cells. The general model for cancer pathogenesis is that it is a multistage process involving initiation, tumour promotion, and tumour progression.5

Therefore, a carcinogen is by definition any agent that can lead to or accelerate one or more of these stages in the advancement of cancer. A complete carcinogen, such as radiation, is a substance that can produce changes in cells at all stages. Cancer initiators are substances that cause the initial changes or mutations in DNA. Nitrosamines, which come from dietary compounds, are considered cancer initiators. Substances designated tumour promotors may not in themselves be carcinogenic, but appear to act by amplifying the effects of other carcinogens.6 PCBs are presumed to operate as cancer promoters. Finally, tumour progressors are agents that can convert tumours from benign to malignant status.

(For a thorough review of the mechanisms of cancer etiology, the reader is referred to: Baxter CS. Carcinogenesis. In: Basic Science of Environmental Medicine: Mechanics & Principles. Brooks S, Gochfeld M, Herzstein J, Schenker M and Jackson R. (Eds. ) St. Louis: Mosby. 1995. Pp. 78-94.)

While a small proportion of cancers is determined primarily by genetic factors, it is estimated that environmental factors (both alone and in interaction with genetic susceptibilities) are implicated in the etiology of some 80 to 90 percent of human cancers.7 Studies of twins, migrants, geographic populations and time trends in cancer have all tended to identify environmental factors (such as tobacco, diet, alcohol, radiation, toxins, drugs, etc.) as explaining to a great degree increases in cancer risk. In the terminology of cancer epidemiology, "environmental factors" are broadly defined and also include such lifestyle or behavioural factors as listed above.

The multifactorial nature of cancer etiology means it is difficult to clearly determine cause for individual cases. There are only a handful of "signature" cancers known to result from exposure to specific agents. For example, the associations between mesothelioma and asbestos, lung cancer and tobacco smoke and malignant melanoma and ultraviolet (solar) radiation are all reasonably well established. The vast majority of cancers, however, are of undetermined cause.