The Search for Environmental Carcinogens

In the search for carcinogens, competing interests can complicate and compromise the case.

[Why is it so difficult to pin down environmental causes of cancer?] 

In December 2011, the Institute of Medicine, which advises the government on medical issues, released a much-anticipated report about breast cancer and the environment. Among other findings, the report’s authors acknowledged a lack of human evidence linking many environmental exposures to cancer. But, as the astronomer Carl Sagan was fond of saying, the absence of evidence is not evidence of absence.

[View the IOM's Breast Cancer and the Environment]

Regulatory agencies are supposed to base their decisions on science. The problem often is that the science can be open to interpretation. Take the Long Island Breast Cancer Study Project, which accumulated a vast database and led to more than 10 studies. The short answer, as summarized on the NCI website, says that the main study “did not identify specific environmental factors as a cause for breast cancer on Long Island,” but the actual picture is more complicated.

[What defines a cancer cluster?]

Two of the major chemicals examined on Long Island were polycyclic aromatic hydrocarbons (or PAHs) and organochlorine compounds (common to pesticides, such as DDT). PAHs are formed from the incomplete combustion of carbon-containing materials and are ubiquitous in the environment as part of air pollution, cigarette smoke or even the sizzle from your barbecue grill.

Research studies did not find a strong association with signs of organochlorine exposure in blood samples, but the results did suggest that PAH exposure may be associated with a higher risk of breast cancer. “We can’t rule that out,” says Marilie Gammon, PhD, of the University of North Carolina in Chapel Hill, who led the main study. The answer probably depends on a woman’s genetics; however, other factors may be at play, such as her age, weight gain or loss, diet or even the type of cancer she develops.

This highlights one of the biggest hurdles in investigating environmental exposures—not everyone who is exposed to a potential carcinogen will develop cancer. It depends on a person’s own genetics and other factors, even the time of life when a person is exposed. Researchers, such as Gammon and others, are still using the Long Island data to investigate the idea of “windows of susceptibility,” or those times in life, such as before birth, during childhood or puberty, when tissues may be particularly vulnerable to environmental threats. Furthermore, there is a long interval between the onset of exposure and actual development of cancer, a duration that can sometimes be as long as a few decades. During that length of time, a person is exposed to so many other potential hazards that cannot be controlled many other potential hazards that cannot be controlled but may influence the outcome.

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