A couple of years ago, I reviewed a landmark report on the connection between breast cancer and the environment. State of the Evidence 2008 made headlines with its conclusion that "a significant body of scientific evidence links exposures to radiation and synthetic chemicals to an increased risk of breast cancer."
On the whole, an estimated 80 to 90 per cent of cancers can be attributed to environmental factors. This takes into account physical (sunlight, x-rays, asbestos), biological (viruses, bacteria), and chemical agents in the environment that increase cancer risk. Of course, exposure to some of these agents is by choice, such as cigarette smoking, drinking alcohol, and excessive sun exposure. What makes chemicals unique is that there's only so much we can do as individuals to reduce our exposure to chemicals in the environment.
Chemicals can linger in our environment and eventually make their way into our bodies. Without even knowing it, and despite our best efforts, we come in contact with these pollutants everyday — in our water, soil, air, food, and manufactured products. Many industrial contaminants can be measured in our tissues and blood. Traces of these chemicals have even been found in the blood of Inuit in northern Canada, although they live thousands of kilometers away from the original sources.
Very little is known about the potential long-term health effects, such as cancer risk, of thousands of industrial chemicals in use today. To the extent that industrial chemicals are tested before being introduced into the environment, the assessment is typically focused on short-term possible health effects. At this time, only a minor portion of environmental chemicals are known to cause cancer. From a biological point of view, however, it is plausible that several environmental chemicals could cause cancer.
Much more needs to be done to evaluate chemicals to determine if they increase the risk of cancer. For example, some chemicals mimic the effects of hormones. Since it's known that some hormones increase cancer risk, the US EPA has decided that thousands of environmental chemicals need to be tested for this possibility. Testing chemicals for cancer also needs to expand to incorporate recent knowledge about how exposures can affect tissue development and gene function, sometimes from a very early age. In the area of breast cancer, for example, a group in California including community advocates and scientists wants chemicals tested for the ability to affect breast tissue development.
I suspect that in the next few decades, research will identify more environmental contaminants that cause cancer. And we know that many chemical agents have already been linked to increased cancer risk. These include asbestos, some pesticides, arsenic, some solvents, etc., and several other suspected carcinogens such as air pollution (diesel and gasoline exhaust), heavy metals, dioxins etc. The International Agency for Research on Cancer, and other reputable organizations, have assessed the science and labelled these agents as "known", "probable", or "possible" human carcinogens.
There is good news, too. Many of the exposures that cause cancer are preventable. There are some steps we can take as individuals to minimize our exposure to cancer-causing chemicals. And collectively, we can ban harmful chemicals or restrict their use to prevent them from entering the environment and our bodies.
State of the Evidence 2008 concluded with a call for policy and research initiatives to reduce the public's exposures to toxic chemicals and radiation. Two years later, there remains much to be done on both fronts... and the need is ever-more pressing.
Kristan Aronson is a professor in the Division of Cancer Care and Epidemiology, Queen's Cancer Research Institute, a member both of the Department of Community Health and Epidemiology and the School of Environmental Studies, Queen's University. She conducts research on environmental and genetic influences on cancer, and feels privileged to teach human health research methods to undergraduates and graduate students.