New view of cancer development focuses on subtle, combined effects
Rethinking Carcinogens: Comparing Cancer Risk Factors: Should We Be Concerned?
The reports of the Halifax Project demonstrate that the universe of cancer-causing chemicals may be much larger than the list of known complete carcinogens. There may be such a thing as partial chemical carcinogens that play a role. But even if chemicals and combinations of chemicals can cause cancer, is that a big concern? How many cancers can be attributed to chemical exposures rather than other risk factors such as inheritance and lifestyle choices?
We know that inherited genetic variability – the genes we’re born with – can increase our risk of cancer. For example, breast cancer rates are much higher among women who inherited mutations in the BRCA1 and BRAC2 tumor suppressor genes. However, we are learning that only a relatively small proportion of cancers can be attributed solely to heredity.
One study in twins showed that, among the cancers the researchers investigated, at most 42 percent of cancer incidence was due to heritable factors.32 Studies of people who migrated from one country to another also demonstrate the importance of lifestyle and environmental factors. These studies have found that cancer rates among migrants more closely resemble those in the country they move to than their country of origin.33, 34 Clearly, a large proportion of cancers are caused by something other than inheritance.
In fact, we know that diet, lifestyle choices, viral infections, radiation, environmental contaminants and all things not genetic play a role in cancer causation and may cause up to 95 percent of certain cancers.35 It’s hard to know exactly how many cases of cancer are the result of exposures to toxic chemicals, but occupational exposures alone are estimated to cause 2-8 percent of all cancers.36
The bad news is that the environment in which we live and work is increasing the risk of cancer. The good news is that cancers caused by environmental contaminants are preventable. However, as the reports of the Halifax Project make clear, our understanding of the interplay between cancer and environmental exposures, particularly to chemicals, is incomplete. This has important implications for the regulation of potential environmental carcinogens. As the 2008-2009 report of the President’s Cancer Panel said, federal environmental laws “fail to address the potential hazards of being exposed to combinations of chemicals.”
Another weakness of current environmental laws is that for many chemicals, there is either a lack of political will to enact meaningful regulation or a lack of information on toxicity. Environmental chemicals have been known to cause cancer since as far back as 1775, when an English surgeon, Sir Percivall Pott, made the connection between high rates of scrotal cancer among chimney sweeps and exposure to soot.37 Today the International Agency for Research on Cancer and the U.S. Environmental Protection Agency (EPA) have classified hundreds of chemicals as having carcinogenic potential, but we know these lists are incomplete.
According to the EPA’s Chemical Hazard Data Availability Study, only about 7 percent of approximately 3,000 chemicals produced or imported in high volumes (at least one million pounds a year) have complete toxicity profiles. Even more alarmingly, the Halifax Project team found no existing dose-response information for 22 (26 percent) of the 85 chemicals it examined. The vast numbers of chemicals in the environment that have not been properly tested for carcinogenic potential represents an immediate public health threat that demands an organized approach to prioritize testing of chemicals with the greatest likelihood of causing cancer.
When you consider that there are thousands of chemicals and millions of possible combinations, it is clear that testing chemical mixtures for toxicity is a monumental task.38 We need new tools to help identify chemicals and chemical mixtures that may be linked to cancer. One approach urged by the Halifax Project reports is to use the hallmarks of cancer to identify the chemical mixtures with the greatest potential to cause disease. Screening for chemicals that interfere with hallmark processes could be a highly efficient way to target carcinogenicity testing of the thousands of chemicals for which we lack toxicity information.