By Ted Schettler, MD MPH
Identifying the causes of cancer, in order to help develop preventive strategies, has been of great interest for a long time. Almost 30 years ago, the Office of Technology Assessment of the US Congress commissioned two British epidemiologists, Richard Doll and Richard Peto, to quantify the avoidable risks of cancer in the US. They limited their evaluation to cancer deaths in people under age 65 and, using epidemiologic data, estimated the largest contributors to be tobacco (30%) and diet (35%). Far down on the list were environmental pollution (2%) and occupational exposures (4%).
Doll and Peto were fairly confident about their estimates for tobacco and less so about diet. They acknowledged that estimating other factors, including pollutants, was hampered even more by a number of assumptions, data gaps, and uncertainties. Despite these limits, which other analysts have repeatedly pointed out over the ensuing years, many scientists and policy makers continue to accept Doll and Peto’s estimates as fact. Their numbers have supported arguments against spending time and resources to reduce exposures to environmental contaminants, emphasizing instead the importance of personal lifestyle choices.
The President's Cancer Panel Report
Now the President's Cancer Panel (PCP) has weighed in. Established in 1971 with the purpose of evaluating the National Cancer Program at the National Cancer Institute, members of the PCP are appointed by the president. They are charged with monitoring the National Cancer Program and reporting any delays in rapid execution of the program to the President. LaSalle Leffall MD, oncologist and professor of surgery at Howard University College of Medicine, and Margaret Kripke PhD, professor of immunology at the M.D. Anderson Cancer Center, comprise the panel responsible for this year’s report. A third slot is vacant.
Previous PCP reports have addressed tobacco use, diet, and other lifestyle factors. This year's report focused on environmental exposures—industrial, occupational, and agricultural chemical exposures, indoor/outdoor air pollution, water contamination, nuclear fallout, radiation, and electromagnetic fields. The panel based findings and recommendations on testimony at four meetings from 45 invited experts from academia, government, industry, the environmental and cancer advocacy communities, and the public, as well as additional information gathering. In their heavily referenced report, Reducing Environmental Cancer Risk: What We Can Do Now, the panel says that "previous estimates [of environmental causes of cancer] are woefully out of date…and underestimate significantly the true toll of cancer related to these exposures." Not content with just calling for more research, the panel also strongly recommended additional action, based on what we already know.
The analysis received considerable media attention and was hailed by many of us in the environmental and public health advocacy communities. We agree that emphasis on smoking cessation and diet is extremely important but we have long held that additional measures must be taken to further reduce the burden of many kinds of cancer.
One of the report's important contributions is its succinct summary of many of the complexities of cancer biology that make it extremely difficult to quantify the relative importance of myriad interactive risk factors in the origins of cancer. It concludes, "We do not know the extent to which environmental contaminants contribute to the cancer burden, but we know enough to act." It calls for the "adoption of a precautionary, prevention-oriented approach [to] replace current reactionary approaches to environmental contaminants in which human harm must be proven before action is taken to reduce or eliminate exposures." The panel urged the president "most strongly to use the power of your office to remove the carcinogens and other toxins from our food, water, and air that needlessly increase health care costs, cripple our Nation’s productivity, and devastate American lives."
This new call for a precautionary approach to cancer is welcome. The PCP seems to agree that complex systems, whether biologic, technologic, or ecologic, behave in ways that are difficult to fully understand. Yet, waiting for quantitative proof of cause-and-effect relationships before taking action means that we miss opportunities to prevent harm to the people and things that we care about.
At SEHN we, along with many colleagues, consider the precautionary principle an essential guide for decision making. Precaution requires heeding early warnings and shifting the responsibility for evaluating safety to proponents of an activity before and during its deployment, rather than reacting to damage after it's done. A precautionary approach involves examining alternatives and participatory decision making.
Contrary to caricatures often drawn by critics, a precautionary approach is not anti-scientific nor does it require banning all activities that might cause harm. Rather, it requires a close look at what we know and don’t know, what we suspect, sources of uncertainty, how easy or difficult they are to resolve, who should bear the burden of proof and responsibility, what is at risk, and what the alternatives are; then, what action is indicated. As we watch the oil flow in the Gulf, financial institutions collapse, glaciers melt, childhood cancer rates rise, and loved ones suffer, we are reminded of what is at stake.
The case for precaution
Here's how the PCP came to their call for a precautionary approach to cancer prevention. They began by reminding us that cancer continues to devastate the lives of Americans. Approximately 41% of people in the US will be diagnosed with cancer at some point in their lives and about 21% of Americans will die from cancer. The incidence of some cancers, including some most common among children, is increasing.
The panel explained that attempts to identify excess cancer risk due to environmental contaminants, radiation, and other harmful exposures face extensive challenges, and research efforts aimed at primary prevention are underfunded. Understanding causal relationships is inherently difficult for many reasons, they said. Cancer initiation and progression is a complex, multifactorial, multistage process. Multiple biologic mechanisms are likely to be involved, including inflammation and immune and endocrine dysfunction.
The panel notes that tens of thousands of chemicals used by Americans in their daily lives are poorly studied and largely unregulated, although many have known or suspected carcinogenic or endocrine-disrupting properties. The entire U.S. population is exposed daily to complex mixtures of them—from consumer products and medical, occupational, military, and agricultural sources. Yet, we don't routinely study mixtures in laboratory cancer assays. Inadequate exposure assessments impede research efforts, and medical and public health professionals don’t routinely monitor or even inquire about sources of exposures to potential carcinogens. Furthermore, we have no real understanding of how chemical mixtures interact with other background risk factors, including diet, socioeconomic stressors, and genetic susceptibility.
We know that certain periods of life, such as early development or puberty, are times of increased vulnerability to exposure to potential carcinogens. But we don't know much about exposures during those times and their long-term effects, although we do know that "numerous environmental chemicals can cross the placental barrier. To a disturbing extent, babies are born pre-polluted."
The panel also expressed concern about the rapid increase in radiation exposure from medical imaging, particularly in children, its cumulative effect on cancer risk throughout life, lack of awareness on the part of medical professionals and the public, and inadequate documentation of exposure levels in individuals.
They acknowledged that some groups of people are at higher risk. For example, disadvantaged populations are more likely to be employed in occupations with higher levels of exposure to potential carcinogens and to live in more highly contaminated communities. This is not just a health issue, they said, but a matter of environmental justice.
Shortcomings of Doll and Peto
The greatest shortcoming of the earlier Doll and Peto estimates, the panel says, is that "calculation of attributable fractions does not fully account for the fact that environmental contaminants interact with each other and that all avoidable causes are not known." These are important points. Doll and Peto themselves acknowledged that risk factors interact, and adding the contribution of each risk factor as if it acted independently of others would result in a number larger than 100%. Obviously we cannot prevent more than 100% of cancers.
Nevertheless, for some reason, the attributable risk factors in Doll and Peto's original table of estimates add up to almost exactly 100%. Richard Clapp and coauthors pointed out in 2007 that "although Doll and Peto acknowledged that attributing causes of cancer to percentages that nicely add to 100% was an erroneous exercise, the mainstream of cancer prevention and control has somehow missed this important point."[i]
The PCP adds that since the Doll and Peto estimates were published, environmental exposures have become more numerous and the impact of various exposures, whether individual, simultaneous, sequential, or cumulative over a lifetime, may not be simply additive but rather synergistic.
Act on what we know now
In summary, the panel concluded:
- Despite modest overall decreases in cancer incidence and mortality, cancer continues to devastate lives. The incidence of some kinds of cancer, including certain childhood cancers, continues to increase.
- Exposures to environmental agents known or likely to be related to cancer risk are ubiquitous, complex, poorly quantified, and inadequately studied. The extent to which they contribute to cancer incidence is uncertain but earlier estimates are outdated, invalid, and too low.
- Research challenges are numerous although our understanding of cancer biology has advanced considerably. The importance of interactions among risk factors, exposures to mixtures of hazardous substances, the timing of exposures, windows of vulnerability, and susceptible populations have become increasingly apparent. Long latency periods between relevant exposures and the appearance of clinical disease add to the complexity.
Thus, the panel concludes, while we need to ramp up our research efforts, now is the time to reduce or eliminate exposures to cancer-causing and cancer-promoting environmental exposures to protect all Americans, but especially children. We must do that based on what we know now. Quantifying the contribution of environmental exposures to the nation’s total cancer burden or to cancer risks in sub-populations will always be difficult, but it is not required in order to act. This is the essence of a precautionary approach to cancer prevention.
The panel recommends action at several levels:
- scientific research to enhance our understanding and by extension, our ability to prevent cancer and respond to environmental carcinogens;
- enforcement of existing policies and regulations intended to protect workers and the public;
- policy and regulatory reforms that support public health and reduce the burden of cancer; and
- personal action.
With respect to regulatory reforms, the panel concluded that current governmental regulation of environmental contaminants is "rendered ineffective by five major problems: (1) inadequate funding and insufficient staffing, (2) fragmented and overlapping authorities coupled with uneven and decentralized enforcement, (3) excessive regulatory complexity, (4) weak laws and regulations, and (5) undue industry influence. Too often, these factors, either singly or in combination, result in agency dysfunction and a lack of will to identify and remove hazards."
The panel singled out the Toxic Substances Control Act (TSCA) of 1976 as the "most egregious example of ineffective regulation of environmental chemicals." Although this legislation was intended to give the EPA authority to control health risks from chemicals in commerce, it has failed. At the time it was adopted, TSCA grandfathered in approximately 62,000 chemicals and today more than 80,000 chemicals are in use, including 1000-2000 new substances each year. TSCA does not require adequate safety testing before chemicals are introduced into commerce or allowed to stay on the market. The panel recommends a more integrated, coordinated, and transparent system, free of political or industry influence. It should shift the burden of proof onto manufacturers to demonstrate safety of a chemical before allowing it into commerce and include post-market safety monitoring for new and existing chemicals.
A bill to reform TSCA has recently been introduced in the US Senate and a companion draft discussion bill in the House. Analysis of the bills is available on the Safer Chemicals, Healthy Families coalition website. We urge our readers to become informed and join the effort to ensure that this 35-year-old ineffective law is replaced with one that protects public and environmental health.
Nothing in the PCP report should be interpreted to undermine the importance of smoking cessation, weight control, and improved nutrition in cancer prevention. But, as Clapp and coauthors point out, "Even though tobacco smoke remains the single most significant preventable cause of cancer, it has been linked neither to the majority of cancers nor to many of the cancers that have dramatically increased in the US in recent decades including melanoma, non-Hodgkin’s lymphoma, testicular, brain, and thyroid."
Cancer prevention will require removing carcinogens from commerce; replacing them with tested, safer alternatives; and shifting the burden of proof to proponents of an activity to evaluate safety rather than requiring the general public or government to demonstrate harm. This is a cultural shift that makes sense to most people but is often fiercely resisted by those who benefit from the status quo.
We agree that it is more effective to prevent disease than to treat it and that scientific evidence of the effects of environmental exposures on disease burdens are not being adequately integrated into policies and strategies for disease prevention. Moreover, allowing people and the environment to be exposed to known or suspected carcinogens or to untested chemicals more generally is unethical. Primary prevention of cancer and other diseases should be a key feature of health care reform and efforts to control health care costs. Elements of the precautionary principle must be incorporated into these efforts for them to be successful.
[i] Clapp R, Howe G, Jacobs M. Environmental and occupational causes of cancer: a call to act on what we know. Biomed Pharmacother 2007 61(10): 631-639.