“On what grounds do you know it's being grossly underestimated? It's a possibility, but many hypotheses have been proposed, and unless you have real evidence, you can’t say that it is,” said Dr. Michael Thun, vice president emeritus of epidemiology and surveillance research for the American Cancer Society.
Thun said the President’s panel overstates the concern about environmental causes when the best way to prevent cancer is to combat the largest risks that people encounter: tobacco, diet and sun.
But many environmental epidemiologists say quibbling over the numbers becomes a diversionary tactic.
They say the American Cancer’s Society’s statement sounds a bit like a principle espoused by industry groups – don’t act without absolute proof of harm. Many environmental epidemiologists are in favor of moving toward the precautionary principle – reducing people’s exposure to environmental pollutants even if there is uncertainty about the risks.
It’s an "erroneous exercise” to try to assign each chemical or exposure a specific fraction of cancer, said Richard Clapp of Boston University's School of Public Health, who co-authored a 2005 review and 2007 update on environmental and occupational causes of cancer.
"It's estimating a fiction, because nobody knows and nobody can know," said Clapp. "Why do we keep beating this dead horse? If there are things we can move on, let's work on those."
Cancer is the second leading killer of Americans, and the leading cause of death worldwide. Every year, about 1.5 million new cases are diagnosed in the United States and more than half a million people die from the disease, according to the American Cancer Society.
Experts agree that most cancers are caused by lifestyle factors such as smoking, diet and alcohol. Smoking alone accounts for at least 30 percent of all U.S. cancer deaths, and another one-third is attributed to diet, obesity and physical inactivity, according to the American Cancer Society.
But it’s the remaining cancers – about one out of every three – that trigger debate.
A 1981 report by two scientists, Sir Richard Doll and Sir Richard Peto, published in the Journal of the National Cancer Institute, estimated that two percent of cancer deaths were attributable to exposures to pollutants in the environment and four percent to exposures in occupational settings. In 2009, those percentages amounted to about 30,000 U.S. deaths.
“If you looked at the number of deaths per day, if that were a plane crash it would be a national news story,” Clapp said.
The 1981 report only considered deaths, not cancer cases. (About half of those diagnosed with cancer die.) Also, the study only included Caucasians under the age of 65, although many cancers increase with age and many minority groups are more highly exposed to environmental contaminants.
The old two percent estimate for environmentally induced cancers is still commonly used – despite advances in modern cancer biology.
New areas of cancer research are focusing on the potential for pollutants to interact with one another and with genetic factors. Carcinogens can act by damaging DNA, disrupting hormones, inflaming tissues, or switching genes on or off.
Also, exposure to hormonally active agents during critical periods of human development – particularly in the womb or during childhood – may trigger cancer later in life. For example, the risk of breast cancer could be influenced by exposures during puberty.
All these elements make it tricky to calculate the magnitude of environmentally induced cancers.
Scientists now know that getting cancer is like being attacked by a multi-headed monster: How can you really be sure which part did the most damage?