Skeptical reactions to the report, most notably from the American Cancer Society, say that the report's focus on potential environmental risks may distract from known risks with much larger effects, such as smoking, sun exposure, diet and exercise.
"It is always worth making the point that tobacco is the most important exposure to try to eliminate," but that message shouldn't preclude investigation of other exposures, says Shelia Hoar Zahm, deputy director of cancer epidemiology and genetics at the National Cancer Institute.
The report, with its focus on the admittedly incomplete science on environmental cancer risks, helps the U.S. government to keep the broad picture in mind as it continues its war on cancer, she says.
The new report is the most recent in a series of annual reports put together by the three-member President's Cancer Panel. Last year, the topic was "Maximizing the Nation's Investment in Cancer," and the year before, "Promoting Healthy Lifestyles."
Here's a look at those chemicals known to be cancer-causing, as well as some suspect ones, and what you can do to lower your exposure.
According to the National Cancer Institute, tobacco accounts for 29% to 31% of cancer deaths, diet for 20% to 50%, infectious disease for 10% to 20%, ionizing and ultraviolet light for 5% to 7%, occupational exposure for 2% to 4%, and pollution for 1% to 5%. Zahm notes, however, that these numbers are from a 1981 paper reviewing studies from the 1970s, which probably studied exposures dating back to the 1960s and earlier.
Since then, no one has tried to revise the estimates, in large part because determining causative factors in cancer is a very difficult job.
For one thing, a cancer usually results from more than one cause. These insults may include genetic predisposition, immune status, lifestyle factors, other disease conditions and environmental exposures. Light-skinned people, for example, are more susceptible to sunlight-triggered skin damage that can lead to skin cancer, and asbestos exposure is more likely to cause lung cancer in pack-a-day smokers.
Another difficulty in investigating cancer risk factors is that people must be followed long enough in studies for cancers to develop — which can often take decades — or histories of cancer patients must reliably report the presence or absence of a risk factor decades earlier. (Often, such histories are sketchy and unreliable.)
In the case of environmental chemicals, it's nearly impossible to determine the extent of exposure someone had — how much chemical and for how long.
The American Cancer Society takes a pragmatic approach. In a report published last year in the journal CA: A Cancer Journal for Clinicians, the groups says it will target "its finite resources in cancer prevention" on those exposures that have the strongest evidence and cause the greatest amount of harm, with smoking at the top of the list.
The list of 50-plus potentially problematic environmental exposures in the President's Cancer Panel report included many chemicals for which the actual risks were unclear: substances such as herbicides, hair dyes and lead. Also on the list were a few well known to cause cancer, including radon and X-rays, says panel member Margaret Kripke, professor emerita at the M.D. Anderson Cancer Center in Houston.
Radon, a naturally occurring gas, was first linked to lung cancers in uranium mine workers in the mid-1900s. Uranium workers are exposed to unusually high levels of radon because the gas is released from the rock. But it also can accumulate in indoor spaces, such as homes, and is the second-leading cause of lung cancer after smoking, accounting for as many as 20,000 American deaths each year, according to the Environmental Protection Agency.
Evidence that household radon exposure increases the risk of cancer comes from studies such as one of 1,027 Iowa women who had lived in their homes for at least 20 years that was published in the American Journal of Epidemiology in 2000. Women who had higher radon levels in their homes were 50% more likely to have lung cancer; even with levels at the threshold of safety — 4 picocuries per liter, per the EPA.
Medical X-rays, such as CT scans, mammograms and radiation therapy, are used to diagnose or treat disease, and their use is on the rise. The more X-rays a person is exposed to over his or her lifetime, the greater the risk of developing cancer, such as leukemia and thyroid, breast and lung cancers, studies have found.
A 2004 study published in the Lancet estimated that medical X-rays account for 0.6% of a person's cancer risk over a 75-year lifetime in Britain. In Japan, where people had the highest exposure rates, the risk was 3%.
Other chemicals such as formaldehyde and pesticides, also on the President's Cancer Panel list, have been shown to increase cancer risk in occupational settings, where regular exposures to relatively high levels occur. But it's not clear how relevant such data are for non-work-related exposures, which are generally much lower.
Formaldehyde is used in manufacturing building materials such as plywood paneling and particleboard and household items such as carpets and drapes. Occupational exposure has been linked with leukemia and nasopharyngeal cancers, although some researchers say the evidence is far from conclusive.
The pesticides that Americans use on their lawns and garden also contain carcinogenic chemicals. Studies in farmworkers have linked exposure to pesticides, such as phenoxy acid herbicides, organochlorine insecticides and organophosphorus compounds, to increased rates of leukemias, lymphomas and soft-tissue sarcoma, although increases are not always observed.
Also in the President's Cancer Panel report were chemicals that are known endocrine disruptors — meaning they interact with hormones in the body — such as phthalates, bisphenol A (BPA) and triclosan. Phthalates are found in cosmetics and plastics, BPA is found in baby bottles and food can linings, and triclosan is an antibacterial agent added to soaps, deodorants and cleaning products.
Exposure is not the question with these chemicals — all have been found in treated wastewater, and BPA and triclosan have been found in the blood of a majority of people tested. However, evidence that these compounds cause cancer at current levels of exposure has not been established.
There are a few simple things you can do if you want to cut down on these risks, real or potential. Test your home for radon and, if high levels of the gas are present, seal cracks and increase ventilation to fix. Ask for your doctor's help in limiting your exposure to X-rays by avoiding unnecessary tests. To cut down on exposure to endocrine disruptors, you can filter tap water, avoid using BPA-containing plastics, use glass or ceramic containers to microwave food and use regular soap instead of antibacterial soap to wash your hands. Air new carpets and drapes to avoid formaldehyde and wear protective gloves and clothing when using pesticides.
But you'll get the biggest bang for your cancer-prevention buck by avoiding tobacco smoke and the midday sun, eating fresh fruits and vegetables, choosing whole grains over refined ones, and working out for at least a half-hour, five days a week.