...pinning cancer on trace levels of poisons in the environment or even in the workplace is turning out to be a vexing task. There has been recent progress in addressing the issue, but the answers that many people believe must be out there remain elusive.Of course "advocates" will have none of it, and want to make everything illegal. Besides, Whatever doesn't kill you might make you stronger.
"It's an area where there's certainly been a lot of heat and not a lot of light for some time," said Robert Hoover, director of the epidemiology and biostatistics program at the National Cancer Institute. For the most part, Dr. Hoover said, "we are down to speculations based on some data but without having the information we need."
Members of advocacy groups agree that there is much to learn, but they say the questions are too important to brush off by saying the research is difficult or the questions complex.
...While most scientists think that only a tiny fraction of cancers might be caused by low levels of environmental poisons, these are cancers that could, in theory, be avoided.
"All it takes is the political will to ban them or impose regulations to minimize exposure, and the cancers are gone," Dr. Hoover said.
The problem is to decide which chemicals might be causing cancer, and in whom.
Some scientists, like Aaron Blair, an epidemiologist at the National Cancer Institute, see hints that environmental pollutants like pesticides, diesel exhaust in cities and workplaces and small particles in the air may instigate cancer.
But, Dr. Blair says, there is a huge problem in following up on these hints because scientists need to figure out who was exposed to what and when the exposure occurred. Asking people is not much help. Most people do not know what they were exposed to, and even if they think they know, they often are wrong, he said.
So Dr. Blair and his colleagues decided to try for the greatest possible rigor by focusing on one group, farmers, that is not only routinely exposed to pesticides that may increase cancer risk, but also keeps excellent records of exposure.
The effort, a collaboration involving the cancer institute, the National Institute of Environmental Health Sciences and the Environmental Protection Agency, began in 1993 and includes nearly every farmer and farmer's spouse in Iowa and North Carolina - 55,000 farmers and 35,000 spouses.
Investigators have been asking the farmers what pesticides and herbicides they used, when they used them and how much they used, and have been obtaining information on other risk factors like smoking. Then they use the medical records from tumor registries to determine who developed cancer and what type was developed.
"We're now just in the period of time where we can look at outcomes," Dr. Blair said. So far, the researchers have found a few associations, but nothing that is definitive.
"I would call it, at this stage, interesting leads," Dr. Blair said. "None are large enough for any regulatory agency to take action or to say they are a human carcinogen. They are leads." They include, for example, a slightly higher rate of lung cancer and leukemia in farmers who used the insecticide diazinon and a possible increase in prostate cancer among farmers who used methylbromide to fumigate the soil.
The investigators looked for an association between pesticides and herbicides and breast cancer, but they did not find one, Dr. Blair said, adding that one pesticide, atrazine, was under particular suspicion because it causes breast cancer in rats and has estrogenlike properties.
Even if the study finds that some chemicals have increased farmers' cancer rates, it remains unclear what that means for the general population, where exposures are usually much lower. Also unclear is whether those chemicals should be banned.
Dr. Blair noted that such decisions were difficult because they were, in part, political, balancing the costs of getting rid of the chemical against the benefits...
..."People differ very greatly in their response to chemical carcinogens," [Gerald N. Wogan, a chemist at the Massachusetts Institute of Technology] said. "Almost all chemicals, with relatively few exceptions, have to be converted from what they are into something more chemically active to be carcinogenic.
"If you encounter one of these compounds, most of it is converted to less toxic material that is excreted," he continued. "Only a tiny amount is converted to a form that could cause cancer. A small fraction of 1 percent gets converted. And people can differ enormously in their genetic ability to do these metabolic conversions."
Further complicating the issue is that a person's diet, or components of the diet, can increase the activity of enzymes that convert chemicals into carcinogens. And other dietary components can inactivate enzymes that detoxify chemicals.
The calculus grows so complex that it can be virtually impossible to predict what will happen in an individual person exposed to low levels of a possibly toxic chemical. For example, Dr. Wogan said, "The same food, broccoli, can affect both types of enzymes."
Added to this are the effects of chronic infections, like hepatitis B, in which the immune system releases chemicals that can magnify the effects of carcinogens.
In theory, Dr. Wogan said, there is hope for untangling the mess....
In the meantime, he and others say they take comfort in cancer statistics that do not indicate a cancer epidemic. Rates of cancer have been steadily dropping for 50 years, if tobacco-related cancers are taken out of the equation, said Prof. Richard Peto, an epidemiologist and a biostatistician at Oxford University.
What appear as increases in cancers of the breast and prostate, Dr. Peto added, are in fact artifacts of increased screening. When healthy people are screened, the tests find not only cancers that would be deadly if untreated, but also a certain percentage of tumors that would never cause problems if let alone.
His analysis of cancer statistics leads Dr. Peto to this firm conclusion: "Pollution is not a major determinant of U.S. cancer rates."