December 12, 2005


Environment and Cancer: The Links Are Elusive

When Mike Gallo learned he had cancer, a B cell lymphoma, two years ago, his friends and relatives told him that they knew how he got it.

His cancer, Dr. Gallo's friends said, was obviously caused by the dioxin that he had worked with for three decades in his laboratory. After all, the Environmental Protection Agency classifies dioxin as a probable human carcinogen. And among the cancers that it may increase the risk for, in high doses, is lymphoma.

Dr. Michael A. Gallo, director of the National Institute for Environmental Health Sciences Center of Excellence at the Robert Wood Johnson Medical School in New Brunswick, N.J., tells his well-meaning advisers that he does not think so.

"I say, 'No, I know my blood levels of dioxin,' " Dr. Gallo said, explaining that he measured them when he worked with the chemical. His levels, he said, are low. And there is no way to make a leap from such low levels of dioxin to his cancer.

Yet many of his friends and relatives remain convinced.

"That's the way people think," Dr. Gallo said. "If you get cancer, there has to be a reason."

And there may be a reason, he and other scientists say. But pinning cancer on trace levels of poisons in the environment or even in the workplace is turning out to be a vexing task. [...]

Gerald N. Wogan, a chemist at the Massachusetts Institute of Technology, takes a different approach. He, like most other scientists, worries that the public is overly concerned about cancer risks from the chemicals they are exposed to. But, he says, the question of how environmentally induced cancers arise is a puzzle that he would like to solve.

Dr. Wogan became interested in pollutants and cancer when he began studying the effects of aflatoxin, produced by mold on peanuts. The toxin caused liver cancer in rats and, Dr. Wogan and others showed, it also causes liver cancer in people. But exposure to aflatoxin was just part of the risk.

Dr. Wogan studied men in Shanghai who were eating foods with high doses of the toxic chemical. They ended up with four times the risk of liver cancer. Another cause of liver cancer, hepatitis B infections of the liver, increases the risk by a factor of seven.

Then Dr. Wogan noticed something that astonished him. The risk of liver cancer was increased 70 times in people who met both criteria; they ate contaminated foods and they were infected with hepatitis B.

"It was like a model system for the environmental causes of cancer," Dr. Wogan said.

The two cancer-causing agents were amplifying each other's effects. He went on to study the mechanisms of cancer causation and discovered that the more he looked at environmental pollutants the more complex and individualistic the biochemical pathways leading to cancer turned out to be.

"People differ very greatly in their response to chemical carcinogens," Dr. Wogan 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.

"If we knew how to identify exactly which factors or agents or dietary factors were responsible and if we were able to identify their effects in people, then, in principle, cancer is preventable," he said. But, he added: "It's so tough. It's so very tough to do."

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."

There's a group of doctors up here who have done eye-opening work on what screenings are really telling us and what they aren't.

Posted by Orrin Judd at December 12, 2005 11:14 PM

On the other hand, the BBC is reporting this morning that "Colds may trigger child cancers".

See link

Here .

Posted by: Bruce Cleaver at December 13, 2005 7:12 AM

In the old days the people blame trace chemicals like dioxin for the cancers would have blamed witches and evil spirts and Satan instead. Same ol' superstitiously ignorant mindset.

Posted by: Raoul Ortega at December 13, 2005 1:07 PM