Environmental factors the major cause of cancer, says report

Posted: 29 June 2004

Most cases of cancer are linked to environmental causes, US government scientists report. Cancer is the second leading cause of death for Americans after heart disease.

Cancers linked to environmental causes make up at least 80 per cent of all cancer cases, according to a second new report by the National Cancer Institute, this one published with the National Institute of Environmental Health Sciences. Environmental causes include exposure to agents in the air and water as well as lifestyle factors such as smoking and diet.

Dr Aaron Blair. Photo: American College of Epidemiology
Dr Aaron Blair. Photo: American College of Epidemiology
Dr Aaron Blair evaluated the risk of non-Hodgkin's lymphoma, leukemia, and multiple myeloma among farmers in the first case-control studies to obtain detailed information on pesticides used and application practices.© American College of Epidemiology
"Most epidemiologists and cancer researchers would agree that the relative contribution from the environment toward cancer risk is about 80-90 per cent," said Dr Aaron Blair, the chief of the Occupational Epidemiology Branch in NCI's Division of Cancer Epidemiology and Genetics. "There is very solid evidence that environmental factors are the major cause of cancer," he said.

Environmental factors

"When I use the word environmental, I mean it in a broad sense to include both lifestyle factors such as diet, tobacco, and alcohol, as well as radiation, infectious agents, and substances in the air, water, and soil," said Dr Blair in the June 17 issue of the NCI publication "Benchmarks."

Evidence comes from studies of people who migrate from an area of high cancer risk to an area of low cancer risk, or the reverse, Dr Blair said. They invariably take on the cancer rates of the country to which they move.

"Since the gene pool changes only after many generations, this means that these changes must be environmental, not genetic," said Dr Blair. "And so, the migrant studies very clearly tell us that the wide range of cancer rates is largely driven by environmental causes."

For decades, Dr Blair said, scientists have been conducting epidemiological investigations of the causes and distribution of cancers, looking at a variety of environmental and host genetic risk factors. "Almost always," he said, "the cancer burden is much greater for environmental causes than just the hereditable genetic factors."

Surviving cancer

But more people diagnosed with cancer are living longer today than ever before. A new report by the Centers for Disease Control and Prevention (CDC) and the National Cancer Institute (NCI) shows that 64 per cent of adults whose cancer is diagnosed today can expect to be living in five years.

To prolong life for cancer survivors, avoiding further exposure to cancer causing factors is critical. Dr Blair says the specific environmental factors involved differ by tumor.

Tobacco smoke is the major cause of lung cancer, he says, "But there is a long list of other chemicals that cause lung cancer - arsenic, asbestos, PAHs (polyaromatic hydrocarbons), and chromium, to name a few."

"For breast cancer, hormone use is one of the major factors affecting risk. Prostate cancer has nothing that reaches the level of evidence of lung or breast cancer, although there are a number of strong leads. Physical inactivity is strongly linked to colorectal cancer, as well as a number of dietary factors - low fibre is probably implicated," said Dr Blair.

Tobacco as a cause of cancer is easy to study, Dr Blair says, because it is a single agent. Diet and environmental pollutants are more difficult, he said.

"The major limitation is that it is so difficult to characterise a person's diet over time," he said. "Typically it's not the diet today that is important for the cancer diagnosed today. We need to know what people ate in the past, and that is really hard to determine."

Environmental pollutants are linked to the incidence of cancer and its recurrance, although this field is also difficult to study, said Dr Blair. "Researchers are beginning to focus on potentially hazardous substances in the water and air," he said. "This is a difficult research area and is every bit as hard to study as diet."

Copyright Environment News Service (ENS) 2004. All Rights Reserved. This is a shortened version of a report which first appeared on the website of ENS.

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