The evidence to support the idea that environmental exposures are associated with cancer risk comes from many types of studies. These include studies of migrant populations that often acquire the cancer patterns of their new countries; changes in cancer incidence over time; studies of twins; variation in cancer risk across geographic areas; increased cancer risk among certain occupational groups; and evidence from basic and toxicological research.
A Conversation With
A Conversation with Drs. Deborah Winn and Shelia Zahm about Environmental Cancer Risks
A report on environmental cancer risks was issued last month by the President’s Cancer Panel, an independent advisory board that monitors the development and execution of activities of the National Cancer Program, and reports directly to the President. To learn more about research in environmental cancer risks, the NCI Cancer Bulletin sat down with Drs. Deborah Winn and Shelia Zahm, who are, respectively, the deputy directors of NCI’s Division of Cancer Control and Population Sciences and Division of Cancer Epidemiology and Genetics.
What is meant by environmental exposures?
Dr. Zahm: The environment can be defined broadly as anything other than genetics, but the recently released President’s Cancer Panel report focused on chemical agents and physical agents, such as environmental contaminants and radiation. Tobacco, diet, and viruses, for example, might also be considered environmental exposures, but these were not a focus of this year’s report. The President’s Cancer Panel addressed these exposures in previous reports.
What is the evidence linking environmental exposures and cancer?
Dr. Zahm: Many lines of evidence support the idea that environmental exposures are associated with cancer risk. (See sidebar) But, while there is no doubt that some environmental exposures cause cancer, there’s also a lot we don’t know. We have more to learn about the effects of low doses of environmental exposures, about interactions among multiple exposures, and about newly introduced exposures, for example. We also need more research on the periods of time over a lifetime when a person may be most susceptible to the effects of environmental exposures.
What are some of the challenges in studying these questions?
Dr. Winn: One of the challenges is that people do not always know what their environmental exposures have been. For instance, people are much less likely to be able to report the levels of radon in their homes than their smoking habits or medical conditions. In addition, because of the long time it takes most cancers to develop, we need information about exposures in the distant past.
Are researchers working on this problem?
Dr. Winn: Yes, and we’re having success in several areas. NCI is a partner in the NIH Genes, Health and the Environment Initiative that includes research to develop new methods for monitoring personal environmental exposures and exposures where people live and work. The research has led to the development of new sensors that people could wear on a lapel, say, to measure environmental exposures during daily activities. This could capture exposures in real time and over time. Meanwhile, some groups are using geographic information systems, mapping data on exposures to maps of cancer cases, which could give us clues about hazards.
Does a person’s age play a role in the risk from exposure?
Dr. Zahm: Studies have shown that there are windows of susceptibility. For example, women exposed to radiation below age 20 are at higher risk of radiation-associated breast cancer than those exposed at older ages. This finding came from studying women with scoliosis or tuberculosis who were monitored by X-ray or fluoroscopy, a procedure that involves radiation exposure.
Thyroid cancer is another example. Studies of atomic bomb survivors in Japan and the population near the Chernobyl nuclear accident have shown that persons exposed as children are at higher increased risk of thyroid cancer later in life than persons exposed as adults.
What can you learn by studying people in high-exposure areas?
Dr. Winn: Studying people with high exposures, either in the United States or internationally, has led to important discoveries about cancer risk factors. These high-exposure areas have been extremely important for discovering cancer-causing agents in the environment and for understanding mechanisms of action. NCI has for many years partnered with international investigators in places where air pollution is extremely high or heavy occupational exposures occur in relation to mining or manufacturing.
Dr. Zahm: In another example, DCEG has collaborated with the Chinese Center for Disease Control and Prevention on studies of occupational exposure to benzene. This work has greatly increased our understanding of the carcinogenic risks of benzene and helped set standards for benzene exposure in the United States and abroad. Because the exposure levels in China included a broad range, investigators were able to evaluate whether risks increase with increasing exposure—which supports a cause-and-effect relationship—and also to study effects at levels relevant to the regulatory standard targets in this country.
How are you studying exposures in this country?
Dr. Zahm: Using the Atlas of Cancer Mortality in the U.S., we can identify geographic areas with elevated rates for certain cancers and target epidemiologic studies to investigate the risk factors responsible. For example, with respect to the high rates for bladder cancer in New England, the exposures of interest include arsenic, which is present naturally at relatively high levels in the groundwater. We are also studying occupational groups that have unusual cancer patterns, such as the farmers in the Agricultural Health Study, miners exposed to diesel exhaust, and workers exposed to formaldehyde. Another important area of research underway in DCEG is determining the long-term risks associated with the recent dramatic increase in medical radiation.
Is it ever possible to determine the cause of an individual cancer?
Dr. Zahm: It is not usually possible to determine on an individual basis whether a particular tumor is caused by a specific exposure based on histologic or molecular genetic markers. There are a few exceptions, such as mesothelioma associated with asbestos fibers that persist in the lung, but generally we can only talk about increased risk on a population level.
How do you study multiple exposures?
Dr. Winn: We have techniques for human population studies that can help us unravel the effects of different exposures people experience. Each person may behave in ways that increase or decrease his or her health risk, for instance, through diet or smoking behavior, and genetic or other predisposing factors may also influence cancer risk. The goals in population studies are to capture and understand all of the risk factors, and, once the other known factors are accounted for, to identify the environmental exposures that might be responsible.
What proportion of cancer is due to environmental exposures?
Dr. Zahm: The exact proportion is not known and, in fact, varies from country to country and as exposures change over time. Our understanding of the proportion also changes as we do more research. Tobacco is undoubtedly the most significant cause of cancer, but other environmental exposures are important and avoidable causes of cancer as well.
What is your view of the future of environmental cancer research?
Dr. Winn: The more research we do, the more we learn. Along with the research, the development of improved methods for capturing data on environmental exposures is critically important. Training the next generation of environmental scientists is also key.Dr. Zahm: As the excitement surrounding research on genetics continues to grow, it will be important to continue to focus attention on the role of environmental exposures. The way we will learn the most about carcinogenesis, I believe, is by looking at genes and environment together and how they interact.
—Edward R. Winstead