Photo by Dean Forbes
Scientists have found additional evidence that environmental exposure to specific chemicals and industrial waste byproducts may be associated with increased risk of non-Hodgkin's lymphoma.
By comparing blood levels of certain organochlorines, including polychlorinated biphenyls (PCBs), dioxins and pesticides in 100 pairs of healthy volunteers and non-Hodgkin's lymphoma patients, Center researcher Dr. Anneclaire De Roos of the Public Health Sciences Division's Epidemiology Program and colleagues found that higher levels of three specific molecular forms of PCBs were linked to an increased risk of developing cancer that starts in lymph tissue. The research appears in the Dec. 1 issue of Cancer Research.
The types of chemicals researched by DeRoos — organochlorines — contain carbon and chlorine atoms joined together. Organochlorines can be harmful because they do not break down easily and stay in the environment and in our bodies for a long time.
The study also showed a first-ever correlation between risk of non-Hodgkin's lymphoma and higher blood levels of total dibenzofurans, or furans. Furans and dioxins form as byproducts of waste incineration and other industrial processes and are present in the environment at lower levels than PCBs.
"Previous studies have seen an association between PCBs and lymphoma, and we wanted to confirm that," said De Roos, who is also an assistant professor of epidemiology at the University of Washington. "Finding that the furans were also associated with lymphoma was surprising."
Lymphomas are cancers affecting cells of the lymphatic system — the network of vessels and nodes that carry infection-fighting white cells throughout the body. According to the American Cancer Society, nearly 54,000 Americans will be diagnosed with the disease this year, and 19,000 will die from it.
Incidence of non-Hodgkin's lymphoma in U.S. white men rose to unprecedented levels during the latter half of the 20th century. However, the rates seem to be tapering off in recent years. So far, the only clear explanation for some of the rise in lymphoma cases in the 1980s is the AIDS epidemic, as there is an association between HIV and lymphoma.
Exposure to PCBs
The rise in incidence also corresponds with the same time period in which synthetic PCBs were manufactured and released into the environment in this country. PCBs were used as lubricants and coolants in electric transformers and the like because of their non-flammable properties. Although production was banned in the United States in the 1970s due to concerns about their toxicity, PCBs persist in the environment because they break down slowly. PCBs are still in use today in some developing countries.
"The residual environmental levels of PCBs have been decreasing, but as with all organochlorines, they stick around in the environment for a very long time and accumulate in the food chain, so they end up in fish and fatty foods like dairy, eggs and meat," De Roos said. "That's where we get most of our exposure today."
Nonetheless, the presence of PCBs in the environment and even in the blood of humans doesn't necessarily mean that these substances cause cancer.
De Roos said, it's definitely hard to prove environmental exposure is a culprit. "The doses you get from the environment are low, so the relative risks are expected to be low, as well," she said. "It is difficult to prove these associations compared to something you might be exposed to every day in high doses, like medications".
"It might be hard to pinpoint an exact cause, but there are now several studies showing some association between PCBs and non-Hodgkin's lymphoma — so the evidence is accumulating."
While the current report adds more evidence about PCBs and cancer, it was not designed to produce the "smoking-gun" evidence that defines the molecular chain of events that instigate cancer. Also, studies of workers with high occupational exposure to PCBs have not detected higher than usual lymphoma rates, adding uncertainty to the relationship.
De Roos recently received funding to do further analysis of the study data to look at where the study participants lived and identify industrial activity around those areas.
Limiting fat and certain fish
Can people lower their exposure to environmental toxins? Yes, said De Roos. "Among the many benefits of eating a lower-fat diet, it will decrease your exposure to organochlorines," she said. "There are also specific health advisories about limiting fish intake from certain bodies of water, like the Great Lakes."
The study was conducted through the National Cancer Institute's intramural research program. The Center was one of the study sites, led by principal investigator Dr. Scott Davis, head of the PHS Radiation/Environmental Exposure Studies group. Davis is also one of the paper's co-authors.