People with non-Hodgkin lymphoma have higher levels of environmental contaminants in their blood than people without the disease, say researchers at the B.C. Cancer Agency.
That suggests the chemicals may play a role in the disease.
The cancer agency collected blood samples from almost 900 British Columbia residents, 422 of whom had non-Hodgkin lymphoma. They tested the samples for exposure to different polychlorinated biphenyls (PCBs), and organochlorine pesticides.
The cancer patients had higher levels of almost every chemical they investigated, reports lead author John Spinelli, a senior scientist at the agency.
Those who had the highest exposures to PCBs showed twice the risk of developing the lymphoma compared to those with the lowest exposure.
The strongest association was found with oxychlordane (a by-product of the pesticide chlordane). Those with the highest levels of oxychlordane had 2.7 times the risk of lymphoma compared to those with the lowest exposure.
"We know that the incidence of non-Hodgkin lymphoma has been steadily rising for the past 30 years worldwide, but there hasn't been clear evidence to explain the increase," Spinelli said. "Our study helps to provide answers to this puzzle by showing a strong link between these specific environmental contaminants and this particular type of cancer."
The study will be published in the International Journal of Cancer.
Most organochlorine pesticides, such as DDT, have been banned in Canada, but many are still in use in other parts of the world.
The use of PCBs is strictly limited, but in the past they were more widely used, as flame retardants, in hydraulic fluid and lubricating oil, as well as in pesticides and paints.
Spinelli says the findings are surprising, given that so many of the chemicals tested have been banned in Canada for so long. But it's long been known that many of the chemicals they examined have long half-lives and can accumulate in fat cells.
"What we can say is that levels increase with age; the older we are, the more exposure," he told CTV News. "Part of that is just that these contaminants build up in our bodies and the other part of it is that the levels were much higher 30 years ago, when these chemicals were in the greatest use."
Many of these chemicals are still finding their way into our food supply, primarily in fish and meat, Spinelli says.
"So even though they're not used anymore and they're not produced in Canada, they're still present and that's where we get most of our exposure these days is through diet; about 90 per cent of our organic chlorine exposure comes through diet," he says.
"Organochlorines are stored in fat, so it's fatty foods that have the highest levels. The highest levels in Canada are found in fresh water fish in the Great Lakes. But any animal product that has fat will contain some amount of organochlorine."
Non-Hodgkin lymphoma is a large group of cancers of the lymphatic system, which helps to protect the body from disease. One of the suspected risk factors for developing the cancer is exposure to pesticides. But how those chemicals lead to cancer is still unclear.
"Really, we don't know the answer," says Spinelli. "There are many hypotheses, one is that these toxins in the body suppress the immune system and one of the known causes of non-Hodgkin lymphoma is suppression of the immune system."
He notes, though, that genetics may play a role as well.
"Looking strictly at environmental factors won't provide the full picture," he said. "Our next step is to identify genetic factors that make individuals more susceptible to these environmental contaminants.
"So we're looking at the interaction between genes and the environment and that may give us a much better clue as to what the actual mechanism that these chemicals cause non-Hodgkin lymphoma."
