A new study by a University of Georgia researcher, to be published soon in the Journal of Toxicology and Environmental Health, however, has found that many of the PCBs in the environment in this country may not be as carcinogenic as previously thought.
"We have better tools now than we did a few years ago to assess what is a carcinogen and what is not," said Dr. Mary Alice Smith, a UGA environmental toxicologist and author of the study. "This study raises an important issue -- how can we incorporate new knowledge gained from science into federal regulations quickly and easily?"
The project was funded in part by the Georgia Agricultural Experiment Stations and by a consulting firm working for the General Electric Corporation.
The issue of how to assess the risk accurately from PCBs is complicated, since there are many formulations of PCBs, from some that are not cancer-causing (using current definitions) to those that are serious threats, based on animal tests. Scientists made original estimates of how dangerous PCBs might be in the environment through the use of baseline information gathered on the most toxic PCBs. The problem, Smith asserts, is that most PCBs released in the environment aren't as toxic as those used to define baseline standards years ago.
Beginning in 1929, PCBs were manufactured for a variety of uses, especially as insulating fluids in transformers and capacitors but also for use in adhesives, inks, paints, lubricants and plastics. More than 750,000 tons of the oily liquids may have been released into the environment. PCBs are a family of some 209 chemical compounds composed of carbon, hydrogen and chlorine and are extremely stable over a variety of conditions. Unfortunately, that stability came back to haunt scientists when it was discovered that PCBs caused cancer in laboratory animals and may cause human health problems. Because the compounds persist under many conditions, they were found to be almost pervasive in the environment -- especially in soils, due to leaks from transformers and industrial accidents.
Starting in the early 1970s, PCBs were found in fish, wildlife and in human organs, blood, fatty tissue and milk. Worse, PCBs tend to become more concentrated as they move up the food chain from microbiota toward humans, where they can cause skin lesions, liver disease and may cause birth defects or cancer. More than half the people in the United States have some PCBs in their bodies, researchers say.
Despite the ban on production of PCBs, Smith said there are likely many transformers insulated with PCBs still on utility poles.
"I decided to look at the issue of carcinogenicity again because new scientific information over the past few years has changed how we classify cancers and tumors," said Smith. "Pathologists now use a different set of criteria to call a cell a cancerous cell."
Because of the new standards, a group of pathologists brought together by the Food and Drug Administration, the Environmental Protection Agency and business in 1990 took the original slides of tissues from laboratory rats examined in the 1970s and 1980s and re-evaluated them. The study results showed that all PCBs are not equally potent in causing cancer, according to Smith.
The EPA uses a test called a cancer slope factor to estimate the lifetime probability that an individual will develop cancer as the result of exposure to a particular level of a potential carcinogen. Smith's study was in part a response to see if the panel's 1990 reassessment of PCB toxicity would change the cancer slope factor. She also wanted to see if three PCB formulations of different chlorine content were equally potent tumor producers.
"What we found was that animal test results were based on the most toxic kind of PCBs, while less toxic forms were most often used in the environment," said Smith.
"This means that most environmental PCBs are probably less carcinogenic than we thought. And we are starting to see a change in focus regarding PCBs. At one time, we focused all our regulations on how carcinogenic PCBs are, but maybe some other things need to be driving the regulations now."
Despite the finding, PCBs remain a serious environmental threat. If anything, Smith's research may refocus attention on other effects of PCBs beside cancer-causing ones.
Cleanup of PCB-contaminated sites is expensive, since contaminated soils have to be dug up and taken to detoxification sites. (Even there, it is not easy, since heat-degraded PCBs produce chemicals called dioxins, which have problems all their own.) Since 1973, a number of microorganisms that can degrade PCBs have been isolated and characterized, but so far this method of clean-up has been relatively unsuccessful.
One study cited in Smith's paper claims that it could cost as much as $100 billion to remove PCBs from the environment. Smith believes that clean-up efforts should be based on the specific PCB formulation found at a site, not based on overall recommendations made more than two decades ago.