The risk of cancer from long-term exposure to low doses of radiation could be as much as 10 times higher than previously thought, according to controversial findings of a federally mandated study of workers in a U.S. atomic research laboratory.
The study, tracking 8,318 employees of the Oak Ridge National Laboratory over 40 years, unexpectedly found high rates of leukemia deaths and an overall cancer death risk related to radiation dose that is 10 times higher than has been seen in atomic bomb blast survivors.
The findings, published today in the Journal of the American Medical Assn., run counter to those of numerous shorter-term studies. If confirmed, they could call into question existing standards intended to protect workers from radiation dangers.
"If it turns out that very low levels are dangerous, or carcinogenic, the policies in place may be doing nothing or little to reduce the number of (cancer) cases," said Steve Wing, lead author of the article.
Current protection standards are based largely on data from atomic bomb survivors.
Numerous scientists, including the authors, said they were surprised by the results and reluctant to draw definitive conclusions. They pointed out that the researchers had no information on other carcinogens the workers may have been exposed to.
"Although I can't say with 100% certainty, 'No, I don't think these results are real,' . . . I would be surprised if these really were a reflection of a larger than expected radiation effect," said Geoffrey R. Howe of the National Cancer Institute of Canada.
The study tracked white males hired between 1943 and 1972 by the Tennessee lab, which performs research and development in fission, fusion and other energy technologies. White males were chosen by the researchers because they had higher radiation exposures and more deaths. They followed them through 1984, by which time 1,524 had died.
By examining death records and radiation exposure records, the researchers concluded that the leukemia death rate was 63% to 123% higher than in the general population. One cause of leukemia is exposure to radiation, a well-known carcinogen.
Similarly, they found that radiation exposure levels were related, to a surprising degree, to death from cancer in general and death from other causes. Radiation has been linked to solid tumors, such as cancers of the breast, lung and thyroid.
Finally, by comparing workers exposed to different doses of radiation, the researchers found that the cancer death risk related to radiation dose was 10 times higher than has been estimated from studies of survivors of the bombings of Hiroshima and Nagasaki in Japan.
Asked why their findings appear to contradict those of previous studies, the researchers noted that they followed their subjects over a longer period. Most of the leukemia deaths occurred among men exposed during the earliest part of the period studied, when radiation exposure levels were highest.
As for the atomic bomb survivors, Wing pointed out that there were no accurate measurements of the amount of radiation to which they were exposed. It is also difficult to estimate the effect of protracted, low-dose exposure using data from single, high-dose exposures.
"There are lots of reasons to question the validity of the results of this study, just as there are reasons to question the A-bomb data or any other data," Wing said. "That's one of the challenges--to figure out what to do when we have uncertain information."
One of the limitations of the study is the absence of information on other chemicals or carcinogens to which the workers may have been exposed. Similarly, the researchers had no information on workers who developed cancer but did not die of it.
"It is important to note that workers in the Wing study have been exposed on and off the job to other potential causal factors, such as hazardous and toxic materials," said Dr. Paul L. Ziemer of the U.S. Department of Energy, which runs Oak Ridge.
"We have to be sure that there is not some confounding factor that may be present in this study that is not detectable," said William R. Hendee, a physicist with a background in radiation medicine who is a vice president of the American Medical Assn.
In an interview Tuesday, Hendee said knowledge of the health effects of radiation has emerged gradually. Long-term effects were not recognized until the 1950s. It has gradually become apparent that certain radiation is more carcinogenic than thought.
In December, 1989, a panel of scientists reported that the risk of developing cancer from low levels of radiation could be three to four times greater than thought and that children exposed to radiation in the womb were at greater risk of mental retardation.
"I don't think the average person should worry about the next time they go to the dentist to get an X-ray or that sort of thing," said Harvey Checkoway , a University of Washington researcher who headed an earlier phase of Wing's study.
"There may be some demonstrable effect on (workers') risk over a lifetime," he said. "That effect may be small when you stack it up against things like smoking or reckless driving. But that effect may be real."