07:45am Wednesday 20 September 2017

Chernobyl Studies Offer Perspective on Radiation Risks

However, the amount of radiation released in Japan thus far is very small in comparison, and officials and workers still hope to prevent more catastrophic damage. The spread of radioactive contaminants into the atmosphere from the Chernobyl accident was detected globally.

Stuart Heard

Stuart Heard

While the spread of radiation from Japan might also reach detectable levels in the United States, many state and federal officials quoted in the news media have indicated that they expect any amount of radiation that reaches the United States will be too low to result in increased health risks.

Nonetheless, there are many reports of customers cleaning out pharmacy shelves and internet suppliers in a rush to buy potassium iodide tablets.

The tablets offer some protection when taken immediately before or during an exposure to radioactive iodine by competing with radioactive iodine for a place in the thyroid gland. But there is no point in taking potassium iodide in the absence of an exposure, health officials say, and individuals who are allergic to iodine or shellfish may have adverse reactions.

Lydia Zablotska

Lydia Zablotska

“People should not be taking potassium iodide now,” says Stuart Heard, PharmD, executive director of the California Poison Control System and professor with the UCSF School of Pharmacy. “It can have side effects. It’s not appropriate for everybody. People should not take it unless they are properly advised.”

Most Iodine Contamination Was from Food

Studies of the health effects of the Chernobyl disaster in 1986 are still underway and may shed light on potential risks faced by workers and many Japanese in the region of the damaged nuclear reactors.

UCSF epidemiologist Lydia Zablotska, MD, PhD, has published several research studies on health impacts due to radiation released during the accident. Her recent, ongoing studies have focused on thyroid cancer among those exposed as children or adolescents in Ukraine and Belarus, and on leukemia and other blood disorders among clean-up workers.

The incidence of thyroid cancer due to release of the short-lived radioactive isotope iodine-131 rose as a result of the Chernobyl accident. Zablotska and her colleagues from the Radiation Epidemiology Branch of the National Cancer Institute have observed that most of the exposure to radioactive iodine was due to consumption of contaminated foodstuffs within a few months of the accident — although there was some airborne exposure while nuclear fuel burned and smoked and was spread by winds.

“The portion of the dose that came from inhalation was minimal, and it really was limited to the first week after the accident,” Zablotska says. Most of the exposure was due to ingestion of milk, other dairy products and leafy vegetables contaminated with this radioisotope during the two months after the accident, she says.

No Risk to Thyroid Detected in Adults

Zablotska has been studying 25,000 individuals who were children living in Ukraine and Belarus at the time of the Chernobyl accident. Local scientists measured thyroid radioactivity in hundreds of thousands of children during the first six weeks after the accident.

Zablotska and her research team traced them and invited them to participate in the study. The researchers screened participants several times for thyroid diseases, starting in 1998. They also gathered additional information on food-borne exposures through in-person interviews with research subjects or their mothers.

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“Our own studies have conclusively shown that those exposed before age 18 years are at risk for developing thyroid cancer and select other thyroid diseases in relation to exposure to radioactive iodine,” Zablotska says.

The thyroid gland absorbs much more iodine in growing children. Radiation-related risks decreased with age at exposure, according to Zablotska, and no studies to date have shown any increase in risk for those exposed as adults.

Other radioisotopes released in much smaller amounts after the accident included long-lived cesium-134 and cesium-137, and strontium-90. Strontium can compete with calcium for absorption into bone, Zablotska says. She has been studying 110,000 workers exposed to penetrating gamma radiation from these radionuclides during clean-up at the Chernobyl plant that took place between 1986 and 1990.

Zablotska is careful to emphasize that all accounts indicate that Japanese nuclear workers, in contrast to clean-up workers at Chernobyl, are being monitored for exposures and routinely use protective clothing and gear.

Similarly to the United States, “The nuclear industry in Japan is very highly regulated and workers are closely monitored,” Zablotska says. In her study of 54,000 US nuclear power plant workers she reported that average total exposure over an average of 12 years of monitoring was 25.7 millisieverts.

By comparison, a full-body, medical CT scan may expose an individual to about 12 millisieverts of radiation, she says. Individual absorbed doses to some organs could be as high as 30 milligrays from one CT scan.

Organs differ in their sensitivity to radiation energy. The millisievert is a unit of radiation used to measure a dose averaged over all tissues of the body, and often is the measure used in studies of workers exposed to penetrating gamma radiation. A milligray is a measure of the dose received by specific organs. One-thousand millisieverts is equal to one sievert, and one-thousand milligrays is equal to one gray.

Zablotska estimated that the exposure to Chernobyl clean-up workers in her study averaged 76.4 millisieverts. There is a significantly elevated dose-dependent risk for leukemia among the workers, she has found.

“At this point, you really can’t predict what risks there might be to workers or to the local population in Japan until we know more about the radioactive releases from the damaged reactors,” she says.

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