Boulder, Colo. — Confirming earlier assessments, new results from ultrasensitive medical tests and related dose analyses indicate that none of the 29 personnel tested to date for internal exposure to plutonium as a result of a June 9 incident at the National Institute of Standards and Technology (NIST) is expected to experience any clinically significant health impacts.
"The TIMS tests are a much more sensitive method for detecting plutonium exposure and confirm earlier testing results that showed low levels of exposure," said Dr. Edward Cetaruk, a toxicologist with the University of Colorado Health Sciences Center, who is providing medical care and consultation to the affected individuals. "Since they have a lower limit of detection, the new tests have identified some additional personnel with evidence of exposure. However, as these levels of exposure are even lower than those found in the initial round of testing, we still do not foresee any significant health impact."
On July 10 NIST announced that results from a technique known as alpha spectroscopy showed that a small number of individuals had received internal plutonium exposure as a result of the June 9 spill. The precise number was not given to protect the medical privacy of the affected individuals.
The latest tests used a technique called thermal ionization mass spectrometry (TIMS), which directly detects individual atoms of plutonium and is significantly more sensitive than alpha spectroscopy. About half the 29 TIMS tests detected plutonium. However, almost all of these associated doses are extremely low, a cumulative radiation dose below 25 millirems (0.25 millisieverts) from plutonium in the body over a 50-year period, according to radiation health physicist Richard Toohey of Oak Ridge Associated Universities. The remaining small number of TIMS tests showing detectable doses are below 350 millirem (total dose over a period of 50 years).
For perspective, while the doses quoted above are for a 50-year period, the average annual exposure of the general public to natural background radiation in the Boulder, Colo., area is about 450 millirem (4.5 mSv) (22,500 millirem over 50 years), based on estimates from the National Council on Radiation Protection and Measurements. Natural background radiation comes from cosmic rays, the ground (e.g., rocks) and the atmosphere (e.g., radon gas).
Bioassay results and dose analyses were received and communicated by a physician to all the affected individuals.
TIMS results for six additional personnel whose bioassay samples were received later than the 29 already reported are expected in about a month, according to NIST.
"We hope that having these definitive tests confirm that there are no significant health effects expected will reassure those affected and their families," NIST Deputy Director James M. Turner said. "Nevertheless, we continue to regret that any exposures occurred. Meanwhile, we are aggressively moving to strengthen NIST's safety system from top to bottom."
The latest round of bioassays including the TIMS tests were performed by Los Alamos National Laboratory, part of the U.S. Department of Energy (DOE) National Nuclear Security Administration. In addition, advice and consultation on radiation emergency medicine was provided by the DOE Radiation Emergency Assistance Center/Training Site (REAC/TS).
Additional information on the NIST Boulder plutonium incident is available online.