Friday, March 25, 2011

Fallout at Fukushima

Fallout at Fukushima
What risks does Japan face as a result of radiation leakage from the nuclear power plant hit by the recent earthquake and tsunami?

[Published 22nd March 2011 03:31 PM GMT]

Technicians in Japan struggle to contain breeches in cooling and containment apparatuses at the Fukushima Daiichi nuclear reactor in eastern Japan, which was hit by the massive earthquake and subsequent tsunami on March 11th. Though considerable uncertainty remains concerning the exact amount of radioactive material that has leaked from the facility thus far, low level radiation has turned up in crops grown in the vicinity of the plant, and the danger of a widespread catastrophe lingers. This week, The Scientist examines the latest research on the effects of radiation and explores some of the worst-case-scenario health and environmental effects of a nuclear disaster in Japan.

The acute effects of radiation

Late last week, a skeleton crew of about 50 workers at the Fukushima Daiichi plant was urgently attempting to cool the reactor core, as specially-fitted helicopters tried (and failed) to drop tons of seawater on the failing facility. Early this week, reports from Japan indicated that the last workers trying to save the facility from catastrophe evacuated as smoke billowed from two of the reactor units. Radiation levels currently being reported by Japanese officials are still quite low, and the early public evacuation reduced the concern for community health risks, said William Schull, emeritus professor at the University of Texas Health Science Center in Houston and an expert on the health effects of the atomic bombs dropped on Hiroshima and Nagasaki in the 1940s. But the workers at the nuclear plants still risked acute radiation exposure and serious health problems as a result, he added.

Upon direct exposure to ionizing radiation, anemia -- the loss of red blood cells -- and leucopenia -- the loss of white blood cells such as those important in fighting off infection -- can result, increasing susceptibility to disease. In addition, someone directly exposed to radiation may display other symptoms of acute radiation syndrome (ARS), such as vomiting, diarrhea, excessive bleeding brought on from the death of hematopoietic stem cells in bone marrow, and hair loss. Such symptoms, however, are caused by exposure of 1-10 grays (Gy), a unit of absorbed radiation dose. The doses of radiation leaking from the Japanese reactor are far below this: On Saturday, Japanese news outlet NHK News reported that the plant's operator, Tokyo Electric Power Company, detected radioisotope iodine 131 at about 5.9 milibecquerels per cubic centimeter, or about 0.0003 Gy/hour. (Click here for an infographic comparing the radiation dose absorbed by humans engaged in various activities.)

But "things could quickly worsen," Schull said. If radiation continues to seep from the reactor, officials could use a recently devised classification system to assess the health of those exposed. The Radiation Injury Severity Classification (RISC) system estimates three sets of clinical and haematological parameters to calculate "a combined score [that] gives you a pretty accurate estimate of what's going to happen to this person," said University of Pittsburgh biostatistician Richard Day, who collaborated in the creation of RISC. Applying the system to 59 workers in a Russian nuclear fuel production facility, Day, Niel Wald of the University of Pittsburgh, and coauthors estimated threshold values for some ARS symptoms, including vomiting (∼1.5 Gy), severely low white blood cell count (∼3.5 Gy), and mortality (∼6-7 Gy).
"In the roughly 115 years since Roentgen discovered X-rays we have learned a lot about the values and hazards of exposure to ionizing radiation," Schull said. "But we still have a hell of a lot to learn."
-- Bob Grant
Radiation and the immune system
Although most cells in the body can withstand considerable doses of radiation before dying, immune cells begin to react at even small doses of radiation. While recent reports suggest that the workers at the Fukushima Daiichi plant have so far only been exposed to relatively low levels of radiation, their exposure could trigger immunological reactions -- though depending on the dose, not all of them may be harmful.

According to several reports, workers at the Fukushima plant have been exposed to radiation levels ranging from 200-400 millisieverts (mSv, a measure of radiation absorbed by a person) per hour -- levels that the human body can withstand with minimal damage, said Richard Wakeford, visiting epidemiology professor at the Dalton Nuclear Institute of the University of Manchester. Once doses reach levels of 500 mSv or more, however, the number of lymphocytes, white blood cells involved in immune response, is cut by half within a few days, and there is considerable damage to stem cells in the bone marrow, said Yoichiro Kusunoki, chief of the department of radiobiology and molecular epidemiology at the Radiation Effects Research Foundation in Hiroshima, Japan, in an email.

At high doses, these effects can be long lasting. According to studies of atomic bomb survivors, T-cells never fully recover, neither in number nor effectiveness, although stem cells and other immune cells bounce back to normal levels in about two months. The body compensates for these shortfalls by increasing levels of inflammatory cytokines -- a pattern that resembles the immune systems of the elderly, suggesting that the immune system may age more rapidly after radiation exposure.

But the production of inflammatory cytokines can be seen even at "a relatively low dose (several mSv) that does not trigger apoptosis of any types of cells," added Kusunoki. This short term inflammation could initially be protective by helping clear cells damaged by the radiation. However, researchers studying radiation exposure during cancer radiation therapy suggest that low doses of radiation that trigger inflammation could also initiate the kind of chronic inflammation that leads to cancer.
Read more: Fallout at Fukushima - The Scientist - Magazine of the Life Sciences

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