Kathleen Farhang, BS
Peter Faulhaber, MD
Raymond F. Muzic, Jr., PhD
Nancy Oleinick, PhD
Pejavar Sridhar Rao, PhD
click here for more information about the speakers
On March 11, 2011 a 9.0 magnitude earthquake produced a tsunami that hit the Fukushima Daiichi nuclear power plant and caused nuclear meltdowns and release of radioactive materials. The release was about was about twelve-thousand times greater than that of the Three Mile Island (March 1979) incident and about one seventh of that in the Chernobyl disaster (April 1986). Although three workers died from the Fukushima disaster, there were no immediate deaths due to radiation exposure. (In comparison, the earthquake and tsunami killed almost 20,000 people.) A 20-km evacuation zone was created and tens of thousands of people were displaced. The immediate concern was the release of I-131 which humans could ingest by drinking milk from cows that eat contaminated grass. Ingested I-131 accumulates in the thyroid where it can damage the thyroid cells’ DNA, potentially causing thyroid cancer, until it has decayed away (with a half-life of 8 days). This is a significant concern for children, who consume more milk and whose thyroids are more radiosensitive than adults. By now, the I-131 has decayed so the concern is the remaining Cs-137 which has a much longer half-life (~30 years). Because Cs-137 emits strong gamma-radiation, it can damage the DNA of people who walk on contaminated ground and if ingested remains in some tissues essentially as long as the person is alive. Even so, the impact on the US population has been negligible. For example, even at its peak (early April), the activity concentration in milk – measured using highly sensitive laboratory instruments – in the San Francisco area was so low that one would have to drink thousands of liters of it to achieve the same radiation exposure as a single round-trip flight from Washington DC to San Diego California. Such exposure is also less than 2% of typical annual background radiation exposure in the US.
What exactly is the annual background level you might ask. It is about 3 mSv in the US. At such low levels the effects, if any, are very small and hard to measure. Some experts consider a linear-no-threshold model; e.g. If an exposure of x has biologic effects y, then an exposure of x/1000 has biologic effects y/1000. Others think that our bodies are adapted to living in an environment with low levels of radiation so that below a certain exposure threshold there is no biologic effect. A few even think that a little radiation may be good. Regardless, most agree that if the effects were not so small, we would could make conclusive statements from available data. This is not possible, though as the experiment would require exposing a very large population to radiation on purpose.
What are the risks of radiation exposure? High level - like radiation workers in Fukushima - could have radiation sickness, cataracts, and even die. These are called deterministic effects. What is far more relevant to all of us is low-level exposure. For these there are no deterministic effects. Rather we think in terms of probabilities (stochastic effects) of getting cancer, for example. Such risk is commonly estimated using the linear-no-threshold model to extrapolate from high doses and this is highly speculative at doses less than 100 mSV, about 40-times the US background. Nevertheless, an example of this sort might forecast that a young person is 10 times as likely to have a fatal car accident than to get cancer, fatal and non fatal, from a CT scan over a 50-year period. Hence, when used wisely, the value of the CT to your health is well-worth the risk.
What are sources of background radiation? These may be broadly classified as from natural sources, approximately 80%, and man-made, approximately 20%. Radon gas accounts for about half of the naturally-occurring and medical accounts for about 80% of the man-made.
Some of our food is naturally radioactive. Bananas are high in potassium some of it it is potassium-40 (even in absence of pollution and contamination). Thus, people have whimsically introduced the concept of banana equivalent dose BED. This is the radiation dose equivalent due to eating one banana. This amounts to about 0.1 microSv. On this scale, background radiation in one year amounts to approximately 30,000 BEDs.
This Science Cafe will introduce the concepts of radiation dose, type of radiation, half-life, and other parameters that must be considered in assessing the risks of exposure to incidents such as the Fukushima event.
American Nuclear Society:
WHERE: The Market
25th Street next to the West Side Market, Cleveland, Ohio)
WHEN: March 12, 2012
discussion starts around 7:00 pm
WHO: Sponsored by Case Western
Reserve University chapter of Sigma Xi, WCPN
ideastream, and the Market
Click here to download a pdf of the event flyer