At Rice University, we want to educate our students to be prepared for the challenges they will face in the real world. With that in mind, I have been teaching a course called “Physics in Society” that teaches nonscience students the physics that may affect policy and business decisions. In the couple of months leading up to the events in Japan, our class learned about earthquakes, tsunamis, radiation, nuclear reactors and meltdown of nuclear reactors. Little did I know how relevant the material was going to become after March 11.
One of the things our class has learned about is the danger of radiation. It is well known that accumulated exposure to radiation can increase you risk of getting cancer. As a rule of thumb, one assumes that an exposure of 2500 rem (25 Sieverts) — a measure of the biological risk of a radiation dose — gives one a 100 percent chance of getting chance of cancer. Then one uses a linear extrapolation to calculate the risk of cancer from lower doses. For example, if you are exposed to 1 rem (0.01 Sieverts), your chance of getting cancer is increased by a factor of 1/2500. What this means is that if a population of 100,000 people is exposed to 1 rem (0.01 Sieverts), 40 of them will get cancer from this radiation exposure. While this calculation is based on approximations and the linear hypothesis may overestimate risk, it is better to err on the side of caution. About 20,000 of these people would have developed cancer anyway (the cancer rate is about 20 percent on average), but it is U.S. policy that subjecting a general population to the increased cancer risk from 1 rem of radiation is unacceptable. The EPA guideline states that if a population is projected to be exposed to 1 to 5 rem (0.01 to 0.05 Sieverts) of radiation, they should be evacuated.
This takes us to events in Japan. Today, NHK World has reported that for the second day in a row, radiation levels 30 km northwest of the Fukushima reactor are elevated. The readings today are 0.015 rem/hour (0.00015 Sieverts). While that looks low, the risk comes from the total exposure; at these levels, within 3 days people will have received more than 1 rem (0.01 Sieverts) of radiation. In fact, the inhabitants of this region have likely already received 1 rem (0.01 Sieverts) of exposure, since these radiation levels have been present for a while. Whereas the U.S. policy would suggest evacuations with the projected dose at this level, in Japan, people outside the evacuation zone have already been exposed to this level of radiation.
NHK reports that Japanese government is saying this level of exposure does not present an “immediate health threat.” That is a true statement — it represents a long-term health threat. Perhaps the radiation levels will go down, but there is a real danger that the levels will go up significantly — we just don’t know. This is why the U.S. Nuclear Regulatory Commission has advised U.S. citizens to keep more than 80 km away from the reactor. This seems like very prudent advice.
Paul Padley, Ph.D., is a professor of physics and astronomy at Rice University.