Like radiation, extrapolation is also everywhere

I enjoyed a recent New York Times article immensely. It covered the current discussion surrounding the potentially world-wide radiation pollution caused by the recent nuclear crisis in Japan. Everyone agrees that the leaked radioactive materials are low in amounts, compared to the ocean to which they were released to. The debate was about whether low or very low radiation equals no risk or harm.

To answer this question, we need empirical evidence. Data! However, the effects of low radiation have not been statistically established. According to the article, available results on the relation between radiation and health risks have all been obtained on radiation level greater than 10 rem. "Current estimates by government agencies for risks from low doses rely on extrapolation from higher doses." Wow, extrapolation! Isn't this more dangerous than the radiation itself?

As we repetitively preach in W1111, the danger of extrapolation is that the regressional relation beyond the scope of observed data can be much different from that of the observed data. Andrew shared a story from his study on radon. They were faced with the same problem. Because there were very few sample points with very low radon levels, without affecting the model fit on the observed data, the predictive model of Y (risk) can be linearly decreasing, diminishing towards zero, or even curve up (meaning that a little bit of radon is good to you) when the radon level goes to zero.

Another interesting aspect of this story is in the potential long-term impact of the Japan nuclear pollution. Current studies on radiation only help us understand the effects of short-term yet high dose radiation exposure. The situation surrounding Japan's nuclear crisis requires extrapolation in two directions: into low level of radiation (as we discussed above) and into longer time of exposure. Double-extrapolation!

Biologists provide their insights on this according to DNA mutation models and their relation with cancers. Basically, radiation (strong enough radiation, maybe) causes DNA mutation. The simplest mechanism would be that radiation cuts up DNA, and the sequence will then get messed up during replication and beyond repair. Such mutations tend to cumulate in our body and cause cancers and other disorders in a long run. Based on such biological belief, it is then hypothesized that low radiation, no matter how low, poses health threat.

So help us Data!