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How Much Radioactive Contamination Is Too Much?

by Michael Levi
March 23, 2011

First it was radioactive material in milk and spinach near the Fukushima reactors. Now it’s radioactive iodine in Tokyo tapwater that exceeds limits for infant consumption. All of which makes me think back to some work that I did in 2002 on dirty bombs. (Even if you aren’t interested in dirty bombs, read through; there’s a lesson for the current situation here too.)

My colleagues and I wanted to estimate the consequences of a dirty bomb attack. To do that, we simulated the dispersal of radioactive material and then determined the area over which contamination levels would exceed established safety limits. The results were disturbing: large swaths of a city could be put off limits by a relatively modest attack.

Some smart people pushed back. Contamination was considered unacceptable under existing regulations if continued exposure raised the risk of death from cancer by more than one-in-ten thousand. But the background rate of death from cancer was already one in five. If a large area became contaminated in a dirty bomb attack, they asked, wouldn’t authorities relax the limits? Faced with a choice between abandoning chunks of a city and accepting, say, a one-in-a-thousand increase in the cancer fatality rate, wouldn’t people pick the latter?

Over the years, I became somewhat sympathetic to that argument, but I wasn’t entirely convinced. In particular, I was pretty sure that changing the safety thresholds after an event (rather than developing a set of alternative rules in advance) would be tough. Few people have the technical knowledge to judge for themselves what’s safe or not. In the aftermath of an ugly incident, they may also lose trust in authorities, which means that they won’t trust revisions to the rules either. Authorities may be stuck with the preexisting guidelines even if there’s a more rational alternative.

That dynamic may come into play in Japan. I don’t want to claim any authoritative knowledge of the situation in Tokyo. That said, to the best of my understanding, the levels of iodine being seen in water exceed the threshold for chronic exposure. [UPDATE: After some more research, this is even more of a muddle. The thresholds seem to be consistent with FDA limits for chronic exposure through drinking water, but much more lax than EPA limits for the same. Like I said, I don't want to claim any authoritative knowledge.] No one is yet at risk of chronic exposure. But authorities are still telling people not to drink the water. That may not be a rational response to the actual threat to public health, but it is a perfectly reasonable response given the existing rules.

We will probably learn a lot in the coming months about exactly how much radioactive contamination people are willing to live with. The initial lesson I take away is that once you reach the point in a crisis where the radiation rules have become potentially counterproductive, it’s probably too late to change them.

Post a Comment 2 Comments

  • Posted by Benjamin

    You have an error in the third paragraph:

    “If a large area became contaminated in a dirty bomb attack, they asked, wouldn’t authorities would relax the limits?”

    [ML: Thanks, corrected.]

  • Posted by Tim

    “Contamination was considered unacceptable under existing regulations if continued exposure raised the risk of death from cancer by more than one-in-ten thousand. But the background rate of death from cancer was already one in five.”

    This is an important issue. I am not optimistic that our political leaders can improve this situation as there is too much political and financial muscle behind the right to sue. The general ignorance of probability in the population makes the situation worse.

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