Saturday, April 2, 2011

What should the risk tolerance be? (CBCNews)

Radiation: How risky is the risk?

bob_60x60.JPGBy Bob McDonald, Quirks & Quarks

Headlines blared this week with news that radiation levels in the ocean outside the Fukushima Daiichi Nuclear plant were "4,000 times above acceptable levels." That sounds like a nuclear holocaust, but how bad is it really?

Radiation is a trigger word, where the mere mention of the term conjures up images of nuclear disasters, nuclear bombs, horrors of radiation burns or giant mutants of science fiction lore. While radiation poisoning is a serious issue, we live in a naturally radioactive world and our bodies are equipped to deal with radiation in low doses. Here and here are two charts that put radiation from all sources into perspective. 

In this context, the amount of radiation from iodine 131 and cesium 137, measured in seawater immediately outside the outflow channel of the Japanese nuclear plant,  is around 450 microsieverts, which is the level where symptoms of radiation poisoning begin to show in the body.  But even then, you must be exposed to that level for a year.

Here is another chart that explains exposure time and health effects more completely.

Another aspect of the situation in Japan is that the measurements of seawater were taken very close to the nuclear reactors. Samples taken several kilometers offshore were back down below acceptable levels.

Seawater, which is a natural filter of radiation, does a very good job of dispersing the contaminants, and iodine 131 has a half-life of eight days. That means any contaminated material that makes it to the shores of North America will be so diluted, and at such low doses, they will barely be detectable.


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