300 Times The Limit of Iodine!

You can put your Geiger counter back in the closet.


This is a guest post from Dr. Rob Tarzwell. Dr. Tarzwell is a nuclear medicine specialist (in training) at UBC in Vancouver, and is on the Faculty of Medicine at the University of British Columbia. He is a CFI advisor. Check out his previous post on this subject, here.

What does it mean when Charlie Smith reports that iodine-131 levels are “300 times normal”? It’s actually very basic. First, I-131 is not found in nature. It is a fission product of uranium-235, and its half-life is only 8 days. U-235 decays *very* slowly in nature, and I-131 decays *very* quickly, so the stuff just isn’t around.

So, if you detect any I-131 at all, it’s above the expected background level of zero.

Not surprisingly, after the Fukushima reactors had to discharge radioactive steam to relieve pressure, along with other emergency maneuvres, I-131 was released into the environment. Super tiny amounts were subsequently detected in Canada.

Any school child knows that you can’t divide by zero, so you can’t divide “super tiny” by “zero” and get a meaningful result. So, Health Canada sets a very, very low, but still finite level for I-131 limits of 0.01 millibecquerels per cubic metre of air. That is the equivalent of one radioactive decay every 100,000 seconds. There are 86,400 seconds in a day (60x60x24), so this is a decay about every 28 hours.

In other words, it’s so close to zero that you can basically call it zero.

Now, Charlie Smith may not appreciate the distinction, but the environmental scientist he quotes ought to. She ought to know that the 3.6 millibecquerel level is ridiculously small and is not a threat to human health in any way. Instead, she beats the drum of 300 Times The Allowable Limit (ZOMG!!!!), and he ignorantly provides her a soapbox.

Look at this from a different angle. Since I-131 does not occur in nature, suddenly detecting it without knowing why immediately indicates that either U-235 fission is unexpectedly occuring, or that a hospital’s supply of I-131 has been appropriated and is near a detector. Either of these problems are of immediate concern to public safety and warrant investigation and action. That’s the real reason for setting such a low limit.

But we *know* why we are detecting super-tiny amounts of I-131: Fukushima. Health Canada has tried to reassure the public, but the Straight would rather stoke fear with the testimony of photogenic publicity-seekers who are self-appointed experts.


Photo from Flickr user spike55151 under a CC licence.

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  • Rob Tarzwell

    Rob is a Vancouver based physician with specialty training in psychiatry and nuclear medicine, and he is a Clinical Instructor on the Faculty of Medicine at UBC. He has publicly debated creationists and anti-WiFi activists, lectured on the importance of flu vaccines, and he writes articles in response to medical misinformation. He is convinced that, one day, one of those windmills he tilts at will actually fall over.