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Tuesday, August 28, 2012

Professor, That's Not LNT

Here's a short video from last year with Radiological Physics (think medical physics) Professor Cynthia McCollough.  I hadn't seen it until today:

Her broad points are correct.  People shouldn't be fearful or even concerned with low level radiation doses (she's referring to a CT scan or two, each can range from 1 to 10 mSv depending on what's being imaged).

She's also correct that people aren't very well educated about radiation and there is a certain mystique to it.

But then she veers off into non-science.  She says that the levels have never been shown to have any negative health effect.   Yet we have human cell studies and animal studies that show negative health effects.  Our human epidemiological studies are limited, but we don't ignore the other evidence we have.  See my page on "LNT Model & Radiation Studies", upper right.

Our best human epidemiological study is that of the Japanese A-bomb survivors.  When we apply Maximum Likelihood Estimation analysis to the data, we get a threshold of zero. Are you denying statistics, Professor?

She compares radiation to cigarette smoking.  With smoking the "effect is huge", she says.  No it isn't, it depends on the number of cigarettes smoked.  The radiation effect is huge, too, just not at CT dose levels.  Dr. S. Fred Singer, of the George C. Marshall Institute (deniers of all the sciences and whom I've mentioned many times before) argued against the linear no-threshold theory as applied to cigarettes!  The fewer cigarettes people smoke, the more difficult it is to discern an effect from epidemiology.  But we make inferences from other evidence, we don't ignore that evidence.  And we reach similar conclusions for cigarettes and radiation.

She says if the effect is there it's so small we'd have to study a couple of million people (two cohorts, those who get exposure and those who don't).  If only it were a couple of million.  Using the Japanese A-bomb survivor results, we see statistically significant epidemiology down to about 0.1 Sv with about 74,000 people (I'm not including people who received higher doses).

We can use a power law with this information.  We expect that this will be a function of the inverse variance of the dose distribution.  Variance is a squared function, so the power law acts like the inverse square law of photon dose (double the distance from a photon source and the dose rate goes down by 4). Since the lowest dose at which we see a statistical effect is 0.1 Sv with the Japanese study, we can estimate that to be able to discern it down to 0.01 Sv (10 times lower than 0.1 Sv, at the upper end of a CT scan dose estimate) it would take 100 times (10 squared) as many people.  Since there were about 74,000 people in the low dose part of the A-bomb study,  we expect it would take around 7,400,000 exposed people in order to discern any effect epidemiologically.  A couple of million people would be a walk in the park!

She says our cells "know" how to repair damage.  Our cells don't know anything, they don't have brains.  Our cells respond to damage, and they have very high fidelity for repair.  But they're not perfect, and that's why there's a risk of cancer, and why there has been cancer in humans, long before CT scanning was invented.  It's good that DNA is mutable, otherwise life wouldn't survive, and we wouldn't be here.

Then as icing on the cake, she says, "the linear non-threshold hypothesis is that theory that we use to do those calculations", yet biologically we don't think that's true.

I covered LNT as a hypothesis, model and theory here.  It is a theory, because we have an explanation which incorporates all the evidence and there isn't any alternative explanation which is as powerful.  It describes what we think it's true.  We don't develop theories around what we think is not true.  That's just ridiculous.


  1. One of you powerful statements here is that radiation is like cigarettes.

    Could we also say that radiation is like salt. A lot of salt will kill you. A lot of radiation will kill you. Therefore, a little salt is bad for you.

    How do I choose between cigarettes and salt as the way radiation at low doses acts?

  2. Cigarettes and radiation are carcinogens (cigarette smoke contains natural radioactivity and other chemical carcinogens). Cigarettes are not necessary for survival. As far as we know ionizing radiation isn't necessary for survival, but since we've always been immersed in it, we can't say for certain.

    Salt (NaCl)is not a carcinogen and is necessary for survival. Your body's electrical signaling requires dissolved salt (freeing the Na+ and Cl- ions).

  3. The big difference between the way radiation (and cigarettes) work, and the way salt (or aspirines, or why not water) work, is in the mechanism.

    We know that what makes aspirin (one of many drugs you can overdose on) dangerous: it inhibits the clotting of blood and thus makes internal bleedings more likely. As long as the blood loss is small, the body can handle it; but when it becomes too big, the body is overwhelmed. So we see that the drug is safe up to a critical dose, and then rapidly becomes dangerous and then lethal.

    Acute radiation can work in the same way as aspirin: it can overwhelm the body's reponses. Then you suffer radiation sickness, which can be fatal. The chronic carcinogenic effect is very different. It's more like a lottery: the more lots you buy, the more chance to hit the jackpot. And while the body tries to fix the damage to the cell's genome in a number of ways, all those mechanisms 'leak', and some fraction of the damage gets through.

    The giveaway is that for the whole dose range where we can observe effects epidemiologically, the risk of disease due to radiation, or cigarettes, increases with dose, in as linear a fashion as one can tell. Just like in a lottery. Even buying one lot together with fifty mates gives you a nonzero chance!