Hacker News Reader

Comment by 08-15 | Hacker News Reader

> In the published estimates shown, studies have utilised a methodology termed the ‘linear no-threshold model’ (LNT).

LNT is neither a method, nor is it a model. It is an assumption, which happens to be convenient for regulatory purposes. Let me quickly explain how it works.

Caffeine is poisonous with an LD50 of around 15g or about 100 cups of coffee. If two people each drink 100 cups of coffee, one of them is expected to die. If we now apply the "method" of LNT, it follows that if 200 people each drink one cup of coffee, one of them is expected to die. Moreover, since 85% of adult Americans consume on average 164mg of caffeine daily, which is roughly 30t in total, we can conclude that caffeine kills one million Americans every single day.

Now you can evaluate for yourself whether LNT makes any sense.

View on HN

My apologies, but your analogy is flawed. An adverse effect of ionizing radiation is that it could cause a mutation that replicates (before it's caught by the immune system) - cancer. It's therefore intrinsically probabilistic: if you're exceedingly unlucky, a small amount of ionizing radiation could get you there. That does not apply to your caffeine analogy, which actually does have a threshold.

Now, there's indications that at least the linearity of the LNT model is off in that there IS any effect of the immune system on the outcome, but that at high doses, it may be suppressed thus making the response non linear. I'm sure there's other effects of that nature and I don't think this is particularly well understood. But I dare say the linear no threshold model is actually somewhat more supported by common sense than you imply. Your distinction between the words "model" and "assumption" is moot by the way. A model is an attempt to (typically mathematically) describe a complex process. That's always an assumption in that sense. :)

(Former high energy physicist. Not an MD. Not a medical physicist.)

> An adverse effect of ionizing radiation is that it could cause a mutation that replicates - cancer. It's therefore intrinsically probabilistic.

This statement betrays a very naive understanding of cancer and complete ignorance of DNA repair pathways. Don't take it personally - nobody understands cancer completely. However, we know that a single mutation will not cause cancer. Cells have "defense in depth" against adverse mutations. There are multiple repair mechanisms, DNA exists in four copies usable for reconstruction, and there are crowbar mechanisms that kill a cell when it detects damage, and there is an immune system which can kill rogue cells. Therefore, to cause cancer, a number of mutations is needed (probably around ten). Clearly, the risk of cancer isn't linear in the dose, it grows approximately with the tenth power.

There are other mechanism absent from your naive view. Ionizing radiation typically doesn't damage DNA directly (alpha radiation can cause double strand breaks, but beta and gamma don't). Rather, it creates reactive oxygen radicals which can oxidize DNA, causing miscoding lesions. (These are repairable, to some extent.) But cells contain compounds that scavenge such radicals (the most important one is glutathione). If glutathione has be exhausted before oxidative damage occurs, that creates a threshold effect.

So response to ionizing radiation is likely nonlinear and has a threshold. LNT is twice wrong.

> Your distinction between the words "model" and "assumption" is moot by the way.

Then I will make it more clear. A model is something that is tested and discarded if it turns out to be no good. An assumption is something that is kept despite failing the tests. LNT was based on nothing (Herman Muller made it up, claiming knowledge about how DNA is affected; and that was before the structure of DNA, let alone the existence of DNA repair was known), which may be forgivable. But it should have been discarded after its predictions didn't match the reality of the Nuclear Navy dock workers study, or the Taiwanese radioactive high rise buildings, or how low dose full body irradiation suppresses cancer instead of causing it.

I shouldn't have said assumption. Let's be honest and call it a dogma.

> Don't take it personally

No offense taken! https://xkcd.com/386/ :)

Thank you for the clear example that drives the point home.

LNT for radiation made sense in 1950 before we had sufficient data on how the body responds to radiation. Today, after carefully studying irradiated populations for 70 years, we know enough to conclude that it no longer makes sense.

This is a ridiculously bad analogy. It’s so bad, I would expect it only from right-wing disinformation efforts who are self-parodies of incorrectness.

There is no correspondence between an acute chemical poisoning mechanism and the mechanism of ionizing radiation induced illness for longer-term effects.

A more correct analogy for LNT, indeed, a “model”, would be a rifle fired at random in to a room. If there is one person in the room, there is say a 1 in 5,000 chance the bullet will hit them. If they are hit, there is a 1 in 10 chance they will die.

If you fire the rifle a million times in to a million rooms, you get a linear scale-up and expect 200 people to be hit, and 20 of them to die. It scales linearly, with no threshold below which the mortality rate declines.

If you pack the room with more people the probability of hitting someone goes up, etc.

Now there are indeed well-based arguments to say that there are indeed thresholds of effects of ionizing radiation, including studies where low ambient levels are present in populations, to studies attempting to show radiation hormesis effects. Those studies have not displaced LNT thus far because they are not clearly conclusive in understanding what they are saying.

LNT has been the standard for 70 years not because it was promulgated by idiots, but rather because it is a conservative protective measure designed by the best minds of the time, using the best information that was reliably known.

So please, discuss hormesis, show alternative models, etc., but please do not try to impugn the model as stemming from idiots - the Manhattan-era physicists were certainly miles smarter than anyone reading HN today - and has not been displaced yet because no one understands a better model of how to protect people from injury and death.

The way LNT works with radiation is that we knew a death-dose curve from the atomic bombs in Japan at extremely high dose. This was acute radiation poisoning (comparable to chemical poisoning). This response curve is precisely the thing that has been extrapolated down to miniscule doses just above background. The analogy you are critiquing with acute chemical dose is right on.

The rifle fired at random in a room analogy is vastly more flawed and does not take into account how the body deals with radiation. There are cell damage events all the time, and there are repair mechanisms that fix them in the body at a baseline rate. These repair mechanisms make radioactive cell damage look a lot like chemical damage at low levels.

Radiation has been a part of the universe forever. 50% of the internal heat of the earth is from radiation. Space is radioactive. Cosmic rays rain down on us all day and all night. The body can and does deal with this, up to a point, after which it becomes inundated.

Thus, a small boost in radiation, far below natural background, cannot be expected to increase deaths.

> It’s so bad, I would expect it only from right-wing disinformation efforts who are self-parodies of incorrectness.

That's not very helpful. It would be helful, if you explained how it is a bad analogy for ionizing radiation. (By the way, that wasn't even the point. I merely explained LNT in a context where everyone immediately recognizes it as ridiculously wrong.)

> do not try to impugn the model as stemming from idiots

I would never! The guy who came up with LNT ten years after the conclusion of the Manhattan project, Herman Muller, was not an idiot!

He was a fraud.

Reply on news.ycombinator.com