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CasioTheSane comments on How to use human history as a nutritional prior? - Less Wrong Discussion
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You have to be careful what you mean by carcinogen. I suspect what you're saying is true for some chemical carcinogens and ionizing radiation in single burst exposures, but not for other time varying patterns of ionizing radiation exposure.
It hasn't been confirmed in humans, but ionizing radiation exposure most certainly doesn't have a linear dose response with in vitro cell lines. Well, it does for single doses- but initial doses to be protective against future doses (hormesis): http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1533272/?tool=pmcentrez
1: that happens for quite significant doses only (1 centigray = 0.01 gray = 10 millisievert for gamma and x-rays = 1000 hours of exposure to 10 microsievert/hour background)
2: on single cell level there is no small doses, there's small probabilities of 1 particle track through nucleus (edit: or 1 Compton scattering electron track for gamma rays). Non-linear response to probability would indeed be quite extraordinary.
Ionization of DNA molecules is going to be linear in response to radiation, but you can't assume that will result in a linear cancer risk. Cells can up and down regulate their DNA repair mechanisms under varying conditions, allowing for a much more complicated relationship between radiation and cancer risk.
In this case, cells appear to up-regulate poly (ADP-ribose) polymerase (PARP) in response to past radiation exposure, increasing the chance of an accurate repair after an ionization event damages one strand.
//edit: The nucleus is also not the only potential site of cancer inducing mutations, the mitochondrial genome is another possibility.
Before cell can regulate it's DNA repair due to radiation, it has to detect the radiation, which at low levels in question (say, up to ~10x natural background, that's ~ 1..5 microsievert/hour, 2 000 .. 10 000 times less than centigray of gamma per hour) means detecting the probability before anything happens.
The issue with radiation is that people don't understand the units. You read the study you linked, you see, 1 centigray of x-rays, that's a 'low dose' they say, in god knows what context (Radiation therapy? Sure it's a low dose there). That's a 10 milliSieverts, okay? The average background dose a human receives per year, is , or 1/4 of that. Nobody's been proposing that 4 years worth of normal dose in a hour are going to still be linear.
edit: or actually, we do. We interpolate the low dose effects from the doses of somewhere around 0.1 Sv and up, based on various real world human data. Meaning that, if the effect outlined in your link is real, and there are some defence mechanisms activating at 0.01 Sv which prevent some of the DNA damage (at some other expense) - then we are underestimating the carcinogenicity of radiation at the low (near background) level, at which those defence mechanisms are not active. That is kind of scary to think about, in terms of potential extra cancer deaths.
Okay, I was misunderstanding what you were saying, and it makes sense now.
To paraphrase: Cancer risk in response to radiation levels can only be non-linear when the cell sees past radiation damage signaling it to mount a response. At low doses a given cell is unlikely to see any DNA ionization events, and therefore the risk must be linear.
That's a great point about the potential problem with extrapolating low doses from high dose data. That should really be investigated more carefully... if true "minor" radiation exposures could be a lot more risky than existing estimates suggest.
Yes. Instead what is happening, the nuclear lobby is citing studies like the one you linked as evidence of "radiation hormesis" i.e. generally beneficial effects of "low doses" of radiation (in your study the cells and the controls have to be irradiated with high dose afterwards to show any benefit; i'm very dubious that there exist any benefit versus the background), and lobbies for removal of strict EPA limits.
It gets even worse than this; the definition of Sievert and the procedures for calculating exposures of people rely on linear model - if the effects are linear then the average dose is all you need - and what the lobby wants is to use linear-model justified average doses together with threshold model, which just doesn't make sense. There literally can not be any consistent non-linear response to doses in Sieverts because the concept of dose and concept of Sievert presume linearity when it does not matter how the dose is distributed in space and time (well, up to organ scaling factors).
I've certainly seen several politicians argue that "radiation is actually good for you," but I've yet to hear any actual radiation health physicists argue that point...
Well, one doesn't usually see any actual radiation health physicists argue anything. I sure seen various engineering type people argue its good, and there are entire countries (Japan) where the linear-no-threshold model is evidently not adhered to.
Plus there is something weird going on with wikipedia articles on the subject all trying to present the pre-LNT views as something new that's challenging the LNT, complete with editing out of highly relevant historical references. Then there is "radiation hormesis", a hypothesis, that the radiation is good for you. Not "because of such and such specific response, radiation is good for you" - just a hypothesis that it is (which incidentally is the first "hypothesis" that comes up when a new exotic poison is found: someone hypothesises it to sell it in small amounts as a cure). Except that its presented as something new. Complete with a laundry list of rationalizations of how it might be so. That's terrible, and misleads people a fair lot.
I dunno if I should go ahead and write article on the topic.
Is this a fair description of the history and science behind hormesis?
See for yourself:
Is it more like some new outcome like 'ohh, there's new method by which the cell would know the radiation doses at low near background level, even for alpha particles a single of which does giant damage! Some new exciting physics discovered - the quantum probability can be measured before event happens!. That got to be useful for something, maybe for defence response. Ohh, there is the defence response, and its so strong.... I wonder if low doses of radiation are good for you?'
Or is it more like like 'okay, suppose the radiation is good for you, let's think and come up with justifications, okay, the untapped powers of organism that will be'.
The former is the process of scientific enquiry, the latter is the process of pseudoscience - start with desired effect, make up vague cause, later on perhaps think up a zillion specific causes, good luck proving them all wrong. I wonder why we even take obvious products of entirely backwards reasoning at face value as if they were not fundamentally different from products of forward reasoning?
Also try calculate how many people are required to find LNT-predicted dose effects at 10x the background. There aren't going to be direct evidence. There will be very strong indirect evidence, such as difficulty for the cell to measure doses near background, and generally low prior probability for some magical untapped powers of organism.