The bottom of the Wikipedia page on K-40 says, “Potassium-40 is the largest source of natural radioactivity in animals including humans. A 70 kg human body contains about 140 grams of potassium, hence about 0.000117 × 140 = 0.0164 grams of 40K; whose decay produces about 4,300 disintegrations per second (becquerel) continuously throughout the life of the body.” Potassium-40 - Wikipedia
I checked the math and I agree with the 4300 disintegrations/second. This seems like a lot to me. Why isn’t this more dangerous than it is?
More info: “About 89.28% of the time, it decays to calcium-40 (40Ca) with emission of a beta particle (β−, an electron) with a maximum energy of 1.31 MeV and an antineutrino. About 10.72% of the time it decays to argon-40 (40Ar) by electron capture (EC), with the emission of a neutrino and then a 1.460 MeV gamma ray.”
Since a Geiger counter wouldn’t show me to be greatly radioactive, I assume most of the beta decays are staying in my body and being absorbed. Is that true? What about the ~430 gammas/s?
Because you have 37 trillion cells. And almost all of the cell is other components than the nucleus. For the most part, damage to anything else in a human cell will only be temporary - all proteins will be replaced eventually, all damaged RNA strands won’t matter because soon a new RNA will be made.
And even if the radiation hits a nucleus, most of the DNA in any given cell is not presently in use. And even if there’s damage to a key area, most such mistakes will cause the cell to just die and not cause cancer and you can afford to lose thousands of cells per day.
And finally, even if the damage is to a region that might lead to the cell become precancerous (it takes multiple mutations to cause cancer generally), there are various error detection molecules that human cells have that may or may not trip as a result of the damage. These molecules can cause the cell to commit suicide instead of potentially becoming cancerous.
On top of all this, most of your cells have intentionally short lifespans - the key so called “stem cells” are the important ones that presumably are the type that can form tumors, while most cells have various mechanisms preventing replication.
TLDR : it takes an incredible run of bad luck for any given bit of radiation exposure to lead to a fatal illness. Unfortunately, the dice are rolling your entire life, and apparently in older humans many of these protection mechanisms break down.
I guess I’m still not sure exactly what we think about these 4300/s decays, ~3900 beta and 400 gamma. Are most of them getting stopped in our body, since a Geiger counter wouldn’t detect them?
Also, you are inadvertently answering another question I’ve had over the years about the age of cells. If all our cells are not the same age as each other (since our conception) then we must say their age is when they divided last. But you’re saying stem cells make all (/most) of the new cells, is that right? So the age of the cell is how long it’s been since it got made from a stem cell? Then I guess we’re trying to figure out why the stem cells don’t get old.
Stem cells get old too; what’s different about them is that stem cells can make multiple types of cells when they divide, and that a later generation stem cell has the same capabilities as an earlier generation did. A non-stem cell is either capable of making cells like itself (with or without errors), or not capable of making cells.
Any time a cell makes new cells, the original cell dies. This is true for stem cells as well as for other cells.
As for the radiation, we’re talking about either electrons (beta) or light (gamma rays). The electrons are just like any other electron, they eventually find a nice atom to get attached to (this atom may be in the original body or not). The biggest differences between them and the other bajillion electrons currently in our body is, one, that they’re travelling a longer-than-usual distance between “original atom” and “new atom”. Two, that most of the electrons in nature are paired: these are traveling alone and will eventually turn the atom they join (and any molecule that atom is part of) into a radical. How much this is “a problem” will depend a lot on what atom, what molecule: if a chunk of metal gets an extra electron, the vicinity barely notices; organic radicals can be relatively aggressive but we’re still talking about one radical surrounded by zillions of regular molecules.
The light will, when it’s absorbed by an atom, either take it to an excited state or kick an electron out of it (oh hi beta particle, see above). An excited atom or molecule will be more reactive than it was in its ground state; if it doesn’t react with something else, it will eventually go back to normal by emitting light again. It can be in multiple steps, in which case each photon will be less energetic than the original gamma photon.
Beta rays are fairly non-penetrating, so a beta formed somewhere within the body probably won’t make it out of the body.
Gamma rays are much more penetrating, so a gamma formed somewhere within the body almost certainly will make it out. But by the same token, it’s also likely to just fly right through the Geiger counter without interacting with it.
No, in cellular proliferation there is no “original” cell and “new” cell. Cells divide into a pair of daughter cells, both of which are equally “old”.
There is a slight difference with stem cells where there is a requirement to maintain the pool of pluripotent stem cells. They either undergo asymmetric division, where one daughter cell is differentiated and the other retains pluripotency; or some stem cells divide into two differentiated cells while others divide into two pluripotent stem cells to maintain the pool.
4300 disintegrations per second sounds like a lot, but the energy of each disintegration (33.5 mega-electron-volts) is minuscule by everyday standards. If you could somehow harness all the energy from all the K-40 disintegrations inside one person over a 100-year lifespan, it would suffice to raise the temperature of a six-ounce cup of water by only 0.1 degree Celsius. If you wanted to make a cup of hot tea just once a year by this highly inefficient method, you’d need to harness all the energy of the combined bodily K-40 disintegrations of the entire population of Duluth, Minnesota.
I was looking at the wrong column. The energy of each disintegration is only 1.3 MeV, one person over a lifetime raises the temperature .003 degrees Celsius, and it would take the population of Houston, Texas, to make a cup of tea a year.
I realize that the total energy is small, but 1.3 MeV is in the range of breaking chemical bonds (and potentially altering DNA), which is the major concern of radiation exposure, right? I suppose that the reason that it’s not that dangerous is first because it is not concentrated in any one area. No matter how penetrating the decay is, since the K-40 is spread all over your body no one place gets a lot of radiation. As SamuelA said, any change the radiation makes probably won’t be a big deal and it can even be repaired, since the repair process can easily keep up with the destruction. If you ate 0.0164g of pure K-40 (in KCl) I suppose it’d be worse for you, though perhaps still not all that worrisome?
That’s what I meant, yeah: the old cell makes two new cells. It dies in the making, whether it’s making two cells which are of the same kind as the original (now-dead) cell or one of the same kind and one not, or both of a different kind (does sexual reproduction still work like that, or is that another thing which doesn’t work like we were told when I was in school?).
Saying that the original cell dies, though, is a bit misleading, because it makes it sound like there should be a dead cell lying around somewhere, instead of the original cell’s “remains” being two living cells.
As you said, the beta radiation would not leave your body except when emitted from an exposed surface and happening to go outwards. And you can detect radiation from your body with a Geiger counter, there just wouldn’t be a lot of those gamma rays going in the direction of the detector. And a Geiger counter doesn’t register every gamma ray, since they are good at going through things, including Geiger counters.
There’s about 30 grams of DNA in the average person, or about 0.04% of the mass of a 75 kg person. Assuming DNA and potassium is evenly distributed (not true but close enough) that means only 1-2 decay events per second will end with an electron interacting with DNA. Even then, every interaction won’t necessarily damage DNA.
More importantly, DNA damage is happening all the time, most of it due to reactive oxygen species that are by-product of normal metabolism. These happen at a rate that is many orders of magnitude higher than any damage conceivably caused by potassium decay - about 10,000 oxidative DNA damage events per cell per day. Since there are about 3*10^12 nucleated cells in the human body, that works out to 3*10^11 events per second. Which makes the contribution of radioactive decay absolutely trivial, as long are you aren’t eating radioactive waste.
Fortunately, DNA has built-in redundancy, and there are multiple damage sensing and repair pathways. Unfortunately, repair pathways aren’t always able to keep up, which is why almost half of all people will get cancer.
OK, so… first you say that the original cell isn’t alive any more and then that it doesn’t die? We have different definitions of life, death and existence, I’m afraid… but that would go to a different forum
Anti-particles can only annihilate with a regular particle of the same type. I.e. antiprotons can only annihilate with ordinary protons, etc. Since your body doesn’t have any ordinary neutrinos, there’s nothing in your body for it to annihilate with.
OK, there’s many neutrinos passing through your body at any one time, so it could annihilate with one of them. However, the chances of that are very low. They also have to hit each other just right to annihilate.
