So…Dr. Z…I was asking my doctor if the nuclear medicine scan I’ve got to take is going to hurt me and after she answered my question she said “Besides, we’re all radioactive anyhow, so it’s not a big deal.” I know she meant it to be reassuring – but it really isn’t. Why are we all radioactive? Is it from nuclear weapons testing or Chernobyl or Fukushima – or is it from something else? How do I protect myself?
Well, to start, I’m guessing your doctor’s medical skills are better than her communications. Having said that – she’s got a good point, even if somewhat clumsily phrased. Let’s see if we can clear this up a little bit – and I should mention that this has nothing to do with nuclear weapons testing or reactor accidents – it’s all natural.
I recently wrote a piece about primordial radionuclides (https://www.ntanet.net/primordial-radionuclides-origin-importance-and-health-impact/) – radionuclides that have been around since the Earth first formed. One of those primordial radionuclides is potassium, which is important for our bodies to function properly. Potassium is part of the chemistry that helps every cell in our bodies to function; among other things, our muscles (including our heart) won’t function without the proper amount of potassium. And it turns out that a tiny fraction of potassium is radioactive.
The radioactive potassium nuclide is K-40; an atom with 19 protons and 21 neutrons. Potassium-40 has a half life of more than a billion years – 1.25 billion years to be exact – and it emits both beta and gamma radiation. To get a little into the weeds, 89% of K-40 atoms decay by emitting a beta particle with an average energy of 560 thousand electron volts (560 keV); the rest of the time it emits gamma radiation with an energy of 1460 keV (or 1.46 MeV). I should say, too, that you don’t need to worry about the units – just that anything less than a few hundred keV is relatively low-energy, a few to several hundred keV is moderate-energy, and anything in the MeV range (1000 keV = 1 MeV) is high-energy. So the K-40 beta is moderate-energy and has a high-energy gamma.
Radiation exposure from the K-40 inside our bodies is sort of interesting; the gamma radiation has a higher energy than the beta, but K-40 decay produces nine times as many betas as it does gamma rays. Not only that, but the high energy of the gammas means that they’re more likely to escape from the body without depositing all of their energy. Put this all together and it turns out that most of the radiation dose from our internal potassium comes from the lower-energy beta radiation it emits – about 15 mrem each year for the typical person. That’s about half the exposure we get from radioactivity in the rocks and soils and is also close to what we receive from cosmic radiation (about 20 and 30 mrem, respectively), and it’s much less than the 200 or so mrem we receive from radon each year. And, more importantly, it’s radiation exposure that humans have been receiving for our entire history as a species, and it’s less than what our ancestors in the distant past received from this same source.
But there’s more than just potassium! Potassium-40 is one of the two most significant sources of natural internal radiation; radioactivity from dust is the other.
Most of the radioactivity that we ingest and inhale comes from the dust. Specifically, traces of uranium and thorium and the nuclides they decay into (I touched on this recently when I wrote about primordial radionuclides). The best-known of these decay series nuclides are radon, radium, and polonium (which was used to poison former Russian spy Alexander Litvenenko in 2006). These elements are found in the rocks and soils and in the dust that forms when they weather and when the wind blows them into the air. We breathe them in and we ingest them when they settle onto the food we eat or into the water we drink; some of the radionuclides can work their way into our bones while others pass through our bodies as quickly as yesterday’s lunch. All in all we’re exposed to about 13 mrem annually from these other primordial radionuclides and the nuclides in their decay chains.
And then there are a few minor radionuclides, both formed as a result of cosmic ray bombardment of the atmosphere – what are called cosmogenic radionuclides.
Two of these minor radionuclides are both formed in the Earth’s atmosphere when cosmic rays slam into atoms in the air. When a cosmic ray neutron strikes a nitrogen atom, for example, it bounces a proton out of the nucleus and the nitrogen atom turns into an atom of radioactive carbon – C-14 to be precise. If you’ve heard of carbon dating to learn the age of a piece of buried wood (for example), this is done using C-14 dating. Like potassium, all living organisms use carbon as an essential part of their biochemistry and this carbon includes traces of C-14. Carbon-14 has a much shorter half-life than does K-40 (“only” about 5700 years) and it emits a relatively low-energy beta particle; as a result we receive only about 1 mrem each year from C-14 in our bodies.
The other cosmogenic radionuclide that’s incorporated into our bodies is a radioactive isotope of hydrogen that has two neutrons to accompany its single proton; H-3, which is called tritium. Unlike C-14, tritium is formed when a high-energy cosmic ray hits an atom hard enough to eject a tritium nucleus in what’s called a spallation reaction. As with C-14, the H-3 mingles with the stable other hydrogen atoms in the atmosphere, in the water, and in the soil; we take in the tritium by breathing, drinking, and eating. Tritium has a fairly short half-life of only about 12 ½ years and it gives off a very low-energy beta particle. Because of this, we don’t get very much radiation exposure at all from naturally occurring tritium.
So that’s where the radioactivity in your body comes from – some you’re born with, some you eat, drink, and breathe; some has been on our planet since it first coalesced from the primordial cloud of gas and dust, some is formed daily from cosmic rays that were born in stellar explosions on the other side of our galaxy. And from all of this, each year we receive about 30 mrem a year from our radioactive bodies – about as much as from an x-ray or two and less than your upcoming nuclear medicine scan (which is also not going to hurt you).
So not to worry!