Are human beings radioactive?

Question

I am reading "The greatest story ever told - so far" by Lawrence Krauss. He starts chapter 9, Decay and Rubble, telling how he was shocked when he realized that humans are radioactive.

When I first learned that we human beings are radioactive, it shocked me. I was in high school listening to a lecture by the remarkable polymath and astrophysicist Tommy Gold, who had done pioneering work in cosmology, pulsars, and lunar science, and he informed us that the particles that made up most of the mass of our bodies, neutrons, are unstable, with a mean lifetime of about ten minutes.

The argument that he uses is particles that make most of mass of our body is neutrons and they are unstable having a mean life of about 10 mins. He argues that within the nucleus, however, neutron is stable and that has got to do with the fact that its mass is 0.1% more than Proton. I am slightly lost in the whole argument. Can someone please help?

Answer 1

While I generally admire Krauss' writing the part of the book you mention is not one of his finest moments. It is certainly true that humans are radioactive, but this is nothing to to with the decay of free neutrons. So his opening paragraph doesn't make sense.

The first sentence of the paragraph is:

When I first learned that we human beings are radioactive, it shocked me.

Humans are radioactive because a small proportion of the atoms in many elements is radioactive. For example you have probably heard of carbon 14 dating. This works because the carbon in all living things contains one radioactive carbon 14 atom for every trillion non-radioactive carbon atoms. Likewise we contain trace amounts of radioactive potassium 40, and so on. I'm surprised anyone would find this shocking, but there you go.

The rest of the paragraph is:

I was in high school listening to a lecture by the remarkable polymath and astrophysicist Tommy Gold, who had done pioneering work in cosmology, pulsars, and lunar science, and he informed us that the particles that made up most of the mass of our bodies, neutrons, are unstable, with a mean lifetime of about ten minutes.

and it is certainly true that free neutrons are unstable. However the neutrons in humans are all in the nuclei of atoms and those neutrons are stable. For more on this see How come neutrons in a nucleus don't decay? The decay of free neutrons is unrelated to the radioactivity in humans.

So the first sentence does not seem to related to the rest of the paragraph.

Answer 2

Yes

All of us have a number of naturally occurring radionuclides within our bodies. The major one that produces penetrating gamma radiation that can escape from the body is a radioactive isotope of potassium, called potassium-40. This radionuclide has been around since the birth of the earth and is present as a tiny fraction of all the potassium in nature.otassium-40 (40K) is the primary source of radiation from the human body for two reasons. First, the 40K concentration in the body is fairly high. Potassium is ingested in many foods that we eat and is a critically important element for proper functioning of the human body; it is present in pretty much all the tissues of the body. The amount of the radioactive isotope 40K in a 70-kg person is about 5,000 Bq, which represents 5,000 atoms undergoing radioactive decay each second.

Second, 40K emits gamma rays in a little over 10 percent of its decays and most of these gamma rays escape the body. A gamma ray is emitted in about one out of every 10 disintegrations of 40K, implying that about 500 gamma rays are produced each second. These will be moving in all directions, some will be attenuated in the body, and the dose rate from these gamma rays outside of the individual's body will represent a very small fraction of the normal background dose rate from all natural sources outside the body.

There are many other radionuclides in the human body, but these either are present at lower levels than 40K (for example, 238U, 232Th, and their decay products) or they do not emit gamma rays that can escape the body (for example, 14C and 87Rb). Radon (and its decay products) is not a significant source of radiation from humans because it is present at very low levels in the body.

(Source: hps.org)