Are Our Bodies Radioactive?
From the radionuclides that are present in our bodies, the average man in the United States receives an effective dose of about 0.3 mSv each year. This is about one-tenth (or 10 percent) of the 3.1-mSv dose that the average U.S. man who weighs 70 kg receives each year from all sources of natural background radiation (not including medical sources). For women and children, the dose is less, in rough proportion to their smaller bodies.
More information is available in the National Council on Radiation Protection and Measurements (NCRP) Report 160, Ionizing Radiation Exposure of the Population of the United States. A pie chart in this report shows dose contributions from various natural background radiation sources, and the contribution from our own bodies can be found by adding the dose from potassium-40 and from thorium and uranium and their decay products (discussed in more detail below).
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.
Potassium-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.
If a person is above average in weight, the dose rate outside of this person's body will expectedly be higher than the dose outside the body of a lower-weight individual. In both cases, however, the dose rate will be extremely small compared to the normal background dose rate. The heavier person will receive a greater internal dose because the decay of the 40K produces other low-penetrating radiation (beta radiation) that deposits its energy within the body. However, the dose to the heavier individual will not be significantly different from that to the lower-weight individual because the energy deposited per unit body mass is the dose-determining factor, and this will be about the same for both individuals.
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.
There is one other very minor mechanism by which the human body acts as a source of radiation: some of the gamma rays emitted by the radionuclides in the environment interact with the atoms in our bodies by what is known as the photoelectric effect. The result is the emission of x rays by these atoms.
Potassium-40 content of the body can be obtained from its natural abundance of 0.0117 percent of potassium and calculating the specific activity of natural potassium (30.5 Bq g-1) using the half-life (1.28 x 109 y). The potassium content of the body is 0.2 percent, so for a 70-kg person, the amount of 40K will be about 4.26 kBq. Carbon-14 content of the body is based on the fact that one 14C atom exists in nature for every 1,000,000,000,000 12C atoms in living material. Using a half-life of 5,730 y, one obtains a specific activity of 0.19 Bq g-1 of carbon. As carbon is 23 percent of the body weight, the body content of 14C for a 70-kg person would be about 3.08 kBq.
According to the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) 1982 Report, "Ionizing Radiation: Sources and Biological Effects," 70 percent of the body content of 210Pb is in the skeleton. The UNSCEAR report estimates the typical 210Pb concentration in the skeleton to be 3 Bq kg-1, which equates to a total skeletal activity of 15 Bq for a 70-kg man. The other 30 percent of the 210Pb in the body, 6.4 Bq, would be distributed more or less uniformly throughout the soft tissues. The grand total of 210Pb in the body is thus 21.4 Bq.
The UNSCEAR report assumes the 210Po concentration in the skeleton to be 80 percent that of the 210Pb concentration. The UNSCEAR report estimates that the soft tissue concentration of 210Po is 2.4 Bq kg-1, which translates to a total skeletal 210Po activity of 12 Bq for a 70-kg man. In the soft tissues, the UNSCEAR report assumes a one-to-one ratio between 210Po and 210Pb. Hence the 210Po activity in the soft tissues would be 6.4 Bq. The grand total of 210Po in the body would be 18.4 Bq.
Levels of 210Po and 210Pb are expected to be lower for women than men and lower for children than adults. Smokers have higher concentrations than nonsmokers.
Radiation can be measured using sensitive detectors in a whole-body counter. These detectors can measure the gamma rays emitted by radioactive materials that are in or on the body. Different radioactive materials will give off gamma rays of different energies, which is one technique to identify the material. Other types of radioactive decay (beta and alpha) cannot be detected in this way, but fortunately, gamma rays often accompany them so most can be detected.
The instrumentation is very sensitive so that the detection limits are well below levels that have a health significance. For example, a whole-body counter can easily measure the amount of 40K that is naturally occurring (0.0117 percent of all potassium) in every person.
Neither 226Ra nor 228Ra is easily measured directly by whole-body counting because neither is a strong gamma-ray emitter. However, both have strong gamma emitters among their decay products that are easily measured by whole-body counting.
For 226Ra, the gamma-emitting decay products are 214Pb and 214Bi, the latter of which emits gamma rays at energies of 0.609, 1.12, and 1.76 MeV. The 1.76-MeV gamma ray, because it is higher in energy than the 1.46-MeV gamma ray from naturally occurring 40K, is usually used for whole-body counting. From these gamma-ray emissions, whole-body counting can measure the body content of 214Pb/214Bi. To derive the body content of 226Ra, some measurement or assumption must be made to determine the retention of 222Rn (the first decay product of 226Ra, and the parent of 214Pb/214Bi) by the body. This can be done by measuring 222Rn exhaled in the breath, but this technique is not readily available. In the long-term follow-up of radium workers, the average long-term 222Rn retention was 37 percent, but this factor could be different for recent exposures (Toohey et al. 1983).
For 228Ra, the first decay product is 228Ac, which emits gamma rays of energies around 0.9 MeV and can be measured directly; since the half-life of 228Ac is only 6.15 hours, it can be assumed to be in equilibrium with 228Ra. Another member of the 228Ra decay chain is 208Tl, which emits a strong gamma ray at 2.62 MeV, and its relative equilibrium with 228Ra can be determined by comparing the measured activities of 208Tl and 228Ac in vivo. Research-quality whole-body counters, such as the one at Argonne National Laboratory-East, which was specifically designed for the detection of 226Ra and 228Ra in former radium workers, have detection limits of about 100 Bq of 214Bi or 228Ac. A commercial whole-body counter would have detection limits several times higher because of higher background levels.
It should be noted that typical environmental intakes of 226Ra and 228Ra, such as from well waters that exceed the U.S. Environmental Protection Agency (EPA) standard for drinking water (185 Bq L-1 for each radionuclide), would be unlikely to exceed the detection limits of a whole-body counter, and indoor levels of 222Rn at the EPA limits (hundreds of Bq L-1 of air) will severely interfere with the measurement of 226Ra using 214Bi.
Of the small fraction of ingested uranium that is absorbed through the gut, most is quickly excreted in the urine and only a tiny amount is excreted in the hair. This is perfectly normal. The hair from different people—or even the same person—will contain varying amounts of uranium, depending on how much is in the water and food that people drink and eat. Some people might have ten or even hundreds of times the amount of uranium in their hair than do others.
Note also that analysis of uranium in hair is neither an accepted nor a reliable method of determining the uranium content in the body. Uranium is a heavy metal and is excreted in the hair and nails, but hair analysis for uranium is subject to inordinately high errors because the analyses also measure uranium commonly found in shampoos, soaps, hair dressings, dyes, and hair treatments of various types. Moreover, since uranium is ubiquitous throughout the environment, the hair sample must be carefully obtained, handled, packaged, and shipped under rigid controls to ensure that it is not contaminated by coming into contact with materials containing environmental uranium, which could be transferred to the hair sample.
Erroneously high results can also occur if analytical procedures are not rigidly controlled and performed with scrupulous care. Controls include appropriate washing of the sample to remove possible surface uranium and use of special, certified, ultra-pure reagents. Labware must likewise be free of uranium; uranium may leach from glassware and contaminate the sample, leading to erroneously high readings. Since the hair samples are so small, even a tiny amount of uranium contamination may give a grossly exaggerated and erroneous result.
There are few, if any, data in the peer-reviewed scientific literature relating to what normal levels of uranium in hair are or how these levels relate to uranium intake, amount in the body, and the amount excreted in the hair. Thus, there is a paucity of data regarding the uranium content of hair and what constitutes the "normal" range. There are no generally recognized established standards for uranium in hair. Background levels of uranium in hair are highly variable from person to person and region to region, depending in large measure on dietary factors, as most of the uranium in our bodies comes from the food that we eat.
Toohey RE, Keane AT, Rundo J. Measurement techniques for radium and the actinides in man at the Center for Human Radiobiology. Health Phys 44(1):323–341; 1983.