Primordial terrestrial radiation accounts for a much larger share of the back­ground radiation we are all exposed to. But what is primordial radiation and where does it come from? Primordial terrestrial radiation comes from extremely long-lived radioisotopes that were present when the earth was formed. The three principal primordial radioisotopes that contribute to terrestrial background radiation are uranium (238U), thorium (232Th), and potassium (40K) (2). 238U has a half-life of 4.5 billion years, 232Th has a half-life of 14 billion years, and 40K has a
half-life of 1.3 billion years. Natural uranium also consists of 0.7% 235U, which has a half-life of 0.7 billion years, and of course is the isotope that is used in nuclear reactors. These elements are widespread in the earth’s crust but are much more prevalent in some areas than others. Uranium and thorium originally formed in stellar explosions known as supernovas and were spread throughout the universe. Potassium is formed in our sun and similar stars. When the earth coalesced out of interstellar matter, it contained these radioactive elements in its crust and in its core. The radiation from the primordial uranium and thorium accounts for much of the heat in the center of the earth that makes it molten.

Uranium and thorium are the beginning radioisotopes in a long series of decays that eventually end up as stable isotopes of lead. 238U and 232Th decay by emitting a particles, creating new radioisotopes that further decay by either a or p decay, along with emitting у rays. The decay scheme of 238U is particularly interesting because it leads to the most important component to our background exposure—radon (Rn) (Figure 8.3). The arrows that move down and left represent a decay QHe nuclei) in which the atomic number changes by 2 and the atomic mass changes by 4. Arrows that move to the right represent p decay where a neu­tron is converted into a proton, changing the atomic number by 1 (see Chapter 5 for more details on radioactive decay processes). The half-lives are roughly rep­resented by the size of the arrows. After several decays, 238U is converted into radium (226Ra), the radioisotope made famous by Marie and Pierre Curie. Radium a-decays to radon (222Rn), which has the unique distinction of being a gas with a short half-life of 3.8 days.

Internal Primordial

These primordial radioisotopes are widespread in the earth’s crust. For example, granite in the mountains in Colorado can have relatively high levels of uranium and thorium. Other areas on earth can also have high levels of thorium, partic­ularly monazite sands in Brazil and India. Potassium is widespread around the earth, and it is essential for all living organisms. Elemental potassium consists of 93.3% 39K, 6.7% 41K, and 0.01% 40K, but only the 40K is radioactive. We get a dose from 40K primarily through the food we eat—bananas are particularly high in potassium, as are Brazil nuts and red meat. It is not too much to worry about, though. You would have to eat 600 bananas to get the same dose as a chest X-ray! Smaller internal doses come from uranium and thorium that can also be ingested from various foods. The average US dose from ingestion of radioisotopes, primar­ily 40K, is 0.29 mSv/yr.

External Primordial

The a particles from uranium and thorium decay do not contribute to an external dose because they cannot travel far in air and cannot penetrate through clothing or our skin. The principal dose comes from the у rays that are emitted by radio­isotopes in the decay series. The distribution of external background radiation varies widely throughout the United States and indeed the world. It is particu­larly high in mountainous regions of the West, and that is a major factor con­tributing to the higher dose in Colorado compared to coastal areas (Figure 8.4). For Colorado communities at elevations lower than 2,000 meters (6,600 ft.), the

average terrestrial radiation dose is 0.79 mSv/yr, and for communities at eleva­tions above 2,000 meters, the average terrestrial dose is 1.12 mSv/yr (4). The com­munities at higher elevations get a higher dose due to the closer association with granitic rock containing uranium and thorium. Of course, they also get a higher dose from cosmic rays. Shielding from houses will reduce this dose, so the actual dose depends on how much time you spend outdoors. The average actual dose for Colorado is 1.25 mSv/yr from cosmic rays and terrestrial radiation (5). This com­pares to 0.54 mSv/yr for an average US citizen (Figure 8.1). The dose in Colorado is adding up!

Radon

We still have not gotten to the main source of background radiation that we are exposed to—radon. The average annual dose from radon is 2.28 mSv, but this also varies widely. Radon (222Rn) is formed from the a decay of radium (226Ra), but radon a-decays in 3.8 days to an isotope of polonium (218Po), which itself quickly a-decays through a series of p-emitting daughter products with half-lives of min­utes to another isotope of polonium (214Po), which in turn a-decays in a fraction of a second to a longer-lived isotope of lead (210Pb) and finally to stable lead (206Pb) (Figure 8.3). The unique aspect of radon is that it is a neutral gas that is formed in rocks and soils from the uranium decay series. External exposure is not a problem with radon because the a particles cannot penetrate the skin. Instead, radon con­tributes to an internal dose to the lungs. Radon itself is not particularly harmful because it is breathed in and out and doesn’t remain in the lungs. The danger from radon is actually the polonium daughter products that are also a emitters and are charged atoms (ions). They can readily attach electrostatically to small particles of negatively charged dust that can get trapped in the lungs. The a particles emitted by the polonium isotopes then irradiate cells in the lung tissue, and damage to these cells can lead to lung cancer (6). Recall from Chapter 7 that a particles are 20 times more damaging per Gy than у rays and that the lungs are among the most sensitive tissues in the body; thus a particle irradiation of the lungs is particularly dangerous.

Another unique aspect of radon is that the danger is not primarily from expo­sure outdoors but rather in our houses. Radon is formed as a gas in the soil and can percolate up to the surface through cracks and fissures in the soil. That is not much of a problem outdoors because it dissipates in the atmosphere, but if it leaks into a house through basement cracks and openings, it can remain trapped in the house and build up to dangerous levels. Many communities now require radon measurements in houses before they can be sold.

The level of radon is measured by the concentration of radioactivity1 in a cubic meter of air (Bq/m3). It is not simple to convert this to a dose in mSv/yr, so it is usually just specified as Bq/m3. The average concentration of radon in houses in the United States is 45 Bq/m3; the median is 24 Bq/m3, but the distribution is not normal—there are many houses with low levels but a few with much higher levels of radon, which skews the distribution. Furthermore, levels can vary substantially

from one house to another in the same community. The EPA recommends that action should be taken to mitigate radon if it is above 150 Bq/m3 (4 pCi/l). This can be done by sealing cracks in basement areas and venting the soil to the atmosphere.

As you might expect, radon concentration varies by region of the country and is generally higher in mountainous areas and lower in coastal areas (Figure 8.5). The overall average radon dose to a person in the United States is 2.28 mSv/yr. In Colorado the average radon dose is 2.87 mSv/yr (5) and in Fort Collins it is about 2.94 mSv/yr (7). In Leadville the average radon dose is 3.44 mSv/yr (5). Once again, we get a higher dose than elsewhere in the country, and people in Florida or Texas get a lower than average dose. According to the Texas State Health Department, the average radon dose in Texas is about 1 mSv/yr.

The US National Academy of Sciences did an extensive study of the health effects of exposure to radon (the BEIR VI report) in 1999. They analyzed 11 dif­ferent epidemiological studies of 68,000 underground miners who were exposed to radon, with 2,700 deaths from lung cancer. One of the major difficulties with these studies is the confounding problem of smoking, which is quite prevalent in miners. Radon and smoking work in a synergistic way, with a much greater risk for getting lung cancer after exposure to both of these carcinogens rather than just radon or just smoking. Using two different dose risk models, the report estimates that 10-15% (15,400 to 21,800) of cancer cases annually in the United States are due to indoor radon exposure. However, the large uncertainties suggest that the number of cases could range from about 3,000 to 33,000. The report also esti­mates that if all houses above the action level of 150 Bq/m3 were mitigated, about one-third of the radon-attributable lung cancer cases would be avoided; that is a reduction of 4% of all lung cancer cases (6). Since about 95% of cases of lung cancer occur in past or present smokers, a far more effective approach to reducing lung cancer would be to convince people to quit smoking!

The linear no-threshold (LNT) model is generally used to estimate the pos­sibility of getting lung cancer from exposure to radon. But is it really the best model? That question engenders strong conversations among radiation biologists. A recent assumption-free statistical analysis of 28 different scientific papers on lung cancer incidence from radon concluded that there is no evidence that radon causes lung cancer below a level of 838 Bq/m3, over five times the level at which the EPA recommends mitigation. The LNT model did not prove to be a good fit to the data (8). Thus, it is likely that the EPA recommendations for mitigation are extremely conservative.