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Our world is radioactive and has been since it was created. Over 60 radionuclides (radioactive elements) can be found in nature, and they can be placed in three general categories:

  1. Primordial - from before the creation of the Earth
  2. Cosmogenic - formed as a result of cosmic ray interactions
  3. Human produced - enhanced or formed due to human actions (minor amounts compared to natural)

Radionuclides are found naturally in air, water and soil. They are even found in us, being that we are products of our environment. Every day, we ingest and inhale radionuclides in our air and food and the water. Natural radioactivity is common in the rocks and soil that makes up our planet, in water and oceans, and in our building materials and homes. There is nowhere on Earth that you cannot find Natural Radioactivity. Radioactive elements are often called radionuclides, just nuclides or incorrectly radioactive isotopes. There are over 2000 different radioactive nuclides. Often, radionuclides are symbolized based on the element and on the atomic weight, as in the case of radioactive hydrogen or tritium with an atomic weight of 3 is shown as H-3 or 3H where the latter is preferred in the present text. As another example, Uranium with the atomic weight of 235 would be shortened to U-235 or 235U.

Primordial Radionuclides

Primordial radionuclides are left-over from when the world and the universe were created. They are typically long-lived, with half-lives often on the order of hundreds of millions of years. Radionuclides that exist for more than 30 half-lives are not measurable. The progeny or decay products of the long-lived radionuclides are also under this heading. Here is some basic information on some common primordial radionuclides:

Table 1: Primordial nuclides
Natural Activity
Uranium 235
7.04 •108 a
0.72% of all natural uranium
Uranium 238
4.47 •109 a
99.2745% of all natural uranium; 0.5 to 4.7 ppm total uranium in the common rock types
Thorium 232
1.41 •1010 a
1.6 to 20 ppm in the common rock types with a crustal average of 10.7 ppm
Radium 226
1.60 •103 a
0.42 pCi/g (16 Bq/kg) in limestone and 1.3 pCi/g (48 Bq/kg) in igneous rock
Radon 222
3.82 days
Noble Gas; annual average outdoor air concentrations range in the US from 0.016 pCi/L (0.6 Bq/m3) to 0.75 pCi/L (28 Bq/m3)
Potassium 40
1.28 •109 a
Soil - 1-30 pCi/g (0.037-1.1 Bq/g)

Some nuclides like 232Th have several members in its decay chain. You can roughly follow the chain starting with 232Th:


or starting with 238U:

Some other primordial radionuclides are 50V, 87Rb, 113Cd, 115In, 123Te, 138La, 142Ce, 144Nd, 147Sm, 152Gd, 174Hf, 176Lu, 187Re, 190Pt, 192Pt, 209Bi.

Natural Radioactivity in Soil

How much natural radioactivity is found in a volume of soil that is 1 square kilometer and 50 centimeter deep? The following table is calculated for this volume (total volume is 5 • 105 m3) and the listed activities. It should be noted that activity levels vary greatly depending on soil type, mineral make-up and density (~1.58 g/cm3 used in this calculation). This table represents calculations using typical numbers.
table 2: Average Natural Radioactivity in an upper crust volume corresponding to one square kilometer by 50 cm deep, data taken from the table next page. We suppose equilibrium between 238U and 226Ra and an emanation factor of 222Rn of 0.5.
Uranium 238
36 Bq/kg (1 pCi/kg)
1500 kg
28 GBq (0.78 Ci)
Thorium 232
44 Bq/kg (1.2 pCi/kg)
7900 kg
34 GBq (0.93 Ci)
Potassium 40
850 Bq/kg (23 pCi/kg)
2528 kg
670 GBq (18 Ci)
Radium 226
36 Bq/kg (1 pCi/kg)
0.78 g
28 GBq (0.78 Ci)
Radon 222
28 kBq/m3 (0.76 nCi/m3)
2.4 µg
14 GBq (0.38 Ci)

>774 GBq (>21 Ci)

Human Produced Radionuclides

Humans have used radioactivity for one hundred years, and through its use, added to the natural inventories. The amounts are small compared to the natural amounts discussed above, and due to the shorter half-lives of many of the nuclides, have seen a marked decrease since the halting of above-ground testing of nuclear weapons. Here are a few human produced or enhanced nuclides:

Table 3: Human Produced Nuclides
12.3 a
Produced from weapons testing and fission reactors; reprocessing facilities, nuclear weapons manufacturing
Iodine 131
8.04 days
Fission product produced from weapons testing and fission reactors, used in medical treatment of thyroid problems
Iodine 129
1.57•107 a
Fission product produced from weapons testing and fission reactors
Cesium 137
30.17 a
Fission product produced from weapons testing and fission reactors
Strontium 90
28.78 a
Fission product produced from weapons testing and fission reactors
Technetium 99
2.11•105 a
Decay product of 99Mo, used in medical diagnosis
Plutonium 239
2.41•104 a
Produced by neutron bombardment of 238U
( 238U + n external image 83e37b7246fdfcb99b2754210ebeae27.png239Uexternal image 83e37b7246fdfcb99b2754210ebeae27.png 239Np +β- external image 83e37b7246fdfcb99b2754210ebeae27.png239Pu+β-)