Natural radioactivity is present in the air, water and soil around us. Some of the radioactive isotopes were created in with the universe. Others are generated by radioactive decay of other elements. Cosmic ray from space create still others by interacting with terrestrial elements. The half-lives of these naturally occurring radionuclides vary from days to billions of years.
Uranium 235 makes up about three quarters of one percent of all natural uranium on Earth. It has a half-life of seven hundred million years.
Uranium 238 has a half-life of four and one half billion years and makes up over ninety nine percent of all natural uranium. Common rock types contain from one half to five parts per million of uranium.
Thorium 232 has a half life of fifteen billion years. Common rocks have on average ten parts per million of thorium.
Radium 226 is present in minute quantities in limestone and igneous rock. It has a half-life of one thousand six hundred years.
Radon is a noble gas which has a half-life of about four days and is found in tiny amounts in soil and air.
Potassium 40 is found in the soil in small concentrations and has a half-life of about one billion years.
There are many other natural radionuclides on earth present in minute quantities. The concentration of all these natural radionuclides varies depending on the location on the surface of the Earth. The average concentrations of the most common natural radionuclides in a volume of soil one inch deep over one square mile is:
Two thousand kilograms of uranium emitting thirty one billion becquerels (GBq).
Twelve thousand kilograms of thorium emitting fifty two GBq
Two thousand kilograms of potassium 40 emitting five hundred GBq.
Two grams of radium emitting 63 GBq.
Eleven micrograms of radon emitting seven and one half GBq.
The total of all these natural radionuclides in this patch of soil is six hundred and three GBq.
Carbon 14 is created by cosmic rays and has a half-life of about six thousand years.
Hydrogen 3 or Tritium is created by cosmic rays has a half-life of about twelve years.
Beryllium 7 is created by cosmic rays has a half-life of fifty three days.
Other less common cosmic ray produced radionuclides are present in our environment.
The common radioactivity found in the Earth's oceans is primarily composed of:
Uranium contributes fourty one exabecquerel (one billion billion becquerels) (EBq).
Potassium 40 contributes fourteen thousand exabecquerel EBq.
Tritium contributes three quarters of one exabecquerel EBq.
Carbon 14 contributes six and three quarters exabecquerel EBq.
Rubidium 87 contributes thirteen hundred exabecquerel EBq
The total radioactivity in the oceans attributable to these radionuclides is sixteen thousand and eighty seven EBq.
All together these different naturally occurring sources of radioactivity make up what is known as the natural radioactive background. Until the human race began developing nuclear technology, we were only exposed to the natural background radiation.