Radioactivity is definitely a threat to our health. It has been said that there is no safe minimum dose of radiation but we seem to survive in a natural environment with many different sources of radiation both outside and inside our bodies. On the average, there are over 800 radioactive events in the human body every second. Here is a list of the radioactive isotopes in our bodies.
(There are 28.3 grams in one ounce, one thousand milligrams in a gram, on million micrograms in a gram, one billion nanograms in a gram, one trillion picograms in a gram, and one quadrillion femto grams in a gram, and one thousand grams in a kilogram)
Potassium-40 - 16.5 milligrams - 4,340 disintegrations per second
Carbon-14 - 16 nanograms - 3080 disintegrations per second
Rubidium-87 - 190 milligrams - 600 disintegrations per second
Lead-210 - 5400 picograms - 15 disintegrations per second
Helium-3 - 20 femtograms - 7 disintegrations per second
Uranium-238 - 100 micrograms - 3-5 disintegrations per second
Radium-228 - 46 femtograms - 5 disintegrations per second
Radium-226 36 micrograms - 3 disintegrations per second
The uranium, potassium and rubidium in our bodies were created in stellar explosions before the Earth was formed. The lead and radium isotopes were created by thorium and uranium decay. Helium-3 and Carbon-14 are being continuously created by cosmic rays bombarding the atmosphere of the Earth.
Potassium-40 is present in all the food that we eat in tiny quantities. Potassium is abundant in our environment and plants take it up from the soil. We consume about two and one half grams of potassium every day. It is an essential part of our diet and our bodies maintain a constant level.
Carbon-14 makes up a tiny amount of the roughly 16 kilograms of carbon in our bodies. It is constantly being created by cosmic rays interacting with nitrogen in the atmosphere. All living things breath in tiny amounts of carbon-14 as their bodies constantly replace carbon. Carbon-14 can be used to date the age of a biological material because when something dies, it stops taking in carbon-14 which decays and slowly disappears.
Rubidium has no known biological function but probably mimics potassium in our tissue. Rubidium is a very abundant element in the Earth's crust but rubidium-87 makes up a very small part of all the rubidium.
Most of the lead-2190 in our bodies comes from the food we eat but some of it is inhaled from the air as a decay product of Radon-222. People also absorb lead-210 from smoking cigarettes.
Helium-3 or tritium is present in the water we drink in minute quantities. Originally all the helium-3 was produced by cosmic rays interacting with nitrogen in the air but since the advent of nuclear power, a small amount of helium-3 is release from nuclear power plants in water vapor.
Uranium-238 finds its way into our bodies as a contaminant in the food that we eat. Its decay generates other radioisotopes.
Radium is present in all soils and in the water in some areas. The radioisotopes of radium are present in all the food that we eat and some of the water we drink depending on the area we live in. Radium 228 and Radium-226 decay into other radioisotope which emit radiation as they decay.
There are many other radionuclides in our bodies but they do not contribute much radiation. We may also have iodine-131, cesium-137 and strontium-90 from fallout from nuclear explosions and some will have radioisotopes from nuclear medicine. Radon is always present in the air we breathe in small amounts.
It is difficult to pin down the amount of damage these radioisotopes do in the body. Different isotopes mimic different elements that cause different organs to absorb them. Iodine-137 is taken up by the thyroid gland, cesium-137 winds up in muscle tissue and strontium0-90 and radium-228 and 226 make their way into the bones. Different people have different sizes of organs, different amounts of muscle and different mass in bone. In the bones, a lot of alpha particles from radioisotopes hit bone and do no damage. Others hit cells and kill them. So the actual danger of an alpha particle causing cancer is very low compared to all the alpha particles emitted.
A variety of radioisotopes is present in our environment and in our bodies. It is part of our life and inescapable. Most of it does no damage but a small amount accounts for different illnesses, many of which can be fatal.