Radiation and radioisotopes are ubiquitous in nature existing since the origin of the universe. The sources are called natural background radiation sources.
- Cosmic radiation: The earth’s atmosphere provides shielding from most of the cosmic radiation.
- Cosmogenic radionuclides: 10Be, 14C, 26Al, 36Cl, and 41Ca
- Primordial radionuclides: Natural radionuclides found in earth, soil, rock: uranium, thorium and their decay products, and 40K. The median values of the activity concentrations of 40K, 238U and 232Th in soil are: 400, 35 and 35 Bq/kg respectively (USCEAR 2000 Report). The radiation dose varies depending on the prevailing activity concentration level.
- Radionuclides in gaseous form: Radon (222Rn) and thoron (220Rn) from uranium and thorium decay series respectively. The major contribution of dose from natural radiation arises due to inhalation of radon and its alpha emitting particulate daughters. Exposure to radon has been a major health hazard in uranium mines. Radiological safety aspects of the mine workers are adequately addressed in ICRP/IAEA/WHO documents. The issues are discussed in detail in the reference book on Radiological Protection and Safety - A Practitioner’s Guide.
Radon is also found airborne in homes, particularly in dwellings of cold countries, like United States and Canada where the air in the dwellings and work areas is recirculated.
Radon comes from the radioactive decay of natural of uranium/radium present in trace quantities in bricks, soil, rock, and water. Being gaseous, it gets into the air you breathe. Basement areas are more radon prone due to exhalation of radon from the floor and walls. Radon typically moves up through the basement/ground to the air above and into your living areas through cracks and other cavities in the foundation. There is a guide “Home Buyers and Sellers Guide to Radon” by US EPA which covers all the radon related issues.
Thus, humans on this planet have been continuously exposed externally and internally, to ionizing radiation from natural sources. The human body even contains some of these naturally-occurring radioactive minerals. The natural radiation is the largest contributor to the collective (total) dose of the world population.
Until the invention of X-ray in 1895 by Wilhelm Roentgen, only the radiation in existence was the natural radiation, which was discovered in 1897 by Henri Becquerel. The man-made sources include: The X-rays, artificial radionuclides generated from nuclear weapon tests, use of atomic energy to produce electricity, radionuclides used for medical, industrial and agricultural applications. Medical exposures contribute significantly depending on the level of health care prevailing in a country. According to the National Council on Radiation Protection and Measurements (NCRP), the average annual radiation dose per person from natural and man-made sources in the U.S. is 6.2 millisieverts (mSv). The main contributing source to this higher value in the US is the medical exposures (3.00 mSv/y). The Indian average value for medical exposures is 0.6 mSv/y.
Annual average (world-wide) doses to individuals from ionizing radiation sources
|Source/mode||Annual average Dose (mSv/y)||Typical range (mSv/y)|
|Radon/inhalation||1.26||0.2 – 10|
|Terrestrial/external||0.48||0.3 – 0.6|
|Ingestion||0.29||0.2 – 0.8|
|Cosmic radiation||0.39||0.3 – 1.0|
|Total (natural)||2.4||1 – 10|
|Artificial sources of Radiation|
|Medical diagnosis||0.6||Near 0 to several tens depending on the type of diagnosis.|
|Atmospheric nuclear Testing||0.005||Higher doses at test Sites.|
|Chernobyl accident||0.0002*||From globally dispersed.|
|Public exposure from Nuclear fuel cycle Operations||0.002||Doses up to 0.02mSv for critical groups at 1km from some nuclear reactor sites.|
|Total artificial||0.6||From zero to several tens.|
The radiation dose from natural background radiation differs from location to location depending on the concentration of natural radionuclides in the soil. Contribution from cosmic radiation will be higher at high altitudes from sea level. The value of 2.4 mSv/y is the global average value. In the coastal regions of south India, the levels are much higher due to the presence of monazite sand containing thorium in the beaches. Medical exposures contribute only 2% of the average individual dose in India. This also is an indication of medical care utilization in the countries.