We live in a radioactive world. There are many natural sources of radiation which have been present since the earth was formed. The three major sources of naturally occurring radiation are:
- Cosmic radiation
- Terrestrial radiation known as sources in the earth’s crust
- Internal sources or sources found in the human body
Cosmic Radiation
Cosmic radiation comes from the sun and outer space and consists of positively charged particles, as well as gamma radiation. At sea level, the average cosmic radiation dose is about 26 mrem per year. At higher elevations the amount of atmosphere that shields us from cosmic rays decreases and thus the dose increases. For instance, those that live in the “mile high” city of Denver have an annual cosmic radiation exposure of 50 mrem per year. The average dose in the United States is approximately 28 mrem per year.
Terrestrial
There are natural sources of radiation in the ground, rocks, building materials and drinking water. This is called terrestrial radiation. Some of the contributors to terrestrial sources are natural radium, uranium and thorium. Radon gas, which emits alpha particle radiation, comes from the decay of natural uranium in soil and is ubiquitous in the earth’s crust and is present in almost all rocks, soil and water. In the USA, the average effective whole body dose from radon is about 200 mrem per year. Nearly all rocks, minerals, and soil may contain small amounts of naturally occurring radioactive materials.
Internal
Our bodies also contain natural radionuclides. Potassium 40, crucial for life, is one example. The total average dose from natural internal sources of radiation is approximately 40 mrem per year.
