Most of the amount of radiation we expose daily is natural radiation. According to the year 2000 report of UNSCEAR 87% of total radiation exposing is natural. The dose of natural radiation is very low. Therefore it is not concerned much. But recent researches have proven that there is an effect.
There are several factors to decide the amount of radiation human exposed to. In example, areas of heavy radioactive mineral exist, has high radiation. Radiation also increase with the altitude. Effectiveness also depend on the age and the time exposed.
There are three main categories of natural radiation sources. Those are, radiation comes from the radioactive mineral of earth, cosmic radiation and radiation inside the body.
There were many radioactive minerals in the beginning of the earth. Isotopes which had less half life have decayed but isotopes with long half lives are still staying. Thorium, Uranium and Potassium 40 are the examples. There are several places with high radioactivity have been identified from Australia, Brazil, China, France and Iran.
Some of the minerals have those radioactive elements with them and those came from prehistoric era. When those prehistoric rocks revealed to the earth surface, decay to small pieces because of the rain, water, wind, etc. and gather to shores, lagoons, tanks like lower lands. Therefore we can identify more radioactive minerals near the shores. Comparing to other places sea shores are more radioactive. Highly abundant radioactive minerals on earth surface are monazites, thorionite and thorite.
Effect from the Radon gas
Radon is a noble radioactive gas. It's isotopes naturally forms daughter elements of Thorium and Uranium. Houses made from soil or sand containing Thorium and Uranium minerals have high level of Radon gas inside. When inhale such radioactive gases may cause lung cancers. As the new calculations, annual radioactivity per person of exposing to Radon gas is 2.0 mSv. According to the 2002 report of World Health Organization (WHO) if a person reveals for 20 Bqm-3 home Radon gas for the entire life, 3 of 1000 may get lung cancer. And they also calculated there are 15000 deaths in USA and 2500 deaths in UK according to this matter. They have established rules to reduce Radon gas in homes.
Cosmic radiation
Cosmic rays are a combination of various nuclear particles. Most of the amount is high energetic protons. It is 90%. Others are alpha particles, beta particles, HZE ions, positrons and antiprotons. We cannot identify exact beginning of those but we can identify some types such as galactic cosmetics rays, sun rays and earth rays. Sun flames emit low powered protons to the space. Those protons and other particles come to the earth's atmosphere and subject to the magnetic fields and direct to the north and south poles. Therefore the amount of radiation change on the latitude. In addition going along the earth's atmosphere, these particles are subjected to complicated reactions. Therefore amount of rays are low on sea level. In other words amount of cosmic rays increase with the altitude. Amount of cosmic rays double in every 6000 feet. Annual effect of cosmic rays is about 0.3 mSv.
Radiation inside the body
Radioactive isotopes go inside with foods we consume. This is about 11% of the total radioactivity. Lead 210, Polonium 210 and Carbon 14 are these isotopes. Total radioactivity is 0.3 mSv and most of it generates by Carbon 14 isotope.