BACKGROUND RADIATION
 There are three main sources of background radiation:
  Natural Background
  Medical
  Occupational

     Average natural background in the United States about 300 mrem/yr with the majority, around 200 mrem/yr, due to radon gas, a daughter product of uranium decay.  However, many places on earth receive more than 1000 mrem/yr from radon.  The amount of radon in theair depends on the uranium content of the soil in the area, weather conditions and other atmospheric conditions.  Also, concentrations will be higher indoors than outdoors due to a lack of ventilation and the uranium content of construction materials.
     External gamma or x-ray radiation due to cosmic and terrestrial radiation sources average about 55 mrem/yr in the United States.  In Guangdon Province China, the average is between 300 and 400 mrem/yr, in Kerala India 380 mrem/yr and in Guarapari Brazil it averages greater than 600 mrem/yr.  The cosmic radiation sources are galactic cosmic rays, geomagnetically trapped particles and solar cosmic rays.  The amount of cosmic radiation varies with atmospheric thickness, i.e. altitude and latitude.  The atmosphere provides an effective shield for cosmic radiation.  The gamma and x-ray radiation from the soil is due to uranium and thorium decay chains and naturally occurring ⁴⁰K( Potassium-40 ) and varies with geographical location.  The above table shows the comparison of terrestrial and cosmic radiation by location.
     Internal radiation is due to the ⁴⁰K which is naturally present within the body.   ⁴⁰K occurs to an extent of 0.01% in natural potassium.  A person who weighs 70 kg contains about 140 g of potassium, most of which is located in the muscle.  It follows that the ⁴⁰K content of the body is of the order of 0.1 uCi.
     The following pie chart shows the remaining background sources as determined by NCRP 93. These include medical sources of background radiation from x-rays and nuclear medicine procedures.  In addition to these, we receive about 10 mrem/yr from consumer products such as tobacco products which contain ²¹⁰Po, domestic water containing ²²⁶Ra and ²²⁰Rn, combustible fuels, ophthalmic glass containing thorium for rose tinting, luminous dials and signs containing ³H and ¹⁴⁷Pm and smoke detectors containing ²⁴¹Am.