Humans have been exposed to radiation from natural sources since the dawn of time. The sources include the ground we walk on, the air we breathe, the food we eat and the solar system on the whole. Everything in our world contains small amounts of radioactive atoms like Potassium 40, Radium 226 and Radon 222. These are either left over from the creation of the world (like Uranium and Radium) or made by interactions with cosmic radiation (like Carbon 14 and Tritium). These natural sources of radiation make up approximately 85 percent of the average annual dose to the UK public (Hughes & O’Riordan, 1993).
The National Radiological Protection Board estimates that the average radiation dose to an individual in the UK is 2.6 mSv per annum. The major contributions to this are natural radiation sources that have always been present and are unrelated to the nuclear industry. These include uranium in the earth’s crust, cosmic radiation, various naturally radioactive materials in the diet, and radon in the air. Averaged over Northern Ireland, natural radiation accounts for 2.2 mSv per annum, but varies widely. When artificial sources are taken into account then the total annual dose in Northern Ireland becomes 2.54 mSv. This estimate can be attributed to the different sources of radiation as shown in Table 2.1.
Table 2.1 Average radiation exposure to Northern Ireland population

Figure 2.1 Sources of radiation exposure to U.K population

As seen in Table 2.1 and Figure 2.1, four of the radiation sources, namely radiation from Nuclear discharge, Products, Fallout and Occupational contributes negligibly to the average dose i.e., less than 1%.
A total average annual dose of 2.54 mSv/year to members of the Northern Ireland population is contributed by the other three sources: naturally occurring radiation, medical uses of radiation, and radiation from consumer products. By far the largest contribution (85%) is made by natural sources, threefifths of which is caused by radon and its decay products. The remaining 15% of the average annual dose consists of radiation from medical procedures (x-ray diagnosis and nuclear medicine).
Uncertainties exist in the data shown in Table 2.1. Uncertainties for exposure from some consumer products are greater than those for exposures from some cosmic and terrestrial radiation sources. The estimates for the most important exposure, that of lung tissue to radon and its decay products, have many associated uncertainties. Current knowledge of the average radon concentration, the distribution of radon indoors in Northern Ireland, and alpha-particle dosimetry in lung tissue is limited.