The naturally occurring a activity of food is mainly associated with 226Ra (1599 y) in the uranium series, its a-emitting progeny [especially 210Po (138.38 d)], and the a-emitting daughters of 228Ra (5.76 y) in the thorium series. Both 210Pb (22.6 y) and its daughter 210Po can enter food not only from the soil, but also from the atmosphere, where they are present as the daughters of 222Rn. Examples of atmospheric concentrations at ground level are 148 ^Bq/m3 (4aCi/liter) for 210Pb and 37 ^Bq/m3 (1aCi/liter) for 210Po. For leafy vegetables, the 210Pb and 210Po contents are determined more by "natural fallout and rainout'' from the atmosphere than by the composition of the soil. Food accounts for 80-90% of the intake of 226Ra, with the remainder coming from water. The average daily intake of 226Ra is about 110 mBq (3pCi) in food having 37—370mBq/kg (1— 10pCi/kg). Cereals and other grain products, vegetables, and fruits are the main sources. Specific examples of approximate specific activity [in mBq/kg and (pCi/kg)] are whole-wheat bread 111 (3), milk 9 (0.24), potatoes 56 (1.5), fruit 48 (1.3), fresh vegetables 74 (2), fish 3.3 (0.9), meat 19 (0.5), and poultry 24 (0.8). For 228Ra the intake is half or less than that of 226Ra.

Most of the naturally occurring activity in food can be attributed to 40K (1.27 x 109 y) (0.0117at. %). A typical pair of approximate values [Bq/kg and (nCi/kg)] is 25-74 (0.7-2). A few of the foods with higher specific activities are bananas 130 (3.5), spinach 222 (6), and coconut 222 (6). For growing plants, the availability of potassium is subject to large variation. Depending on the nature of the soil, only a small percentage of the potassium in the soil may be exchangeable and, therefore, available.

In the United States the daily dietary intake of the emitter 210Pb (22.6 y) (uranium series) is about 37mBq/kg (1pCi/kg).

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