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originally posted by: stumason
a reply to: bksl90
Anything with a Bq of 25 is perfectly safe - for example, banana's contain trace amounts of radioactive potassium and have an average Bq rating of around 15 or Brazil nuts, which contain Potassium and Radium, can be as high as 400Bq/kg.
1Bq is an extremely small unit of measurement. It does seem from your OP that you are somewhat confused about the unit, which is possibly what has led to this thread.
The Bq rating is a measurement of radioactivity in a given amount - your statement that "if we're drinking 25Bq/kg of plutonium" seems to imply you think that 1 Bq is different from another depending on the material present - it isn't.
If something gives off 25Bq, it gives off 25Bq, regardless of whether it has radium, potassium or plutonium or in fact any other element in it. It's like saying 1 litre of water is more than 1 litre of petrol.
The major natural source of radioactivity in plant tissue is potassium: 0.0117% of the naturally-occurring potassium is the unstable isotope potassium-40 (40K). This isotope decays with a half-life of about 1.25 billion years (4×1016 seconds), and therefore the radioactivity of natural potassium is about 31 Bq/g – meaning that, in one gram of the element, about 31 atoms will decay every second. Plants naturally contain other radioactive isotopes, such as carbon-14 (14C), but their contribution to the total activity is much smaller. Since a typical banana contains about half a gram of potassium, it will have an activity of roughly 15 Bq. Although the amount in a single banana is small in environmental and medical terms, the radioactivity from a truckload of bananas is capable of causing a false alarm when passed through a Radiation Portal Monitor used to detect possible smuggling of nuclear material at U.S. ports.
The dose uptake from ingested material is defined as committed dose, and in the case of the overall effect on the human body of the radioactive content of a banana, it will be the "committed effective dose". This is typically given as the net dose over a period of 50 years resulting from the intake of radioactive material.
According to the US Environmental Protection Agency (EPA), isotopically pure potassium-40 will give a committed dose equivalent of 5.02 nanosieverts over 50 years per Becquerel ingested by an average adult. Using this factor, one banana equivalent dose comes out as about 5.02 nSv/Bq × 31 Bq/g × 0.5 g ≈ 78 nanosieverts = 0.078 μSv. In informal publications one often sees this estimate rounded up to 0.1 μSv.
A 1978 paper from Oak Ridge National Laboratory estimated that coal-fired power plants of that time may contribute a whole-body committed dose of 19 µSv/yr to their immediate neighbours in a 500 m radius.
A preliminary investigation into the radiological conditions in underground non-uranium mines in Western Australia has been undertaken. Measurements of radon concentration by passive track etch monitors and absorbed gamma dose-rate by thermoluminescent dosimetry were undertaken in 27 mines.
The average annual effective dose across all mines was estimated to be 1.4±1.0 mSv, ranging from 0.4 mSv for a nickel mine to 4.2 mSv for a coal mine. Radon progeny exposure contributed approximately 70% of the total effective dose. The estimated average annual effective dose in three coal mines (employing 297 workers) was 2.9±1.5 mSv. On the basis of this preliminary investigation it was concluded that no regulatory controls are specifically required to limit radiation exposures in Western Australia underground mines.
the average natural background radiation in the United States is 2.6 mSv.