According to its Wikipedia entry, "banana equivalent dose (or BED) is a concept to place in scale the dangers of radiation by comparing exposures to the radiation generated by a common banana."
Many foods are naturally radioactive, and bananas are particularly so, due to the radioactive potassium-40, or 40K they contain. Bananas are radioactive enough to regularly cause false alarms on radiation sensors used to detect possible illegal smuggling of nuclear material at US ports.
A medium sized banana contains about 450 mg of Potassium. 0.0117%, or about 53 μg of this being 40K. 53 μg of 40K produces 14 radioactive decays per second (dps), or 0.00037 μCi of radiation. If the banana is eaten, the dose equivalent is about 0.01 mrem. 0.01 mrem is equivalent to 0.1 μSv.
A radiation dose equivalent of 100 μSv (10 mrem, or 1,000 BED) increases an average adult human's risk of death by about one micromort – the same risk as eating 40 tablespoons of peanut butter, or of smoking 1.4 cigarettes
Here is a link to the full entry: Banana Equivalent Dose.
In the coverage of the Japan's Fukushima nuclear plants SI multiples have been used, including the millisievert and microsievert. There was some confusion about this yesterday in the news as a spike was observed.
A millisievert (mSv) is a thousandth of a sievert and a microsievert is a millionth of sievert. A chest x-ray has 0.1 mSv. (Peter Wehrwein,editor of the Harvard Health Letter, has a good discussion of the topic at the Harvard Health Blog.)
Yesterday's spike at the Fukushima Dai-chi plant was 400 mSv per hour. Sounds like a bunch of bananas.