Conversion of Becquerel to millisievert

The contamination of soil, food and water is given in Becquerel/sq.metre or Becquerel/Kg or litre. [1]

Becquerel
Becquerel (Bq) means one disintegration per second.

Dose Equivalent
A measure of the biological damage to living tissue as a result of radiation exposure. Also known as the " biological dose," the dose equivalent is calculated as the product of absorbed dose in tissue multiplied by a quality factor and then sometimes multiplied by other necessary modifying factors at the location of interest.

Sievert
The dose equivalent, measured in sievert, defined as the dose of absorbed radiation that has the same biological effect as a dose of one joule of gamma rays absorbed in one kilogram of tissue. In the United States the roentgen equivalent man (rem), equal to 0.01 sievert, is still in common use, although regulatory and advisory bodies are encouraging transition to sieverts

Conversion Becquerel/Sievert and Sievert/Becquerel
1 Bq                 = 0.013 microsievert
1 microsievert = 76.923 Beq

The Becquerel/Sievert conversion factor has been extracted from detailed tables of “Dose coefficients to calculate radiation exposure 2001”.  [2]
In these tables, published in the official German Federal Gazette 2001, dose coefficients are provided for different radionuclides related to the intake by aspiration or ingestion by different age groups which enables to calculate organ doses and effective doses (which means the sum of all organs).

The Doses coefficient for calculating exposure for a group aged > 17 years (Adult persons) which ingested 1 Bq Cs-137 1,3E-08 Sv (This equals 0,000 000 013 Sv/Bq).
The reciprocal value, or inverse value is 1 Sv =  76 923 076 Bq  or 1 mSv = 76 923 Bq.

Examples [3]
As a rule of thumb, one can say that eating an amount of food with 80.000 Becquerel Caesium 137 is equivalent to approximately one millisievert. Eating 200 g mushrooms contaminated with approximately 4.000 Becquerel Caesium 137/Kg is equivalent to 0,01 Millisievert. (Rounded figures, for exact conversion, see the foregoing factors)

Radiation exposure
The radiation exposure considers how long the exposure took place. It is given in Millisievert/year.
Example; The natural radiation exposure in Germany  is 2,1 Millisievert/year (0,24 Microsievert/hour) another 2Millisievert are added as artificial radiation (mainly medical devices)

Radiation limits for food
Food  radiation  should not exceed 500 Becquerel/Kg.
Baby food  radiation should not exceed 400 Becquerel/Kg for Caesium and 100 Becquerel for iodine.

[1] Nuclear Regulatory Commission. "NRC Regulations: §34.3 Definitions". United States Government.
http://www.nrc.gov/reading-rm/doc-collections/cfr/part034/part034-0003.html

[2] Dose coefficients to calculate radiation exposure. Published in the Bundesanzeiger No. 160a and 160b of 28 August 2001.of  the Federal Republic of Germany.
http://www.bfs.de/en/bfs/recht/dosis.html

[3]  David Tait Niels Roos (Kiel): ForschungsReport 1/2011. “25 Jahre Tschernobyl. Was ist geblieben?" [Article in German]
http://www.vti.bund.de/fileadmin/dam_uploads/vTI/Presse/PDFs/PM_pdf_2011/FoReport_1-11_Tschern.pdf