The units of Becquerel and Sievert are the main measures of radiation dose. Becquerel is a unit of radioactivity and focuses on where radiation comes from. It is used to express the amount of radioactive materials contained in soil, foods, tap water, etc. The higher the value expressed in becquerels, the larger the radiation being emitted. Sievert is a unit of radiation exposure dose that a person receives and is used with regard to what is exposed to radiation, i.e. the human body. The larger the value expressed in sieverts, the larger the effects of radiation to which the human body is exposed.
1 Sievert is a very large does and usually does are expressed in milli-Sievert (mSv) where 1 Sv = 1000 mSv. Safety limits of radiation for industrial workers are set at 20 mSv per year. Doses of radiation above 5 Sv can be lethal.
1 Becquerel is a small amount of radioactivity and radioactivity levels are often expressed in units of Giga-Becquerel (GBq) = 1000 Million Becquerel or Tera-Becquerel (T Bq) = 1 million million Becquerel. The natural radioactivity of the body of a 70 Kg human being is about 5,000 Bq ( representing 5,000 atoms undergoing radioactive decay each second).
More details are given on the page: Essentials of Nuclear Physics