Bq Becquerel
1 Bq is one radioactive decay per second and is a very small unit. One adult human contains about 4,500 Bq from natural sources. More can be found form World Nuclear
Sv Sievert
The Sievert is a measure of the health effect of ionizing radiation on the human body.
Also here World Nuclear gives a good overview of the situation.
A very illustrative, and apparently reliable, Radiation Dose Chart is provided by Wikimedia.
Other Units
Several other units are still used in different literature:
1 Sv = 1 J/kg = 1Gy = 100 rad = 100 rem = 100 Roentgen
Becquerel to Sievert
There is no clear-cut relation between Becquerel and Sievert.
The only guide found is in a paper from Luckey fig. 10. Here you will deduct: 1 mSv = 1.7 kBq/litter.
Without having checked and fearing being wrong I quote:
1000 Bq/m^3 = 4.45 mJ*hour/m^3 = 6.3 mSv

Damage to humans

  • 500 mSv: Careful monitoring of survivors of the nuclear bombing in Japan showed that people who were exposed to 500 mSv or less had no shorter average lifespan than the rest of the population.
    (500 mSv in a single exposure will cause minor symptoms of acute radiation sickness, and is more than 100 times what is normally permitted)
  • 3000 mSv will result in half of the exposed will die and the survivors vill have an average lifespan three years less than the rest of the population.
  • Contrary to many horror stories and calculation of future deaths, there is only a very little correlation between radiation and cancer.

Most predictions and many atrocity stories are based on calculations.
It is usually assumed that no matter how little, then all radiation is harmful (LNT)
However, this hypothesis is clearly at odds with reality.

Types of radiation

Alpha radiation is fast nuclei of helium.
It is stopped by a layer of paper.
Beta radiation is fast electrons.
Both alpha and beta radiation can cause severe skin-burns, but not radiation sickness unless absorbed through the digestive system, or the lungs.
Gamma radiation is ultra short light waves.
It can penetrate the skin and cause severe radiation damage.
Neutrons appear only in connection with nuclear bomb attack.
However, in conection with accidents in relation to tests and wrong handling of plutonium, there may be serious exposure to neutrons.
(At an experiment going wrong and turning into “criticality” a chemist, Cecil Kelley died from a 200 microseconds blast of Neutron and gamma radiation.)