A curie and a becquerel are both scales used to measure radioactivity. They are both named after the scientists who are credited with discovering radioactivity.
A curie, originally defined in 1910, was a rather vague determination of the radioactivity produced by one gram of radium. With modern analysis methods, this can be more accurately be defined as 3.7×1010 decays per second based upon the activity of the radium isotope 226Ra. A picocurie is one trillionth of a curie.
The becquerel, a term introduced in 1975, is a much simpler concept and does not rely on any one particular radioactive element. It refers to one radioactive decay of a nucleus of an atom in one second.
If you need to convert from picocuries to becquerels you just need to multiply by 0.037. So 4 pCi is equivalent to 0.148 Bq.
Here’s where it gets interesting. The action level in the U.S. is based upon picocuries per litre of air ( pCi/L ) whereas the rest of the world use becquerels per cubic metre of air ( m3 ).
One cubic metre of air contains 1,000 litres. This means that when looking at the radon action levels between the U.S. and the rest of the world, all you need to do is multiply the pCi/L count by 37 ( 0.037 x 1,000 ) to convert the U.S. action level to becquerels per cubic metre.
The action level in the U.S. is 4 pCi/L which equates to 148 Bq/m3 in Canada. The 200 Bq/m3 action level n Canada translates to 5.4 pCi/L.
This doesn’t mean that radon is safer in Canada, just the Health-Canada has opted for a higher action-level than the U.S. on which it asks people to base their mitigation decisions upon.
Interesting to note, based on more recent research, the World Health Organization recommends homeowners take action to remediate the radon level in their home if it exceeds 100 Bq/m3. This corresponds to 2.7 pCi/L.