[ RadSafe ] Current Dose Conversion Factors for Thoron

[Peter Collopy]

Just finished reading an UNSCEAR report (UNSCEAR, United Nation Scientific Committee on the Effect of Atomic Radiation. Annex B: Exposures from Natural Radiation Sources. UnitedNations, p. 104 (2000).) and they showed a conversion for radon and thoron air concentrations to dose as follows:

My question is why is thoron more limiting (i.e. a lower concentration) in these calculations? In 10CFR20 (I know its out of date) and some other publications the ratio is the opposite. i.e. you need more thoron inhaled to deliver an equivalent dose to a lesser amount of radon. 
I assume the total energy delivered is greater for the thoron but why was that not reflected in previous dose/DAC estimates? Peter Collopy, CIH, CHP, CSP 
Director, Entropy Control 
Chaos for the Future 
1777 Bristol Road
Damariscotta ME  04543