ICRP 71 question

["Charp, Paul" <pac4@ATSDTO1.EM.CDC.GOV>]

In ICRP 71 (age adjusted inhalation dose conversion factors) on page 308 is 
the table for U-234 class S.  For the dose to the bone surface of the 15 
year old age range, the DCF is listed as 1.1E-6 Sv/Bq.  This value is 
somewhat higher than the 10 year old or the adult age ranges.  The same 
trend is apparent in the other radioisotopes of different solubility 
classes.  In essence, the highest DCF is for an infant, then decreases until 
the age range of 10 years at which time the DCF increases.  For an adult, 
the DCF again decreases.

On first inspection this seems to be somewhat strange.  Does anyone have an 
explanation for this trend?

thanks

paul charp (pac4@cdc.gov)