MadSci Network: Physics
Query:

Re: what is dac and mpca? how it is related to curie?

Date: Sat Mar 8 00:55:51 2003
Posted By: Gerald Gels, Certified Health Physicist
Area of science: Physics
ID: 1044618305.Ph
Message:

jeetendra:

From your question, I will assume that you know the basics of radiation 
physics.  The DAC stands for Derived Air Concentration.  Since it is a 
concentration, the units commonly used are microcuries per cubic 
centimeter (uCi/cc) or per milliliter (uCi/mL).  It is derived from a more 
basic limiting value, the ALI, or Annual Limit of Intake, which is defined 
for each radioisotope.  The ALI is defined as that amount of a radioactive 
isotope (in activity units such as microcuries) that, if taken into the 
body by inhalation, will result in a dose of 5 rem (the annual 
occupational limit in the US).  The DAC is derived from this limit by 
calculating the concentration of that radionuclide in air which would lead 
to an ALI.  Typically, a breathing rate of 20 L/min is assumed for a 40 
hour work week and a 50 week year.  Thus the ALI would be divided by (20 
L/min X 1000 cc/L X 40 hr/wk X 60 min/hr X 50 wk/yr.)  The result would be 
the DAC in units of uCi/mL (or uCi/cc).  The DAC is defined as "that 
concentration of a radionuclide that, if present in the air continuously, 
would result in a dose equal to the occupational annual limit."  Other 
DACs can be calculated for other limits such as a "non-occupational" 
limit, but in that case, the time of exposure might have to be modified 
from 40 hrs/wk to perhaps even 168 hrs/wk, depending on the circumstances.

The MPCa (where the "a" is a subscript) is the unit that the DAC 
replaced.  It was called the Maximum Permissible Concentration for air.  
It was defined as "that concentration of a radioisotope in air which would 
lead to an annual dose of 5 rem due to inhalation of that isotope."  It 
was essentially the same as the DAC, but use of the term "DAC" makes clear 
that the more basic unit is the ALI from which the DAC is derived.

I hope the above explanation is helpful.  Please feel free to follow up 
with additional questions if you need any clarification.


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