MadSci Network: Physics |
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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