Answer to Question #11233 Submitted to "Ask the Experts"

Category: Radiation Basics — Doses and Dose Calculations

The following question was answered by an expert in the appropriate field:


Question 1: Should we consider respirable dust to find out the internal dose of a worker working in a monazite processing plant based on air activity present in a particular location, or should total dust be taken into account?

Question 2: In the thorium processing industry, is there any quick assessment of internal dose based on air activity?


Answer to Question 1: The dose should be based on the respirable fraction rather than the total dust concentration, because not all the dust in the air will be in the size range that can contribute to the inhalation dose. The specific area where the person is breathing also has to be considered to account for the person's internal exposure. Often a personal air monitor that collects an air sample near the worker's head can be used to help determine particle sizes of the dust and to obtain a quantitative measure of the amount of dust that could be inhaled by the specific worker.

Answer to Question 2: Yes, one method is to measure the airborne concentration of the respirable fraction of 232Th (and uranium, if present in the monazite) in the breathing zone of the worker. The radionuclide air concentration may vary with time, depending on the work location and tasks being performed. Next, account for the hours the person worked in the airborne radionuclide area(s) to determine the fraction of the annual limit on intake (ALI) and to compare to the derived air concentration (DAC).

The ALIs in Appendix B of 10 CFR Part 20 are the allowable annual intakes of given radionuclides by "reference man" which would result in either (1) a committed effective dose equivalent of 0.05 Sv (stochastic ALI), or (2) a committed dose equivalent of 0.5 Sv to an organ or tissue (nonstochastic ALI). The stochastic ALIs were derived to result in a risk, due to irradiation of organs and tissues, comparable to the risk associated with a deep dose equivalent to the whole body of 0.05 Sv.

The ALIs are used to determine DAC values, which are derived limits, intended to control chronic occupational exposures. The relationship between the DAC and the ALI is given by: DAC (Bq ml-1) = ALI (Bq) ÷ [2,000 h working year-1 × 60 min h-1 × (2 × 104 ml min-1)]. So the DAC (Bq ml-1) = ALI (Bq) ÷ (2.4 × 109 ml) , where 2 × 104 ml is the volume of air breathed each minute at work by "reference man" under conditions of "light work."

For example, if a person works in an area where the airborne concentration of 232Th is 1.85 × 10-8 Bq ml-1 for 2,000 h, the person's committed effective dose equivalent would be 0.05 Sv. Thus, the DAC for most chemical compounds of 232 Th is 1.85 × 10-8 Bq ml-1. This also depends on the particle size of dust being inhaled, the respiratory rate of the worker, and the chemical composition of the 232Th.

Another method for assessing internal dose is performing bioassays. The Centers for Disease Control and Prevention have a nice summary of analytical methods for thorium in urine and feces in Toxic Substances Portal—Thorium or Toxicological Profile for Thorium.

John P. Hageman, MS, CHP
Linnea Wahl, MS, CHP

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