Re: Investigation Levels for Extremities

[Michael Williamson <M.Williamson@mailbox.uq.oz.au>]

>         In keeping with our ALARA program we are assigning
>         investigation levels to extremity monitoring results.
>         Typical TLD exposures that would warrant this at our
>         license have come back as 250 - 500 mrem for a 30 day
>         period of wear.  Please comment on how you assign
>         investigation levels for extremity monitoring results.
>         Could one use ICRP-54 as a guide for this ?


The ILs given in ICRP are based on radionuclide intake, rather than on
absorbed dose.  However they do seem to provide a sensible basis for
establishing dose ILs, that is, using 3 tenths of the pro-rata limit.  This
implies that for monthly extremity monitoring you would use 12.5 mSv (1250
mrem) i.e. 30% of the annual limit for the skin (500 mSv) divided by 12.

I think that these ILs are perhaps a bit high for the type of work done in
our labs and would prefer to use the ICRP's "special (operational)
monitoring" levels of 1/10th the pro-rata limit.  This would set the IL at
4.2 mSv (420 mrem).

Another approach would be to set your IL on statistical principles, where
say a departure of ~2 std deviations or so from the long period mean could
be used.  This would probably produce a low IL in a "good" practice but an
indefensibly high one at a "bad" practice. Therefore: stick with the ICRP
for legally defensible monitoring but feel free to apply lower,
statistically based, limits for operational control.

BTW the numbers you quote seem high for typical biochem/DNA profiling type
work.  For people using P32 we get those numbers in micro sieverts! i.e. a
factor of 10 lower.  Maybe your people are doing more work or using larger
amounts?


Michael Williamson
                                        ph +61 7 3365 4504
Radiation Protection Adviser            fax +61 7 3365 1577
Occupational Health & Safety Unit       m.williamson@mailbox.uq.edu.au
University of Queensland
Brisbane 4072
Australia