2 mr in 1 hour limit

["Richard, Mack L" <mrichar@iupui.edu>]

Dear Radsafers:

Carol Marcus indicated that the 2 mrem in any on hour limit was changed to
500 mrem to individuals exposed to patients who had received radioactivity
for medical purposes.  Unless another change that I am not aware of has
occurred, I don't believe Carol's statement is entirely correct.  10 CFR
35.75(a) does allow hospitals to release patients if it can be reasonably
assured that members of the public will not receive greater than 500 mrem
from those patients.  However, it is my understanding that if the patient is
hospitalized for radiation safety purposes (e.g., the 500 mrem limit cannot
be assured), the licensee is required to meet the requirements of 10 CFR
20.1301(a)(1) - 100 mrem to a member of the public in one year and 10 CFR
20.13019(a)(2) - 2 mrem in any one hour limit in an unrestricted area.  Both
of those sections provide relief from the requirements if a patient has been
released under 10 CFR 35.75(a).  On the other hand, if the patient has not
been released under that section, I believe the part 20 limits continue to
apply.  Carol's interpretation would save us a lot of grief, but I don't
think we can apply that interpretation for patients who are hospitalized for
radiation safety purposes (e.g., temporary brachytherapy).  

As an aside, the "100 mrem in a year" limit applies to an individual while
with the "2 mrem in any one hour" limit applies to an unrestricted area
rather than an individual.  While the two limits may appear to be similar,
one can assign an occupancy factor to the 100 mrem individual limit, but not
to the 2 mrem in an hour limit.  

To illustrate this, let me cite an example.  Suppose one measures an
exposure rate outside a  brachytherapy patient's room door (i.e., the
boundary of the restricted area) of 3 mR/h (assumed to be 3 mrem/h).  If the
total treatment time for that patient is 45 hours, the integrated dose
equivalent outside the door would be 135 mrem.  Assuming a very conservative
occupancy factor of 0.25 (i.e., someone stands outside the door 25% of the
time), the integrated dose equivalent to such an individual would be 135
mrem x 0.25 or 34 mrem.  That obviously meets the "100 mrem limit to an
individual" requirement.  Unfortunately, the 3 mrem exposure rate outside
the door exceeds the "2 mrem in any one hour limit to an unrestricted area"
requirement; therefore, some type of precautions must be implemented to meet
that requirement.  That has never made much sense to me, but who said
regulations are always logical.

Regards,
Mack R.  

Mack L. Richard, M.S., C.H.P.
Radiation Safety Officer - IUPUI/Indiana Univ. Med. Cntr.
Phone #: (317) 274-0330   Fax #: (317) 274-2332
E-Mail Address:  mrichar@iupui.edu

************************************************************************
The RADSAFE Frequently Asked Questions list, archives and subscription
information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html