Re[2]: Lead Aprons in Dentistry

[tdc@ehssun.lbl.gov]

>I left off the most important quote from this state inspector:

> That said, the highest leakage instantaneous rate I have measured is about 
> 200 mR/hour at 1 meter, and that was on a brand new dental unit.  One can 
> see it is well below the FDA limit, and from an actual risk point of view, 
> I'm sure the hazard is low.  We became involved because our state limit was 
> written as 100 mR/hr, an instantaneous rate.  Most older units, though will 
> measure less than 5 mR/hr at one meter.  You can tell the heads on the 
> older units are much heavier and contain more shielding.  


Ah yes - that is more in keeping with my experience.

BTW - ONE of the reasons for the reduced weight on SOME of the newer units
is a trend to go from 60 cycle power to high frequency inverter units.
This lightens the transformer considerably!  But then traditionally the
transformer is a big part of the shielding!  All in all, however, - with
less copper windings and more lead shielding - for a given leakage I would
not be surprised at a lighter tube head.

Also - as stated to one responder off line - in my experience currently as
a consultant and formerly as an in house HP at a large university medical
center - I have seen few very new units - ie. less than 5 years old.

Ted de Castro
tdc@ehssun.lbl.gov
University of California Lawrence Berkeley Laboratory
Bldg 75B Rm 112A
Berkeley, CA 94720
(510) 486-5256
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