Re: Filter Collection Efficiency for Personal Air Monitor

["Otto G. Raabe" <ograabe@ucdavis.edu>]

February 24, 1999

>Otto G. Raabe wrote:
>>All filters have a minimum efficiency for particles about
>>0.3 micrometer in physical or aerodynamic diameter. Smaller particles and
>>larger particles are more efficiently collected....

>Mike McNaughton wrote:
>I suppose this does not hold true all the way down to 0.1 nanometer, since
>gases such as tritium are not collected efficiently. So below what diameter
>does the efficiency start to decrease? Graphs that I have seen do not
>continue to extremely small particle size.
**************************************************************
REPLY:	The word particle refers to a solid or liquid cluster of molecules,
not a gas molecule. This is important because a particle that contacts a
surface becomes firmly and usually irreversibly attached to the surface by
surface tension forces, while a gas molecule contacting a surface has a
high probability of being released or reflected off depending on its vapor
pressure.

One of the smallest radioactive airborne solid particles are "unattached"
radon decay products (metallic particles) whose effective diameter (with
clustered water molecules) is about 1 nanometer (10 angstroms)[see: Raabe,
O.G. Concerning the interactions that occur between radon decay products
and aerosols. Health Physics 17: 177-185, 1969]. These metallic particles
are very efficiently (nearly 100%)collected by air filters and display
nearly 100% deposition in the upper respiratory airways during inhalation.
This is the reason that unattached radon decay products deliver a higher
dose to the bronchial epithelium than the same decay products attached to
larger particles.

Otto
		*****************************************************
		Prof. Otto G. Raabe, Ph.D., CHP
              Institute of Toxicology & Environmental Health (ITEH)
		     (Street address: Old Davis Road)
		University of California, Davis, CA 95616
		Phone: 530-752-7754  FAX: 530-758-6140
		E-mail ograabe@ucdavis.edu
              *****************************************************
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