Histonet:

In an article in CAP TODAY, Nov 94, Vol. 8 No. 11 pg
titled Preventing tuberculosis transmission The chaos
over respirators the risk assessment for Anatomic
Pathology go like this.

Fixed tissue (cut-in, embedding, microtomy, staining)
Viable load of MTB is vastly reduced and aerosol
generation is low.
Administrative controls - not required
Engineering controls - not required
Personal protective devices - not required

Unfixed tissue (cut-in, freezing, microtomy, staining)
Viable load of MTB is modest and aerosol generation
is low unless sprayed with coolant.
Administrative controls - not required
Engineering controls - No spraying with coolant is
allowed unless within the confines of a HEPA-filtered,
negative pressure microtome cryostat.
Personal protective devices - not required


Also included in the article are the following
statements:

" ......... it is not necessary to wear respiratory
protection except in settings where administrative and
engineering controls are not likely to protect persons
from inhaling infectious airborne droplet nuclei."

"droplet nuclei one to five microns in size"

My understanding of the issue:

We breathe in all kinds of airborne dust and yet our
lungs don't fill up with dust.  That is because the dust
stays suspended in the air we breath and does not
deposit in the respiratory tree.  In the same way
someone inhaling cigarette smoke exhales virtually
all of it.  The same concept applies to inhaling fine
aerosols of MTB.  It is the one to five micron droplets
which pose an inoculation danger because they don't
stay suspended in the respiratory tree.  As a result of
their relative weight the droplet nuclei develop enough
momentum while being inhaled that they end up
depositing in the lung. Like a car going too fast would
skid out of control into the guard rail.

As I understand it the aerosols produced by cryostat
sectioning may contain a MTB load but they are not in
the one to five micron droplet nuclei range that poses
an inhalation hazard.

Looking forward to more lively discussion on this
posting.


Colin in London, Ontario