hi all, I stumbled onto this info posted like 5 years ago by someone
on the cozy builders forum, and thought it was good enough to share.
Bottom line, you want to *always* wear a P100 respirator (not paper
mask) when sanding our aircraft. Very cheap insurance.


Every once in a while a topic comes along where I actually know
something about how to answer it.  In my real job I create particles
that you're supposed to inhale.  I work on dry-powder inhalers (DPIs),
specifically in what is called Chemistry, Manufacturing, and Controls
(CMC).  When I'm really on my game they look like this:

(dead link)

This is opposed to working on our fiberglass airplanes, where we
create particles you are NOT supposed to inhale.  I am providing
insight from the point of view of someone that makes particles, not as
a medical doctor and not as an industrial hygienist. Sorry for the
very long-winded reply, but this is a SERIOUS safety issue that I
think people take way too lightly.

Inhalation of particles can be completely safe, or absolutely
dangerous, depending on the chemical composition of the particle, the
aerodynamic size of the particles, and the total amount of exposure.
Your airways classify by aerodynamic particle size.  Anything larger
than 25 microns is unlikely to actually make the trip through your
nasal sinus or through the back of your throat without being stopped
by the mucus membranes and expelled.  When you blow your nose when
you're done working and get a tissue full of very white snot, that's
what it is.  Particles less than 10 microns are likely to make it past
the first 3 branches of your bronchi.  Particles less than ~0.5 micron
will make it to your deep lung, and then be expelled right back out.
This is actually what keeps people that smoke relatively healthy for
such a long time.  The particles are so small the vast majority come
right back out.

The critical sizes you care about then are between about 10 and 0.5
micron.  They will go in and stay there.

I'll take each for the case of you inhaling aerosolized glass fibers
and microballoons.

Microballoons supplied by Aircraft Spruce are a 3M product called K20.
You can reference the data sheet that is on ACS's website here:


In it you will find that the baloons are made of soda-lime
borosilicate glass.  You will also find they have a median diameter of
60 microns, an x10 of 30 microns, and an x90 of 90 microns.  This is
all good information as far as inhalation is concerned.  Your lungs
and nasal cavity are a pretty efficient filter of certain aerodynamic
particle sizes.  Because we're talking about spheres, the aerodynamic
particle size and the geometric particle size are nearly identical.
The vast majority of the microballoons are above the cutoffs where you
should be concerned, but 10% of them are in the range that could get
to your deep lung.  That isn't to say that by sanding them you haven't
broken the larger particles down into smaller pieces that will get
into your deep lung and stay there.

>From a chemical point of view, borosilicate glass is a pretty inert
material.  Your body isn't going to absorb it, but that creates
another problem.  The particles go in, and your body doesn't absorb
them, so they stay right there.

The glass fibers are a similar story, with a critical difference.  The
morphology of the particles is, well, fibrous.  The longs strands of
glass (and by long we're talking ~100 microns here) have an
aerodynamic particle size that can be two orders of magnitude smaller
then their lagest physical dimension.  They "fly" like a 2 micron
particle when they can be 100 times that in length.  So they are quite
able to make it well into your deep lungs and then lodge there and
become a serious irritant.  Again, the glass is largely inert, but
this is a double edged sword because that means they will stay there
for quite some time.  These two characteristics (largely inert, large
physical size to aerodynamic size) are what makes asbestos such a

The next thing to consider is the amount of exposure.  For particle
inhalation there are EPA standards for what a person can tolerate on a
day-to-day, 8 hour exposure for job safety issues.  Typically these
levels are such that if you were to look across a well-lit room you
would not be able to see the dust in the air.  Sanding micro and
fiberglass is multiple orders of magnitude above this level.

So, we have a material where at least 10% of the microbaloons, and
potentially much more than that of the fiberglass is able to get to
your deep lung, made from materials that are mostly biologically
inert, so they are going to stay right there for a while.  And you're
exposing yourself to potentially orders of magnitude more than the EPA
considers acceptable on a day-to-day basis.

This is all entirely preventable by wearing a proper dusk mask at all
times.  I put mine on when I get to the hangar, and try to leave it on
until I leave.  I certainly always have it on when cutting class,
sanding, etc.  The type of dust mask makes a big difference too.  The
little paper masks with a single small elastic band are totally
useless.  They will not seal to your face, and give you a false sense
of security.  Buy filtering face masks that are rated NIOSH N95 or
better.  More information here:


You can get them at Home Depot, Lowes, etc.  If you're not paying
about $1 each for them , they are the wrong ones.  Make your life
easier and buy the type with an exhalation valve and you'll be less
tired at the end of a  building session than if you don't have one.
It is fine to re-use them as long as you store them, face up, in a
plastic bag between uses.  If you contaminate the face seal taking it
on or off, throw it out and use a new one.


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