OT: N95? Humbug.

This article tha James posted in the other thread is illuminating on the properties of N95/N99 masks. For instance, I wouldn't have guessed that alcohol will depolarize the electret filter layer.

It's probably very hard to get viruses out of body fluids without a lot of other stuff in the droplet, which dries down to something quite a lot larger than the virus. A 30-um droplet that's 99.5% water dries down into a solid particle of about 30 um * (0.005)**1/3 = 5 um diameter.

BITD I did a lot of work in submicron aerosol detection, and I can tell you that making good submicron aerosols is not something that happens by accident. You start with a medical aspirator, a very well-filtered air pump, ultrafiltered 18.2 megohm deionized water, a tiny small amount of Fisher Triton-X surfactant, and standard suspensions of polystyrene latex (PSL) or colloidal gold spheres. You dilute the spheres until the probability of getting two in one droplet is negligible.

You can make good 0.1 um aerosols that way, but anything smaller is significantly harder--that cube root is tough to fight.

Cheers

Phil Hobbs

How effective is an organic vapor cartridge, as in masks for paint spraying? I know they stop some nasty stuff, such as brake cleaner aerosol, very effectively.

-bill m

Gases diffuse around rapidly, so the air just has to hang around within a couple of diffusion lengths of the adsorber briefly.

Cheers

Phil Hobbs

The cartridge claims to be activated charcoal, goodness I suppose depends on how long it's been exposed to the atmosphere. Still no smell of solvent getting through the old one I have. I store the canisters in a screw-top plastic container most of the time, to keep away from atmosphere. Looking a little further, this thing has an N95 disc in front of the canister. I do wonder about the statement that the N95 masks are for others benefit, since they all have an exhaust path which does not get filtered at all. The filtering, for whatever it's worth, is all on the intake airflow. Best I can see is that exhaust air is directed away from front, down & to sides, but does that really help?

-bill m

It'll help with particles large enough to hit the surfaces

My reusable N99 has valves, probably to prevent it from losing contact during a cough or sneeze.

Cheers

Phil Hobbs

A virus load in an aspirated droplet is an infection danger proportional to the droplet VOLUME, and the smallest droplets are not the important ones. Conversely, an irritant or allergen is active in proportion to the droplet AREA, and smaller particles means lots of area per milligram of material.

A UV sterilization air handler is a good idea for a purpose-built hospital building, but filters are simple cordless devices and UV lamp banks are not. Still, an upflow air system for a public space would have some collection efficiency on aspirated droplets, and keep 'em off of surfaces.

Ooh, how about a powered electrostatic filter? I am curious how the N95 uses a critical electrostatic charge in that nasty fuzzy material while the round pink P100 filters look like a totally different design.

Electrostatic precipitators are used quite extensively. At one time you could put them on your hot air furnace. Unfortunately they tend to generate a fair bit of ozone, which is not a good thing to be breathing and is also hard on the stuff in your house. In an industrial setting (e.g. as part of a stack scrubber) it doesn't matter much because the ozone is highly diluted and dissociates pretty fast (a few weeks). In a recirculating system the concentration is higher and it's what you're breathing all the time.

From a classical physics POV, one would expect electrets not to work. An object with a frozen-in charge imbalance or dielectric polarization should attract surface charge until the field outside it is neutralized.

This is true in the far field. Closer to the surface, though, say within 100 um, you still see the field. This turns out to be because in a solid-state physics POV, there aren't enough available surface states to balance out the frozen-in field. So if you configure them correctly, e.g. in spun fibre form, you can make the equivalent of an electrostatic precipitator.

Cheers

Phil Hobbs

I haven't seen anyone in Safeway using scuba gear, but I'm always optimistic.

???

Cheers, James Arthur

"Captures particles as small as 0.3 microns" it doesn't say how many of those small particles it stops.

Is this internal charge scheme then also the mechanism for HEPA vacuum bags? There has been controversy regarding their use for mask filters.

I've seen in several places instructions for making a mask from a bandanna and two layers of coffee filter. Try breathing through even one layer of coffee filter.

Two layers of duct tape will stop 100% of viruses.

Cheers

Phil Hobbs

That's the motivation for electrets in the N95 -- fewer layers makes for higher filtration with less obstruction.

That's what made me turn to HVAC filters as electret sources -- you can get 'em at Home Depot, and layer 'em up as thick as you want.

But the Dr. of Dr. Phil's video emphasizes that the pedestrian masks we're supposed to wear in public aren't to stop the virus -- that's not how you get it. They're to keep us from touching our faces with contaminated fingers -- that's how you get it. (Slowing out-going payloads is a fringe benefit.)

Cheers, James