N95 masks and odors - what is the science?

Dear Daughter, ICU RN, reported to me today that when she is wearing a properly-fitted N95 mask, she can still smell coffee brewing, and patient’s bowel movements.

Q1: is this cause for alarm?

Q2: if this is expected, what is the high school-level science explanation for how odors can get through, but generally viruses do not? It doesn’t seem logical that odor-carrying molecules are smaller than viruses, but maybe they are.

Note that official protocol for testing the fit of an N95 mask is for you to don it, then they spray some specific substance, and you report whether you can smell it or not.

Viruses are pretty complex. Coffee is pretty complex too, but there are those argue that viruses are alive, sort of, and no one makes that claim for coffee. So, just on that basis, I’m betting that viruses are FAR bigger than coffee molecules.

Furan-2-ylmethanethiol, a significant component of coffee aroma, is made from 13 atoms.

A small portion of COVID-19’s surface contains 200 million atoms. That’s just enough to simulate the behavior of the spike proteins. So yes, virus particles are quite a lot bigger than odor molecules.

Huh. The OSHA fit test for N95 mask involves enclosing the head with the mask on and shooting aerosolized saccharine into the enclosure. If you could taste the saccharine, it wasn’t a good fit. I assume that since it was based off taste and not smell, that smelling something isn’t necessarily a disqualifier of a good fit?

Keeve’s guess was right; thank you Dr. Strangelove; that’s simple enough for even me to understand.

I told my daughter “I’m on this message board, and…someone will know the answer”. I’m glad I was right. And super-fast, too!

Sodium saccharin is a solid at room temperature, so by “aerosolized” it’s likely that it’s ground to dust with a particular particle size. These particles would be much, much larger than their component molecules. It’s not exactly about taste vs. smell, but rather that individual odor molecules can move around (i.e., an N95 is no obstacle), while things that we taste tend to be solid, and lumped into much larger particles.

As was mentioned, the medical mask is designed around stopping the relatively large germs rather than tiny odor molecules. But you can get a mask which even blocks odor molecules. One example is a respirator mask with the two filters on the side. They are harder to breath through and you wouldn’t want to wear one for an extended period of time. Typically they are used in environments with solvents or pesticides that shouldn’t be inhaled. If you’re doing something like spraying the lawn, you’d want to use one of those respirators rather than an N95 mask.

While we’re on the subject…

How do we know that a random piece of cloth, such as might be used for a homemade mask, will trap virus particles? There must be a distance between fibers that is too large to trap them, and a smaller distance that will. But people are being instructed to make masks out of any material that is handy. How logical (and safe) is this?

There have been some studies on how well different materials block tiny particles. In that report, t-shirt material was in the 70% range while a surgical mask was in the 90% effective range. One reason the surgical mask is more effective is that it’s made with special fibers with a static charge which help capture the virus. The surgical mask material doesn’t have to be as tight, so it’s easier to breathe through. Cotton that was tightly woven enough to be 90% effective would be dense and hard to breath through.

But one significant benefit of wearing improvised masks is that it can help block infected droplets from getting into the environment. The mask won’t block everything, but it will mean less will get out and the droplets which escape won’t travel as far. So in addition to providing some benefit against inhaling the virus, it also provides some benefit against an infected person expelling it.

As far as I know, respirators work on a different principle: not by filtering out particles of a certain size, but rather using some chemical effect to absorb the volatiles. For example, activated carbon is a common one, but most likely there are others that are tuned for particular applications. It’s not impossible to filter molecules by size (see reverse osmosis filters), but as far as I know the surface areas and pressures would make them impractical for passive respirators.

Just to underscore the critical word in filmore’s post, for the benefit of anyone who might not have noticed: droplets (which are way larger than viruses)

This was already answered, but I just wanted to comment as someone who did the N95 fitting test today. Between each person donning the hood for the fit test, the staff would sanitize the inside of the hood. While wearing the mask, I could smell the cleaning vapors the entire time, but I couldn’t taste the sugar spray. Perfectly normal.

To expand a little more, perhaps unnecessarily, respiratory masks do not block individual viruses. Viruses, although much larger than most individual molecules, are far too small for any kind of cloth to filter them. When viruses were first discovered, they were called “filterable viruses” because unlike bacteria, they could pass through a solid porcelain filter. A porcelain filter has much smaller pores than any kind of cloth. Luckily, individual viruses are not what we are generally exposed to. We’re exposed to liquid droplets containing virus. The droplets are what is being filtered.

Actually, if you read the procedure, it says the saccharin is to be dissolved in warm water, then aerosolized using a nebulizer. So it is creating fine water droplets, which presumably simulates the aerosol transmission of diseases.

The document also describes a test using isoamyl acetate, but specifies that it’s only for organic vapor respirators, not particulate respirators. N95 masks are particulate respirators.

That makes sense. I should have read the link. Though I wonder if that’s a good proxy for dry particulate matter as well. My tiny personal stock of N95 masks were for filtering fiberglass insulation and other construction-related particulates.

Either way, a water droplet or dry particulate is going to be millions or billions of atoms, not tens as with a volatile solvent. For that, you need something that bind chemically with the material.

“Respirator” is a general term for a well-sealing protective mask, which may or may not be single-use/disposable. Here’s a PDF explaining the diff between a surgical mask and a disposable respirator.

A non-disposable respirator can be fitted with replaceable filters that block particulate matter, or cartridges that absorb volatile organic compounds or acid gases (the latter typically also blocks particulates).

This officer is wearing a half-mask respirator with P100 filters; these filters block virtually all particulate matter, but not VOCs/AGs. They’re lightweight and compact enough to (for example) fit inside the confines of a welding mask.

This guy is wearing a half-mask respirator with VOC/AG cartridges. These cartridges are also P100-rated, so they’re blocking particulates along with those toxic fumes. They’re bulky and heavy, but IME the breathing restriction isn’t typically worse than simple P100 filters.

Some odors seem to travel on particulates, while some are truly gaseous. Example, I cooked a bunch of bacon once while wearing the P100 respirator shown in the first photo above, and I couldn’t smell anything at all unless I took the respirator off. OTOH, if you’re using spray paint, the VOC/AG cartridges are indispensible.

One surprising use I found for the VOC/AG cartridges is when cleaning the shower. The harsh chemical cleansers are truly nasty stuff, but the cartridges block it completely, enabling you to do a thorough cleaning job while not suffering miserable nose/lung irritation.

I should also point out that non-disposable respirators (and some disposable respirators like this one) typically feature checkvalve(s) which ensure that your exhaled air does not go through the filters/cartridges; it is dumped, unfiltered, to the atmosphere. This means that respirators featuring such checkvalves should not be relied on to protect anybody but the wearer.

VOC “filters” typically contain an adsorbent (with a D) with high surface area, e.g. activated carbon.