What’s does the 95 in N95 stand for?

Sometimes we don’t see the things that are staring us right in the face.

The other day, Megan Mansell posted a new version of her thread on N95 masks

Megan Mansell @mamasaurusMeg

My old N95 thread was deleted by accident, so here is an expanded redo, as it was popular. Current minimum infective dose (MID) and virion output calculations show a MID range of 300-1000 virions for transmission, 100,000-750,000 virions output per minute. 1/17

but the most critical part of this thread is the simple fact that we have not been looking at the things in our face… or even on our faces if you’ll pardon the pun.

The 95 in N95 stands for 95% of particles larger than 0.3µm that these masks are both designed and approved to capture under perfect conditions.

These are not N100 masks.

These masks let 5% of the particles through.

When I call it 5% maybe that sounds small to you… how do you feel if I call it 1 in 20?

The next point Megan makes has also been obviously staring us in the face for the past three years… namely that they are designed to capture 95% of particles larger than 0.3µm.

They are not designed to capture particles that are smaller than 0.3µm.

In fact there are issues with capturing very small particles, because the smaller the holes, the harder it is to breathe in and out.

If you tried to make a mask that stops 100% of all particles from being breathed in or out it would not be a mask but a suffocation device!

The mask has to have some holes to let air through.

If the mask doesn’t let enough air through then you will literally blow the mask off your face when you try to breathe out.

Many years ago, I was helping my dad sand down wooden some floorboards in his house. It’s very dusty work, I wore an N95 mask and taped it on my face to stop any dust getting in… and still afterwards there was dust on my face and trapped in natures greatest filtering system, my nasal hair.

So from experience I knew that N95 masks let some dust pass through… but I hadn’t realised it!

Now getting back to Megan’s thread, she starts by mentioning the Minimum Infective Dose (MID). This is the minimum number of infectious particles of a pathogen that one must (on average) be exposed to in order to become infected with that pathogen.

It’s important to know that the MID is a range or a spectrum, the medically vulnerable will probably have a lower MID than a robustly healthy person.

The current estimates for the MID of COVID-19 are in the range 300-1,000 virions for transmission.

Estimates are that an infected person outputs between 100,000 to 750,000 virions per minute.

Megan takes the most conservative figures:

Megan Mansell @mamasaurusMeg

Adults breathe 16-20 times per minute. We will look at the output for a one hour period, an 8 hour period, and by a per-minute basis for each. Most conservative figures: 1000 virions for transmission as MID 100,000 virions of output per minute H/t to @birb_k for graphic 3/

i.e. the least virions output per minute and the highest virions MID:

• 1,000 virions as the transmission MID

• 100,000 virions of output per minute from a symptomatic person

After one hour we will have 6 million virions which could theoretically infect 6,000 individuals.

The N95 mask will -best case- have stopped 95% of those 6 million virions and the remaining 5% would be theoretically able to infect 300 individuals.

So if one is symptomatically infected and sitting in a room wearing a perfectly fit-tested N95 mask for 1 hour then the room’s air will contain at least enough virions to infect 300 people. [Update: Megan points out the 95% rating is solely for the wearer, not for the output, and definitely not for the output if the mask has the exhale vent that a lot of N95 masks have to ease breathing out and prevent loss of fit from pressure build-up on exhale. Part of the reason for this is that there are no approved methods to test the effects for anyone other than the wearer]

But wait, there’s more! Remember how the N95 masks are neither designed nor approved to capture below 0.3µm.

You may have noticed that COVID-19, somewhere between 0.06-0.14µm, is smaller.

Particles this size remain in the air for hours or even days.

And then there’s people who haven’t fitted their mask correctly

Megan Mansell @mamasaurusMeg

Yet for a pathogen with a 99.8% survivability rate, people are chin-strapping or have fully open side gaps, when 3.2% leakage equates to 100% inefficacy. h/t @pettypodcast1 14/

If your mask has just 3.2% leakage, that is equivalent to 100% inefficiency. In other words, if you have a 3.2% gap, you might as well have not bothered wearing your mask…

Oh and did you remember to cover your lacrimal duct?

Megan Mansell @mamasaurusMeg

👩🏻‍🔬Important🥽 “If you have goggles or an eye shield, you should use it.” Anthony Fauci, 07/30/2020 We had heard enough from Fauci by this point to begin automatically tuning out his often contradictory advice. What if we had given weight to this and explored why he said it? 1🧵

And all this for a pathogen with (at least) a 99.8% survivability rate.

I guess the final thing that we all knew but have forgotten is this…

When people face deadly pathogens, they don’t wear N95 masks but rather something more akin to a spacesuit.

Photo by James Gathany, Dr. Scott Smith,

Thank you Megan for pointing out the stuff that most of us already knew but didn’t understand.