Face coverings vs. protection from aerosolized respiratory viruses from a fluid mechanics perspective

I’m not a doctor. Though I bet I know more about fluid mechanics than the overwhelming majority of medical doctors.

Air is a fluid. Not fluid as in liquid – but fluid as in behaves in exactly the same way when flowing as fluids like water behave. Air flow is modeled by exactly the same equations as water flow.

Anyway – it is sometimes confusing to think of air as a fluid – but it is.

There is a pressure drop across any filtration media whenever there is any fluid flow across it. This applies to every filter. Always and everywhere. 3 main conditions impact the pressure drop across the filter:

The more clogged the filter becomes, the higher the pressure drop across it.

The higher the airflow through the filter, the higher the pressure drop across it.

The better the filtration performance, the higher the pressure drop across it (given the same surface area media and material).

You need a really good (read physically massive) filter to screen out particles 0.2 microns in size for any length of time with insignificant pressure drop.

COVID-19 spike protein is said to be less than 0.2 microns. Viruses are not large.

Again – no doctor – but human bodies were designed to to inhale air (oxygen) at a pressure of approximately one atmosphere. Human beings were designed to exhale their “exhaust” (carbon dioxide) into a pressure of approximately one atmosphere.

When we slap humans with “filters” over their noses and mouths, we restrict the volume of air on inhalation, while decreasing the amount of carbon dioxide exhausted to the outside air on exhalation.

During inhalation – the filter causes the one atmosphere air on the outside of the filter to become less than one atmosphere on the inside of the filter. We can observe this pressure drop by the mask “sucking in” towards the wearer during inhalation

During exhalation – the filter causes the one atmosphere of air on the outside of the filter to become more than one atmosphere on the inside of the filter. We can observe this by the mask “blowing out” from the wearer during exhalation.

But wait – it gets even better:

All surgical, cloth, and N95 masks also “filter” (aka restrict) your exhaled CO2. The CO2 concentration inside the filter is several hundred percent higher than it is in the surrounding air.

In fact, it is several hundred percent higher than the CO2 limit OSHA states is too dangerous to work under.

The medical term for restricting your oxygen is “hypoxia.” It is low oxygen in the blood. The medical term for re-breathing your own exhaust gasses is hypercapnia. Excess carbon dioxide in the blood. Masks always and everywhere increase both of these conditions. If you are doing any kind of physical excursion, the pressure drop across the filter increases in proportion to respiration into and out of your lungs. So does the amount of hypoxia and hypercapnia.

Many school districts mandated people playing singles tennis – or running – outside – to wear face filters. Yes – really. This is “the science” talking.

You are working harder to exhaust your waste gasses through an extremely inefficient filter. You’re also re-breathing a good chunk of those very same exhaust gases on your next inhalation.

Sounds healthy. This is a terrible idea. Sorry doctor – I disagree.

Let me get this straight doc – speak slowly so I can understand:

You want to slap people – even elderly and sick people who have trouble breathing even without a respiratory viral infection – with filters that make them even more susceptible to hypoxia and hypercapnia? And you want people and those filthy kids kids to wear these disgusting things all day long?

Sorry – I utterly reject your medical advice. No M.D. required.

Our bodies were not designed to work with filters. Period.

You are not going to stop a 0.2 micron aerosolized respiratory virus with a surgical mask, or a bandanna, or a face shield, or even a well fitted N95 mask. It’s not going to protect the wearer, and it’s not going to work for “source control” (protecting those around the wearer) either.

I don’t need a study to know this. Just by looking at these filter designs I can tell that they will do absolutely nothing to prevent infection or transmission, and they have some very major downsides.

Lucky for me, I’ve read 58 of the 150+ studies in their entirety that conclude the very same thing. And I’ve read the conclusions of the rest. We knew this about aerosolized respiratory viruses and masks decades before anyone had even heard of COVID-19.

If you are that worried about it, get yourself one of the masks that you see car painters wear – with the 2 cartridges on the sides rated for organic vapor, and make sure it covers your eyes. also make sure you have a clean shave if you have facial hair above a very small mustache.

The above type full face respirator has a wide open exhaust valve, which are generally “illegal.”

These still have the hypoxia concern when worn for long periods, but the hypercapnia is not a concern since the exhaust is not recirculated back into the intake.

Not surprising the only type that actually has a chance of working is banned by “the science,” while the less than useless “poser” filters are mandated.

Hint: It’s got nothing to do with science – It is obedience training: Do what I say or else.

Wait – I’m not done slaughtering face filters from a pure design standpoint.

I would contend that the biggest “torture test” for a human filter (aka obedience cloth) would be during a sneeze event. That is when you will have your highest (albeit short-lived) airflow rating – with correspondingly highest pressure drop across the filter.

Surgical masks don’t even stay airtight against the wearer’s face during normal conversation. What’s wrong with that you say? Pressure drop across the filter increases in proportion to airflow across the filter.

During a sneeze, you generate a very high pressure inside the filter compared to outside of the filter. This higher pressure inside the filter pushes the filter away from your face since it can’t physically flow that much air that quickly. Since the filter is also restricting your sneeze, the velocity of the sneeze is increased and escapes the only place it can: Around the sides, top, and bottom of the filter.

For a quick experiment on why restricting any fluid increases it’s velocity, try to restrict a wide open garden hose with your thumb while keeping the sides open and you’ll have a nice demonstration of what happens when you try to block viral aerosols from escaping a sneeze with an obedience cloth. You’ve spared the person 6 inches in front of you from a soak but sprayed water droplets 40 feet in every other direction. Hope nobody was standing anywhere around you…

This means during a sneeze, you are shooting aerosolized particles even further than they would absent any mask.

Congratulations on shooting viral aerosols even further during a pandemic Stop pretending there are no downsides to “masking.”

Well at least that’s the end of the downsides – right?

Wrong. What happens to filters when they get wet? They work even less efficiently than they do when they are dry.

Well – at least that’s the end of the downsides – right?

Maybe not: What is the temperature and relative humidity of the inhaled air inside a face mask? Is there more or less oxygen per unit volume in higher temperature or lower temperature air? Hint: There’s less oxygen in higher temperature air. I’d guess the temperature of the air inside a face mask to be approximately 98.6 degrees. I’d guess the relative humidity to be extremely high as well. Condensing humidity levels for sure based on how well these things tend to fog up eyeglasses.

Again – no doctor – but I would describe an obedience cloth worn for long periods as breath-moistened petri dish resting directly on your mucous membranes. Sounds healthy. Do viruses replicate faster in higher or lower oxygen environments? What about bacteria? What about fungus? Restricting oxygen and increasing CO2 sounds very healthy – especially when you do it all day long – for years on end. Especially when we’re all panicked about a virus that attacks your respiratory system.

And for certain – let’s slap elderly people, many of whom already have trouble breathing, with devices making it even harder to breath.

In summary – face masks result in the following conditions:

  • Lower pressure on the inside of the mask during inhalation results in less oxygen per breath than no mask
  • Higher pressure inside the mask during exhalation results in less CO2 expelled per breath than no mask
  • Insufficient exchange of CO2 to the surrounding air during exhalation results in re-breathing exhaled CO2 at a level that is several hundred percent higher than OSHA’s “safe workplace limit” for CO2 concentration
    • Higher CO2 concentration in a workplace = “unsafe.”
    • CO2 concentration several hundred recent higher than levels unsafe for a workplace = “safe.”
  • Higher temperature air during inhalation decreases oxygen even further
  • Higher relative humidity inside the mask decreases oxygen even further
  • Higher relative humidity and lowered oxygen results in higher probability of bacterial and fungal infections.
  • Zero efficacy against catching or transmitting viral infections. Meaning whether used for wearer protection or “source control” (protecting others).
  • Dangerous to wear during physical excursion due to the very high pressure drop across the filter.
  • Increases velocity of breath expelled during cough or sneeze events, causing aerosolized viral particles to travel even further.

Again – who makes this up?

I can see no upsides.

What’s the first thing that your doctor makes you do when going to see him about – say – breathing difficulties? Force you to restrict your breathing further with a less than useless surgical mask.

It is beyond reckless to suggest that a “face covering” is going to protect the wearer, or anyone around the wearer, from contracting or spreading an aerosolized respiratory virus. If you are at high risk of severe disease – stay home.

There is a very good reason that not a single manufacturer of surgical masks or even N95 respirators is willing to state on their packaging that these devices offer any protection whatsoever from aerosolized respiratory viruses: If they did – they would be lying.

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