Thread by @Poppendieck

A thread on filtration:

Masks, portable filters, and HVAC filters are all the same.

They must account for three things to achieve EFFECTIVE, real-world filtration:

1) Fit
2) Flow
3) Filter Efficiency

(H/T @JimRosenthal4 for topic idea)

1/18

Let's start by clarifying this thread will be about mechanical filtration where the particles are removed via a physical process.

This thread is not about filtration technologies that rely on energy or chemical reactions to remove particles.

2/18

FIT:

Air must go through the filter to be effective. Whether on your face (N95s) or in a duct (MERV13) if there is a bad fit a majority of the air will not go through the filter.

Just like water finding the easiest path down hill,
3/18

FIT:

air will preferentially find the path of least resistance: AROUND the filter/mask.

You can have a mask with 100% filtration efficiency but if it doesn't fit, you may not effectively decrease the particles you inhale.
4/18

FIT:

This is also a big issue in residential or commercial HVAC systems where filters are not always installed correctly, are the wrong physical size or may have a hole in them.
5/18

FLOW:

No flow, No filtration.

As @CorsIAQ says, a table is an excellent particle removal device (just don't clean it for a couple months and see the dust build up). But there is very little flow near/through the table. So we don't sell tables to remove particles.
6/18

FLOW:

Filters in HVAC systems only work when the HVAC fan is on. In residential systems this can be as low as 10-20% of the time.

Portable air cleaners are the same. At low settings HEPA filters deliver only a fraction of their rated CADR (from highest setting).
7/18

FLOW:

In addition, as particles build up on your filters the flow through the filter may drop (fan dependent) due to increased resistance to push air through. This is
one reason filters need to be replaced.

sciencedirect.com/science/articl…

8/18

FLOW:

For masks we seldom realize the flow we are trying to get to go through the mask. Every breathe is nearly a soda bottle worth of air. This is why fit is so important. In addition, the volume of air goes up as we exercise.

https://twitter.com/Poppendieck/status/1476632327589617669

9/18

FILTER EFFICIENCY:

This is what people focus on when discussing filtration (N95, MERV13, HEPA). It is important to note that filtration efficiency is particle size dependent.

https://twitter.com/linseymarr/status/1282737345691897857

10/18

FILTER EFFICIENCY:

There are several physical mechanisms that allow fiber filters to remove a large size range of particles. Direct impaction (like snow hitting a windshield), interception (snow hitting the side of a car) and...
11/18

FILTER EFFICIENCY:

Brownian motion/diffusion (snow hitting the back window.. sort of..) all play a role. These three mechanism combine to remove the smallest (below 100 nanometers) and largest (above 500 nanometers) the most effectively.

https://twitter.com/Poppendieck/status/1484168637359472645

12/18

FILTER EFFICIENCY:

The hardest particles to remove are in the 300 nanometers range. This is why HEPA filters are challenged to remove this size range (they remove smaller particles better). And N95 respirators are tested to remove 95% of 300 nanometers particles.
13/18

In summary, as we get more widespread adoption of better masks and portable filter units we need to make sure we account for all three components to get EFFECTIVE real-world filtration:

Fit, Flow, Filtration Efficiency.
14/18

Without all three you won't be effectively removing particles from the air you breathe.

A well fitting cloth mask:
⬆️Fit
⬆️Flow
⬇️Filter Efficiency
=⬇️Effective Filtration
15/18

A poor fitting N95 mask:
⬇️Fit
⬆️Flow
⬆️Filter Efficiency
=⬇️Effective Filtration
16/18

A too small HEPA filter for a room:
⬆️Fit
⬇️Flow
⬆️Filter Efficiency
=⬇️Effective Filtration
17/18

So next time you put on a mask, buy a portable HEPA filter, or replace an HVAC filter think about the three pillars of effective filtration:

⬆️Fit
⬆️Flow
⬆️Filter Efficiency
=⬆️Effective Filtration

18/18