This episode was made in partnership with Lydall.
They’re dedicated to creating a cleaner, quieter, and safer world.
You can learn more at the link in the description.
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You probably bump into HEPA filters fairly often.
They’re the basis of modern filtration, which is everywhere,
from your vacuum to your air purifier.
What you might not know is that they represent a straight line
from the Manhattan Project to your living room.
This very scientific-sounding acronym really just stands for
High Efficiency Particulate Air filter.
I guess HEPAF was less catchy.
And they do exactly what it says on the tin.
We rely on HEPA filters for keeping our air clean and breathable.
They can handle all sorts of things suspended in the air,
like wildfire smoke, particulate air pollution, and even viruses.
And they were developed to protect people making the first atomic bombs.
It all started back in 1938 when two German scientists
accidentally discovered nuclear fission,
the process of splitting uranium atoms to release a huge amount of energy.
The US government was worried the Germans would use this to their advantage
during World War 2, so they launched their own research into atomic weapons.
This classified research became known as the Manhattan Project,
and ultimately resulted in the first atomic bombs being dropped in 1945,
with devastating consequences.
But it took years of research and development with radioactive materials
to get to that point, and there was a pretty big risk of accidental exposure
for scientists, workers, and the general public.
Like in the manufacturing facilities, where processing the radioactive elements
uranium and plutonium could generate dangerous particles in the air.
And that meant the Manhattan Project had to develop
some kind of filter to stop people from breathing all that in.
To do this, the researchers at the Army Chemical Corps and Naval Research Laboratory
actually based their design on the gas masks worn by German soldiers fighting in the war.
These designs had a special kind of paper inside them, made of cellulose and asbestos.
The cellulose is nothing special;
it’s the main component of most kinds of paper,
and was primarily there to give the filter some structure and support.
But the asbestos proved to be surprisingly effective at filtering out tiny particles,
whether they were from smoke, chemical weapons, or processing of radioactive materials.
So for the Manhattan Project, the researchers simply
made a bigger version of the gas mask filter to be used in buildings.
This was the precursor to today’s HEPA filters.
Today, we know that asbestos is a less than ideal choice for masks or buildings,
as it can cause a rare form of cancer.
But nobody knew that in the 1940s,
and scientists were excited at how well this asbestos filter worked.
See, asbestos is a group of naturally occurring minerals
that are made up of really tiny fibers, just a few microns across.
And when they’re added to the cellulose paper,
those fibers orient themselves in all directions to form a fine mesh.
When air passes through the filter, it needs to wind its way around all of these tiny strands,
so it’s diverted in all directions as it passes through.
And while air is just fine doing that,
the tiny particles that are carried in the air have a tougher time of it.
To make sure the filters were as effective as possible, the Army Chemical Corps
hired Nobel laureate and chemist Irving Langmuir
to figure out exactly what happened to the particles on their journey through the filter.
Langmuir concluded that while the biggest particles get caught up in the network of fibers
like grains in a sieve, this isn’t actually the main trapping mechanism.
Smaller particles have to swerve around the criss-crossing fibers so much
that they eventually collide and stick.
And the very smallest kind of particle, less than a tenth of a micron across,
follows an even more convoluted path.
These particles bounce off the air molecules themselves
in what’s known as Brownian motion.
Sooner or later their jerky random path may bring them into contact
with one of the fibers of the filter, where, once again, they’ll stick.
But in between the largest and smallest particle sizes,
none of the trapping mechanisms work quite as well.
Medium particles are too big to follow zig-zagging Brownian motion,
but small enough that they can dodge the fibers more easily.
So Langmuir concluded that to make the filters as effective as possible,
they should be designed to stop these in-between particle sizes.
Langmuir found that the filters were least effective
for a particle size of approximately 0.3 microns,
and recommended the Army test their designs on particles of this size.
Anything bigger or smaller would then be even easier to filter out.
As it turns out, this analysis from 80 years ago was remarkably accurate.
Today, particles between about 0.1 and 0.3 microns
are described as the most penetrating particle size.
The exact size can vary depending on the conditions,
but those tend to be the hardest to stop with filters.
For the same reasons, they’re also the ones
that will travel deepest into our lungs if we breathe them in. Which is bad.
So the first HEPA filters were designed to stop these penetrating particles,
with big sheets pleated many times, to lengthen the gauntlet that the particles have to run.
In this way, the Manhattan Project created an important safety innovation
with uses far beyond the atomic.
After World War 2 ended, the filtering tech was declassified,
and the HEPA filters spread beyond purely military applications.
And they got some improvements along the way.
They’re smaller now, for starters, and there’s no more asbestos.
Instead, the fibers are made of glass. Which is less hazardous than asbestos.
Modern HEPA filters are designed to stop 99.97% of the most penetrating particles,
and even more of the particles either side of that most penetrating threshold.
So while they continue to stop radioactive particles,
they also stop tiny liquid droplets, pollen, dirt, bacteria, and even viruses.
These filters, developed for the Manhattan project,
touched off a boom in industries from healthcare to electronics.
Yes, even your phone was made with the help of an air filter.
Who would have guessed that it would take a secretive nuclear research project
to come up with something that’s keeping us safe every day?
Thanks to Lydall for partnering with us on this episode.
They believe that it’s what’s inside that matters,
which is why they’re in the business of making the hidden stuff that helps keep us safe.
That’s everything from the components of air filters like I talked about today,
to the filtration layers in N95 and surgical masks
that have been helping to protect us from COVID-19.
And their team of engineers is always focused on addressing current and future threats.
You can learn more about Lydall, and their work making filtration material,
in the link in the description down below.
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