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. [♪ INTRO].

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. [♪ OUTRO].