We’ve all heard it before. Surely you can’t stop a virus with a filter because the gaps in the filter are too big and something as small as a virus would just pass through?

Well hold on! 

It’s not that simple!

EPA (Efficient Particle Air Filter) and HEPA (Highly Efficient Particle Air Filter) really do capture tiny particles as small as viruses with incredible efficiency. Even though the gaps in them are much bigger than they are.

In fact – (fun fact) – the smaller the particle is, the better these filters are at capturing it.

Yes…. Really. You read that correctly. EPA and HEPA filters capture tiny nano particles more efficiently than, say, mere microscopic particles even though these are much, much smaller than the gaps in the filter itself.

It sounds counterintuitive, but these filters make use of a couple of bits of funky physics to achieve this. Here’s an explanation with, of course, an angle related to our unwelcome guest the Coronavirus…..

Viruses And Germs Don’t Travel Alone

A Coronavirus particle is approximately 0.12 microns in diameter, and that is absolutely tiny. In fact, it’s been calculated that all the Coronavirus in the world wouldn’t fill a coke can.

It’s small. Really, really small, but….

Viruses and Germs don’t travel alone. They travel in aerosols which all of us breathe out. The WHO defines an aerosol as anything which can remain suspended in the air including dust or fungal spores. While there’s some debate about the physical definition of an aerosol a particle of 5 microns or smaller has usually been used as the benchmark. That’s bigger than a virus, but still minute and much smaller than the gaps in a filter, so what else does a filter have up its sleeve?

It’s All About The Surface Area

Firstly, filters are pleated to ensure that their available surface area for filtration is enormous in relation to their size.  We don’t necessarily advise you to do this (in fact we’ve done it so you don’t have to), but if you were to unpleat a standard 12 x 20 HEPA filter you would be left with a strip of material that was approximately 9m or 30ft long. (Don’t do it. Just don’t.…)

This is a principle with analogies in nature. Your lungs and your intestines are far more effective at absorbing tiny oxygen and nutrient particles than a simple measurement of their size suggests they should be. This is because their surfaces are packed with “pleats” called alveoli (in your lungs) and villi (in your intestines) which massively increase their surface area.

The filter’s enormous surface area means a tiny particle has to pass through a lot more filter material than it would if the filter were unpleated, drastically increasing the chances that it will be trapped.

Irregular Weave

Secondly, unlike normal fabric, the fibres which make up a filter are packed together irregularly. This means that there is no neat, straight gap in a filter that a particle can travel through. If you want to picture it, think of a particle navigating an obstacle course, blocked by fibres at every turn.

Moreover, the outsized (very large) surface area surface area of the filter means a particle has to effectively pass through several obstacle courses without being trapped before it can find freedom on the other side. Hopefully you’re already forming a picture of why this is becoming unlikely!

Small Particles Don’t Move in Straight Lines And Filters Are Sticky

This is the final trick, and also explains why filters become more efficient at trapping particles the smaller they are.

When air is passed through a filter, the air molecules knock the very tiny particles in it around randomly. This phenomenon is known as Brownian Diffusion, and means that small particles pinball around as they move. Even if you had a straight gap through a filter, small particles would be more likely to hit the sides of it than to pass straight through it.

This is where the final trick comes in. The fibres which make up the filter are made “sticky,” with a long lasting electrostatic charge that holds particles. 

Have you ever rubbed a regular party balloon on a carpet and then watched in awe as your hair stands up when you pass it over your head? If not, try it. 

Rubbing the balloon on the carpet has given it an electrostatic charge, and this attracts your hair.

Electrostatically charged fibres do the exact same thing to tiny particles. Random, Brownian movement makes tiny particles more likely to hit a fibre. As soon as they do they are captured and held fast by its electrostatic charge. 

It’s game over.

In Summary

The huge surface area of the filter, its random weave of electrostatically charged fibres and the random way small particles move make it almost impenetrable. Even to tiny viruses and the aerosols which carry them.

Almost impenetrable…. Of course some get through. That’s how randomness works, but here are some figures which show you just how good this is.


An E11 EPA Filter captures 95% of particles measuring 0.3 microns in diameter in air which passes through it, while an H13 HEPA Filter captures 99.97% of particles of the same size. A Study by NASA showed they’re even more efficient at capturing smaller particles (remember, Brownian Diffusion).

So all in all an Air Purifier with an EPA or HEPA filter is going to remove practically all the tiny particles, including virus and germ containing aerosols that pass through it. Even though the gaps in the filter are much bigger than the particles.

What Is Layered Protection?

Layered protection is a concept borrowed from nature and replicated by humans as a design philosophy to minimise the risk of adverse events.

You may have heard of systems or things described as failsafe, resilient or redundant. 

Our world has been revolutionised by being able to do things which are inherently dangerous because engineers have made things failsafe, resilient or redundant using layered protection. Read on for an example….

Flying On A Plane

You probably take flying on a plane for granted. It’s fair to assume you will take off and land at your destination intact, uninjured and alive a few hours later ready to enjoy your holiday or your business trip.

How can you assume this? Flying is a perfect example of something which is inherently dangerous after all. 

Flying has been made incredibly safe through the application of layered protection. 

Every critical system on an aeroplane is replicated often multiple times, so that if a primary system fails a backup can take over. Often the backup uses different technology from the primary system, to limit the extent to which a single vulnerability can cause a critical failure.

From pilot checklists to triplicated computer systems, safe flying represents layered protection at its most optimised. As a result we can travel from place to place at velocities approaching the speed of sound in incredible safety.

We Got The Idea From Nature


Layered protection is found throughout nature. You don’t need to look far to find examples either. Just look at your own body. 

How many kidneys do you have? How about lungs? And even if the answer is “1” this still proves the point!…. Layered protection is everywhere in nature, and that’s because it works really, really well.

Living With Covid

Living with Covid is, in our view, a great example of something which should be managed with layered protection.

Covid is inherently dangerous, with the potential to cause death and disability. While post-vaccination most people who get it will suffer milder disease, there is a risk of adverse and long-term effects, such as Long Covid. In addition, there is increasing evidence that absenteeism due to Covid and its potential effects is beginning to harm businesses, services and infrastructure – including schools and other educational settings.

The risks it poses are both individual and systemic and are significant.


When you travel on an aeroplane you don’t notice all of the layered protection in place which make 40,000ft in the sky one of the safest places to be. 

It’s unintrusive.

Protections which are inconvenient or require lots of compliance are less likely to be sustainable in the longer term – against a threat which, if we do our job correctly, will be diminished but which could keep coming back.

This is why we think clean air in shared spaces is going to be a critical layer in our protections from this disease. The air can be replenished and cleaned without people noticing, and without them doing anything to make it happen. In fact, clean air has so many benefits besides infection control that if people do notice it, it’s likely to be for positive reasons such as their allergies being less intense!

The Coronavirus is an airborne virus. 

In poorly ventilated indoor spaces we all share air, and this unfortunately is also how we share many common viruses and germs including the Coronavirus.

The virus is spread by hitching a lift on tiny particles called aerosols which are coughed, sneezed or simply breathed out by someone who is infected. These aerosols are so small that they can float in the air for hours, creating an infection risk for anyone who breathes them in.

So how can air purifiers help?

Making Indoor Air As Covid-Safe As Outdoor Air

Outdoors, airborne transmission of viruses and germs is much less of a problem than indoors. This is because wind quickly disperses the aerosols they use to hitch a lift, while strong sunlight degrades the viruses and germs in them. 

Indoors is a different story. In poorly ventilated indoor spaces, aerosols can accumulate and remain infectious for hours.

It is in these poorly ventilated indoor spaces that air purifiers can really help by sucking or blowing the contaminated air through a filter which traps almost all of the aerosols.

In this way it is possible to clean aerosols out of the air, reducing the risk that they spread infection.

Ultimately our aim is to make indoor air as covid-safe as outdoor air, using this and other technologies.

What Type Of Purifier Is Best?

There are 2 things scientists consider when rating an air purifier. 

The first is how efficiently its filter traps the aerosols, and the second is how often the air is passed through its filter. 

Imagine a room with 1,000 aerosol particles floating in the air. Once all of the air in the room has passed through the filter once, then 95% of the aerosol particles have been removed. This leaves only 50 aerosol particles. The second time the air is passed through the filter, another 95% of particles are removed, leaving only 2 or 3 in the air. You can see from this that it is very important both to filter the air efficiently on each pass through the filter, and also to ensure that all the air in the room passes through the filter as many times as possible.

Together, the efficiency of the filter and the number of times air is passed through it in an hour make up the Clean Air Delivery Rate (or CADR) of the air purifier, usually expressed in m3/h.

Studies on ventilation have shown that completely replenishing indoor air 6 times per hour reduces Covid transmission by 82%.

As a guideline, if you are aiming to replenish the air in a room 6 times per hour and have no external ventilation, then calculate the volume of your room by multiplying its length by width by height (L x W x H) in metres. Then multiply this figure by 6. This is the minimal CADR in metres squared (m3) you would need to achieve 6 air changes per hour, or ACH. If you do have some ventilation and know how many air changes this gives you, then this can be added to your number of air changes. As a general rule it is better to have an air purifier that is too big than too small, and as many air changes per hour as you can cost effectively achieve!

Take a look at our calculator to help you decide (Link to Stefan’s calculator)

What’s The Evidence This Works?

Evidence that air purification can work versus Covid is being gathered all the time, but the best evidence is simple physics. Air purifiers can remove almost all the aerosols from the air which the virus uses to spread. If it is filtered out of the air, then it cannot spread.

It is likely that the effect of air purification is greatest on far field transmission. This is transmission that happens over longer distances. We’ve all heard about cases of Coronavirus infecting people in offices, on cruise ships or at restaurants who had no close contact with each other. This is far field transmission.

See our Resources for studies showing how filtration can reduce the amount of Coronavirus (and other viruses and germs) in the air, and how ventilation, a close proxy for filtration, can reduce transmission:

Layered Protection

Air purification is a risk reduction measure. Although we believe it can help a lot, it is one layer in the several layers of protection we should use. Layers of risk reduction are the best way to protect yourself, your family, your staff or your business.

As well as using air purifiers please also get vaccinated, ensure spaces you operate are as well ventilated as possible and adopt and encourage good mask wearing and hygiene practices.

Covid is not going to be easy to live with, but if we are sensible we can reduce its impact on our lives by reducing its transmission. Air purification can be a major tool in achieving this.

Who We Are

The Corsi-Rosenthal Foundation is a Not for Profit Corporation in the US and in the process of registering as a Charity in the UK.

Our mission is to improve human health by supplying and promoting the use of air purifiers to clean indoor air. 

Why We Are Needed

Humans spend up to 90% of our time indoors. 

Indoor spaces are often poorly ventilated and the COVID-19 pandemic highlighted the importance of clean air in preventing the spread of viruses and other pathogens that can cause disease. 

Airborne pathogens are not the only problem. Indoor air can be contaminated with allergens, pollutants and particulates which can cause acute and chronic illness, absenteeism and reduced quality of life.

Air purifiers are the best way to clean indoor air of airborne threats to our health, but often those most in need of air purifiers are least able to pay for them.

The Corsi-Rosenthal Foundation works to ensure the most vulnerable in society can enjoy the benefits of clean indoor air.

How We Do It

We fulfil our mission in two main ways.

We recommend or supply an air purifier to those most in need, either free of charge or with a voluntary donation that helps cover our costs. Those most in need include schools, healthcare settings and clinically extremely vulnerable people.

We also provide open source information including build instructions and kit lists, so people and organizations can build their own air purifiers. Corsi-Rosenthal boxes are the award-winning DIY air purifier named after our founders. This means those who need an inexpensive, powerful air purifier can build one themselves, using locally sourced components for a fraction of the price of a commercially manufactured air purifier.

In addition to this, we provide a general educational resource about air purification, infection control, and research through our website and network of expert professionals.

How We Started

The Corsi-Rosenthal Foundation began as a global grassroots movement to build a low cost, open source DIY air purifier in response to the COVID-19 pandemic.

The DIY air purifier, known as the Corsi-Rosenthal box, is named after Dr Richard Corsi, Dean of the School of Engineering at UC Davis, and Jim Rosenthal, owner of a filter manufacturer and distributer.

The beauty of the Corsi-Rosenthal box is that from inception it was designed to be completely open source, built from different components which builders could select on the basis of price or local availability while still arriving at a consistent end product air purifier with a CADR (clean air delivery rate) of around 1000m3/h and a cost between $70 - 150. This is a phenomenal rate of clean air exchange, as many more expensive commercial purifiers have rates between 200-400 CADR.

This approach made the Corsi-Rosenthal box an immediate hit. It has become widely used in schools in the US, winning awards and attracting the attention of the White House, where, we are pleased to say, you will find Corsi-Rosenthal boxes in use.

Growth of the Network

Building a Corsi-Rosenthal box isn’t just a cost-effective step towards cleaner indoor air, it’s also fun! 

Being open source allows people to get creative, and very quickly a global network of people optimising, improving and sharing their Corsi-Rosenthal boxes on social media started to drive adoption and improvement of the original design.

This network included everyone from members of the lay public following existing designs closely to expert engineers trying to improve or reiterate the original with their own innovations.

The Corsi-Rosenthal box isn’t just a nice idea. It has actually been tested against some of the most stringent air purifier standards in the world using ISO certified equipment by experts in aerosol science and public health. This is how we know the original Corsi-Rosenthal box easily holds its own against some of the most powerful and expensive commercial air purifiers on the market at a fraction of the price.

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