Following the outbreak of coronavirus, millions of people flocked to buy face masks to protect themselves from the virus. Data shows face masks can protect you from the coronavirus, but can air purifiers also capture coronavirus particles?
What’s in Your Typical Air Purifier?
Most air purifiers are made up of two things: a fan and a filter. They’re as simple as that.
Most of us understand how fans work, but how about the filter? The filter – typically a HEPA filter – is the workhorse of any purifier. Turns out they’re nothing fancy either. They were , and they’re just a mat of synthetic fibers. What’s important is that they can capture over 99% of air pollution particles.
How Small Are Coronavirus Particles?
But first we need to know how small the coronavirus is. Scientists have already taken electron microscope images to measure the size of coronavirus (Covid-19) “virions” (or particles). The virions are spherical particles with diameters of approximately 0.125 microns. The smallest particles are 0.06 microns, and the largest are 0.14 microns.
That makes the coronavirus particles smaller than PM2.5 particles, but bigger than some dust particles and gases.
Can HEPA Filters Capture Particles As Small As The Coronavirus?
The definition of HEPA filters on is that they must remove 99.97% of particles “greater than or equal to” 0.3 microns.
It only mentions particles 0.3 microns and above. So what about smaller particles the same size as the coronavirus? Can HEPA filters capture these?
This exact question has led to years of confusion and false advertising claims about HEPA filters, such as this one, so let’s get to the bottom of it.
And this confusion is now leading to more misinformation during the spread of information during the coronavirus outbreak:
Why Our Intuitions About Air Purifier Filters Are Wrong
The answer is surprising. It’s surprising because our intuition about filters is wrong. Like most people, I had the intuition that HEPA filters work like a net.
If a particle is smaller than the holes in the net, it gets through. Makes sense! However, it turns out that particles as small as the coronavirus, and other nano-particles get captured using a scientific phenomenon called diffusion.
Diffusion is surprisingly effective at capturing tiny virus-sized particles. According to NASA, HEPA filters capture “virtually 100% of particulates.”
Researchers at the University of Minnesota tested this question with weaker fiberglass furnace filters and higher-grade HEPA filters. In their test, they shot particles of silver from 3 to 20 nanometers at the filters (that’s roughly 5 to 30 times smaller than coronavirus particles).
The results showed that filters captured 99.99% of particles smaller than 5 nanometers. Bingo! HEPA filters are incredibly effective at capturing nanoparticles–even smaller than the coronavirus.
What Does This Mean for The Coronavirus?
Botton line: HEPA filters can filter out over 99% of the coronavirus in a single pass.
This is important for air travel, because airlines use HEPA filters to clean the air in the cabin.
It’s less clear that this would be useful in a home, office, or school, since there is less recirculated air and viruses often (but not always!) travel on large water droplets, which tend not to linger in the air. However, if there are virus particles in the air entering a purifier, the HEPA filter will filter them out.
Paddy is the CEO of Smart Air, running operations from Beijing. He has a Masters in aeronautical engineering from Bristol University, UK having specialised in aerodynamics. An advocate for open data, free information and transparent business, he spends his spare time promoting honest business and social enterprise.