Air purifiers with HEPA filters are highly effective at filtering out viruses such as the COVID-19 coronavirus.
But viruses can remain on surfaces for scarily long periods of time. When thinking about viruses stuck in an air purifier’s HEPA filters, that sounds scary. It’s also led to incredible-sounding headlines about how long viruses last.
But a “detectable” virus is very different from a “viable” virus. A viable virus is one that’s alive and still able to infect someone. “Detectable” viruses are just that – detectable, but dead.
How Long Can Viruses Last on HEPA Filters?
To answer this question, it’s important we look at how long viruses remain viable on HEPA filters. It’s likely that dead viruses will remain stuck in HEPA filters for a very long time. But what’s more important is how long are the active and able to re-infect for.
Researchers at the University of Hong Kong tested how long viruses remain viable on 6 different surfaces. They sprayed H1N1 flu virus on common household materials including plastic, wood and steel, and tested viability up to 24 hours.
Unfortunately, they didn’t test any HEPA filter material. But they tested something fairly similar: J-cloth material. It turns out this J-cloth material is actually surprisingly similar to the electrostatic material used in HEPA filters.
For the HEPA-like J-Cloth material, the tests found that the virus was completely non-viable in less than 5 hours.
The H1N1 virus remained viable the longest on hard, non-porous surfaces like the stainless steel and plastic. However, the flu was non-viable in under 10 hours for even the worst surface–the stainless steel. The researchers concluded that the H1N1 is “likely to survive up to a few hours.”
Another set of scientists in the UK tested the viability of a range of viruses on HEPA filters, including Bacillus atrophaeus and MS-2 coliphage. They placed the viruses on the HEPA filter, and then blew air through the filter at a constant rate to simulate usage of the HEPA filter.
Their data found that the majority of the viruses survived no longer than 2 days. However, the MS-2 coliphage virus – a non-enveloped virus that has been shown to survive longer than enveloped virus – was able to survive up to 6 days. (The COVID-19 coronavirus is an enveloped virus, not like the MS-2 coliphage virus).
It the Same for HEPAs With The Covid-19 Virus?
The study above is for H1N1 (influenza A), not the coronavirus. There’s little data available specifically on how long the Covid-19 coronavirus lasts.
The CDC says that the coronavirus may remain viable for hours to even days. A recent study (that has not been formally published yet) tested the viability of the new coronavirus (HCoV-19) and the “old” coronavirus, SARS (SARS-CoV-1). They sprayed the two viruses on plastic, stainless steel, copper, and cardboard.
They did not test HEPA filters or cloth materials this time. But the closest material to a HEPA filter surface was cardboard. It took 48 hours for the coronavirus to be completely non-viable on cardboard. On stainless steel, a tiny amount of viable coronavirus remained at 72 hours, That’s 7 times longer than the earlier study found for H1N1.
However, it’s important to remember that these tests were done in laboratory conditions, sometimes with very high doses of viruses which aren’t expected to be seen in real-world circumstances. This means these numbers may be exaggerated, as explain in one paper on the exaggerated risk of COVID-19 transmission.
What’s more, viruses prefer wet materials. They don’t survive as well on dry materials. HEPA filters inside air purifiers are constantly bombarded with high velocity air, which would dry out any viruses.
Based on this, it’s unlikely the COVID-19 virus would last more than a few hours on HEPA filter surfaces. However, it’s always good to err on the side of caution. Based on these two studies, a very conservative amount of time it would take to kill the COVID-19 virus on HEPA filters would be 48-72 hours.
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.