How Do UV Light Air Purifiers Work and Can They Kill Viruses?

Air purifiers with UV light are marketed as effective virus killers, capable of targeting viruses such as COVID-19. UV light technology including far-uv, far-uvc, and UVGI are advertised to kill 99.9% of viruses.

One in every four air purifiers displayed as top results on Amazon is labelled as a UV-C air purifier. But are UV light air purifiers truly able to protect you against COVID-19 and other airborne viruses?

How UV Light Air Purifiers Work

Terms like ‘UV-C sanitizers/sterilizer’ and ‘UV Light Germ Killers’ are typically used to refer to UV light air purifiers. How they work is simple: they use a UV light to kill viruses and disinfect or ‘sanitize’ the air. That’s it.

Air purifier marketers like to use fancy names to describe the technology, but it is really quite simple.

What is UV light and how does it kill viruses? Read our explainer article here →

The Problem With UV Light Air Purifiers on the Market

The question is, are the UV lights in UV light air purifiers and far-uvc technology powerful enough to kill the coronavirus? Scientists have shown a dose of 75mJ/cm2 is needed to kill >99.9% of the COVID-19 virus.

After analyzing China’s biggest e-commerce platform – Taobao, we found data on the UV light strength for 3 UV light air purifiers. This includes one purifier from Samsung:

UV light air purifier comparison power and airflow

The average UV bulb power for these 3 purifiers is 6W. Using a simple rule of thumb, for an average distance of 10cm from the UV light, these purifiers give out on average 6mW/cm2 of UV light.

6W UV light from common household UV light sanitizer kills virus at 10cm distance

At 6mW/cm2, the UV light would need  to shine for at least 12.5s to kill >99.9% of the virus.

That doesn’t sound so long. However, knowing each purifier’s airflow, we estimated that air is passes through the purifiers in around 0.35s. That’s only 1/36th the time required to kill the viruses.

Are UV lights really not powerful enough?

Particles really close to the light (2cm away), would be subject to a much higher dosage – 150mW/cm2. At this distance, it would only take 0.5s to kill over 99.9% of the virus. That’s much better, but still too long for air that’s only in the purifier for 0.35s.

Result: Typical UV light air purifiers aren’t effective at killing COVID-19

But Can Air Purifiers With Strong UV Lights Kill COVID-19 and Viruses?

In direct response to COVID-19, new companies have popped up claiming to offer “incredibly strong UV light air purifiers”. Here’s one:

New powerful UV light air purifier to kill and disinfect COVID-19 coronavirus

This company claims that its UV light air purifier is specifically suited for killing viruses. They’re able to achieve a UV light intensity of 16.8 mW/cm2 due to a ‘special coating’ inside the purifier. That’s almost 3 times the intensity of the ‘standard’ UV light air purifiers mentioned above.

Intensity of UV light in an air purifier to kill COVID-19

At 16.8mW/cm2, viruses would need to be exposed to the UV light for 4.46s to kill >99.9% of the virus. That’s a little better than ordinary UV light purifiers, but still not great. Air would still need to pass through this purifier 15 times before it killed >99.9% of COVID-19.

Result: Even more powerful ‘COVID-19 ready’ UV light purifiers don’t give enough time to kill COVID.

What All UV light Air Purifiers Have in Common: A HEPA Filter

At this point, let’s backtrack. There’s one thing most UV light air purifiers above have in common: a HEPA filter. The reason for this is simple. The UV lights in air purifiers aren’t powerful enough to kill all viruses in one pass. On the other hand, HEPA filters are great at capturing viruses. Data shows they can capture over 99.9% of viruses in a single pass.

HEPA filter

Maybe the UV Light Is to Kill Viruses Stuck in the HEPA Filter?

Let’s give the air purifiers in question the benefit of the doubt. Trying to purifier air that’s moving really fast through the purifier doesn’t seem like a smart idea. So maybe the goal of the UV light is instead to disinfect viruses that get stuck in the HEPA filter? Here’s our super-simple rendition of how this might work:

Using UV light to disinfect HEPA filter from viruses and COVID-19

However, that’s got its own problems. HEPA filters are a complex mat of pleated fibers. It’s unlikely that a UV lamp shining on HEPA filter surfaces can actually disinfect the entire surface.

UV Light Ineffective Disinfect Clean HEPA Filter Virus

What’s more, UV light can also damage materials like masks and HEPA filters. Although there’s a paper shows UV-C treatment does not compromise N95 mask performance or integrity at low doses, long term and higher doses may still degraded your HEPA filters.

That’s not all. There are a few other disadvantages of UV light air purifiers.

Point #1 – Dirty UV Lights Rapidly Lose Effectiveness.

If UV lights get dusty, it has a huge impact on the UV light bulb’s irradiation. That means UV light purifiers will either lose their efficiency quickly, or they’ll need cleaning regularly.

Point #2 – UV Lights Cost More (Long & Short Term)

UV lights will cost you more. Adding a UV light to a purifier will not just make the purifier more expensive, but will also run up higher electricity bills. A typical UV 50W UV-C light will cost you $4.32 a month if running 24 hours per day in the US. That’s wasted money for something that gives you little in return.

Point #3 – UV-C Light is Dangerous

UV-C light is really dangerous stuff. In fact, it’s the most damaging type of UV light out there. Why put something near you that could potentially do more harm to your body? Even if the UV light is sealed inside the air purifier, it can still leak out. The guys selling the fancy COVID-19 purifier mentioned above admitted that UV-C light leaks out.

The CDC’s Stance on UV-Light, UVGI, and Far-UV

The CDC does not recommend UV light as a primary solution to COVID protection because:

  1. UVGI is more costly to install and maintain, and may cause temporary eye or skin damage as well as damage to plants.
  2. Questions remain about Far-UV’s overall safety and its mechanisms for killing organisms. As an emerging technology, there is also little peer-reviewed data to show how well it works, unlike for example HEPA filters.

Read More: The CDC’s Recommended Ways to Reduce COVID-19 Transmission

Use a HEPA Filter Air Purifier to Purify the Air, Not UV-C Light Sanitizers

HEPA filters do an incredible job of capturing viruses. Data shows that they can capture more than 99.9% of particles smaller than the COVID-19 coronavirus in just one pass.

HEPA filters have been used by hospitals and in other medical environments for a long time.

Hospital Blast HEPA Air Purifier
Blast HEPA air purifier used in hospitals

Researchers state that UV lights aren’t a replacement for HEPA filters.

dust really affect the efficiency of UV light

Bottom Line: How Effective Are UV Light Air Purifiers at Killing Viruses

UV light air purifiers are too weak to kill viruses such as COVID-19 quickly enough. What’s more, most UV light purifiers contain HEPAs – the real key in removing 99.9% of viruses from the air. UV lights add extra cost, are difficult to maintain, and can damage HEPA filters. All with little benefit for removing COVID-19 from the air.

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