Common UV Lights Aren’t Great at Killing COVID-19

The Dose of UV Light Needed to Kill Viruses is Stronger Than You Think

UV light has surfaced as a potential solution to killing COVID-19 and other viruses alike. Data shows that UV light can kill COVID-19 on masks without degrading their performance. However, how about disinfecting other things with UV light? And can UV lights be an effective form air purification?

 

Using UV Light to Kill COVID-19

A group of Japanese researchers measured how well UV-C light sources could kill SARS-CoV-2 inoculated on surfaces. In their research (yet to be peer-reviewed), they used an LED to shine UV light onto a 60-mm Petri dish at a distance of 20mm. The intensity of the UV light was 3.75mW/cm2, and they shone UV light on the virus for varying times from 1 to 60 seconds.

The scientists found that 10s of the UV light was enough to kill 99.9% of the COVID virus. 20 seconds killed over 99.9% of the virus.

Using UV light kill virus, the relationship between time and killing percentage rateTable of UV light kill virus

 

With this data, the scientists concluded that the a 75mJ/cm2 dose of UV light is required to kill COVID-19. They got to this number by taking the time required to kill >99.9% of the virus (20s) and multiplying by the intensity of the bulb (3.75mW/cm2).

Using UV light disinfecting virus at a distance of 20mm and dose of 75mj/cm2

How well can consumer UV-C lights kill COVID-19

OK, so we know 75mJ/cm2  of UV light is needed to kill COVID-19, can common household UV light sanitizers give this dose? The UV light used by the Japanese researchers was a 3.75mw/cm2, but how well can common consumer UV lights kill COVID-19?

UV light from amazon and taobao

We checked UV light sanitizers on China’s Taobao e-commerce platform, and on Amazon US. Data on UV light intensity and wattage was not displayed on any of the listings.

After asking customer support representatives, we found that a fairly common wattage for these UV lights was 0.7W of UV-C light. At a distance of 1m, that’s 70μW/cm2.

0.7w uv light with 1 meter distance killing virus

 

The Japanese scientists found that 75mJ/cm2 is required to kill >99.9% of the coronavirus. Doing the math, these lights would need to shine on a surface for 1071 seconds to reach that level. That’s almost 18 minutes.

0.7w uv light with 1 meter distance killing virus with 18 minutes

 

Moving the UV light closer to the contaminated item would reduce the time required to disinfect viruses. UV-C light intensity reduces with the square of the distance from the source. Reducing the distance from 1m to 10cm would require just 11s to meet a dose of 75mJ/cm2.

distance time kill 99.9% virus

11s sounds more reasonable, but it may be difficult to keep the UV light constant across the infected surface, so longer times would be required. We suggest at least 1 minute.

 

HEPA Filters: A Better Way to Remove COVID-19

UV lights seem like a great solution, but this data shows that it may be difficult to ensure over 99.9% of the virus is killed. On the other hand, HEPA filters can filter over 99.9% of virus-sized particles in one pass, and may be more effective at removing COVID-19 from the air.

Read more: HEPA filters are an effective way to filter out COVID-19 →

 

Bottom Line: Killing COVID-19 with Common Household UV Light Sanitizers

A dose of 75mJ/cm2 of UV-C light can kill over 99.9% of the COVID-19 coronavirus. Most common household UV lights give off very small doses of UV-C, and so may require up to 18 minutes to kill viruses. Moving the item closer to the UV light source can reduce this time, however.


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