Thread by @joeyfox85

Intro to ultraviolet light against COVID 🧵

Ultraviolet light has many different applications. Here are 6 that have been used for COVID:

1. In-duct
2. Stand-alone
3. Upper Room
4. Far UV
5. Surface Sterilization
6. Photocatalytic Oxidation

TL;DR - See tweet 16 👇

1/20

1. In-Duct

A UV light is placed in the duct or by the coil of an air handling unit. It will kill virus particles being supplied to the zone.

I'm not in favor:
a. It can damage the filters
b. Better alternatives are: increasing outdoor air % and improving filtration.

2/20

c. Transmission through ducts is long range transmission. Shared room transmission is much higher risk. Better to focus on higher risk.
d. People in the industry have expressed skepticism to me of how
effective this actually is.
e. UV is more expensive than filtration.

3/20

2. Stand Alone UV Unit

This is a box that has a UV light and a fan. The fan brings air through, exposes it to UV light, kills any microbes and sends the air back in the room. This is similar to a portable HEPA filter, just replace the filter with a UV light.

4/20

My opinion is it isn't worth it because #corsirosenthalbox or HEPA filter is cheaper and people in the industry have expressed skepticism about its effectiveness.

Proponents argue that:
a. Virus particles are too small for HEPA filters.

5/20

b. As dust builds up, the CADR slows down
c. Changing filters is dangerous because they contain viruses.

None of these problems exist with UV, but I don't believe they are justified:
a. the virus isn't naked & HEPA filters work below 0.3 microns.

6/20

b. Change the filters on schedule and the flow reduction won't be significant.
c. Use PPE when changing the filter and the risk is low.

I would opt for filtration over UV when it comes to a stand-alone unit. Much cheaper.

7/20

3. Upper Room UVGI

UV light is shined in the upper room and not on people below. This will kill virus particles in the upper room. As the room air mixes, there is a very high rate of virus-free air supplied to the people. If done properly, this is safe.

8/20

This is an important UV technology. There is a long history of it being used showing 90% reduction in flu transmission and strong effectiveness against TB. HEPA filters can add 2-3 air changes/hour. This can add 20-30 ACH! Resources are provided at the end of 🧵.

9/20

4. Far UV-C

Most UV-C for germicidal irradiation (UVGI) is at 254 nm from mercury lamps. With LEDs, now there is UV at 222 nm. It's a perfect wavelength where it doesn't hurt people but can kill viruses. It allows it to safely be shone everywhere in an occupied room.

10/20

There's a ton of promise and it has started being used, but it is not yet been used on a large scale. Hopefully with more awareness, this will change soon.

11/20

5. Surface Sterilization

This can be done in different ways. A UV light is shone into a room when no one is there and it will kill microbes on all surfaces. Alternatively, use a box with a UV light and place things in the box like smartphones or wallets.

12/20

COVID is airborne. Transfer through surfaces is not a relevant driver of the pandemic. UV surface sterilization might be useful for some medical applications, but it is not useful to mitigate against COVID.

13/20

6. UV Photocatalytic Oxidation (UV PCO)

All other methods kill viruses directly with UV light. This is different. It shines UV light on a surface with titanium dioxide (TiO2). It will cause a chemical reaction creating hydroxyl radicals which then go into the space.

14/20

This is a form of electronic air cleaning. Many air quality experts and organizations are against this technology.

medium.com/open-letter-to…

15/20

UV Summary

1. In-duct or standalone - might work, but filtration is less expensive
2. Upper Room - amazing technology that requires more use
3. Far UV - great, but not widespread yet
4. Surface Sterilization - not helpful for COVID
5. UV PCO - Electronic air cleaner-Avoid

16/20

Upper Room UVGI Resources

Here's an amazing presentation which includes @Don_Milton & @WBahnfleth . It gave me a great understanding of this. There's more focus on upper room UVGI, but there's also discussion of far UV.

ghdcenter.hms.harvard.edu/keeping-public…

17/20

Great Time article that discusses both upper room and far UVC:

https://twitter.com/ShellyMBoulder/status/1492500478667616259?s=20&t=gvJKPqLZkO3cBcok-6cqlA

18/20

Some academic articles:

ncbi.nlm.nih.gov/pmc/articles/P…

atsjournals.org/doi/10.1164/rc…

19/20

UVGI Fact Sheet

usgbc.org/resources/scho…

20/20