Thread by @jljcolorado

1/ Air cleaners: the good, the dubious, and the ugly

With pandemic & major contribution of aerosol spread, many ppl wanting “air cleaning devices” to reduce / remove virus. Unfortunately many types, aggressive & misleading marketing of unproven & potentially dangerous cleaners

2/ Critical background: virus is NOT "naked" (0.1 um) in the air. Aerosolization processes in respiratory system produce larger particles, likely mostly 1-10 um (

https://twitter.com/linseymarr/status/1290617594597310465

). Unfortunately "0.1 um virus naked in the air" is frequently repeated misconception.

3/ Best air cleaning: filters; work, no side effects
(a) Filters on building / home HVAC, furnace or AC, MERV13 if possible (ashrae.org/technical-reso…)
(b) Portable HEPA filter, great, but expensive (shellym80304.com/2020/06/15/a-h…)
(c) Cheap fan-filter, $40! (wired.com/story/could-a-…)

4/ For selecting HEPA cleaners, see the link above, also CU-Harvard spreadsheet:

https://twitter.com/ShellyMBoulder/status/1291071670313050112

. Also AHAM certification in US ahamverifide.org, CARB certif. ww2.arb.ca.gov/our-work/progr…, en Espana: eurovent-certification.com/es/technical-i… (But ONLY HEPA, not other types even if cert.!

5/ Many manufacturers of other air cleaners dismiss HEPA air cleaners as ineffective. Excuse my French… but this is BS! Eg this huge error, actually in a paper in press! (sciencedirect.com/science/articl…). Know 1 of authors, is trying to get this corrected as he had not noticed before

6/ HEPA = 99.97% filtering efficiency at 0.3 um, better at any other size. See: epa.gov/indoor-air-qua…

(But this causes pressure drop, not usable in some pre-existing systems, then go MERV13 if possible)

7/ Selecting HEPA cleaner, DO NOT want ones that also have UV, ions or other bells-and-whistles. More $, don’t help, can be dangerous (see below). Filter already capturing the virus, which will decay there. Just change filter carefully, put in sealed bag, wash hands

8/ There could be some infective virus left on filter when changing it, but hard to get it back to the air. Change outdoors, with mask, just to be extra careful.

9/ Germicidal UV: this method is known to work very well to destroy viruses including SARS-CoV-2 (e.g. dhs.gov/science-and-te…).
- But expensive, need maintenance, very dangerous if installed improperly. Experts only recommend when nothing else possible: sciencedirect.com/science/articl…

10/ For UV (& other types below), **very important** to make sure that they do NOT produce ozone, which is very damaging to lungs (cfpub.epa.gov/ncea/risk/reco…). Ozone generators only seem to be a problem in the US, and not Europe or elsewhere (HT Bart Croes).

11/ See CARB list of devices to avoid: ww2.arb.ca.gov/ozone-generati…

If cleaner does NOT certify that it does NOT make ozone, DO NOT BUY!

12/ Other types of air cleaners: ions, oxidation, plasmas etc. Cause some air chemistry. Probably effective at removing or deactivating the virus. But same chemistry that deactivates the virus by oxidation of its organic molecules, can also produce toxic compounds

13/ Toxics formed from from oxidation of volatile organic compounds (VOCs). VOCs are very abundant in indoor air, much more than outdoor air (pubs.acs.org/doi/abs/10.102…).

14/ Manufacturers often brag their products reduce VOCs. Problem is that do not convert them to CO2 and H2O, impossible in small devices. Studied this chemistry in gory detail, applies to air cleaners (e.g. cover article in Chemical Society Reviews, doi.org/10.1039/C9CS00…).

15/ See paper from MIT (krollgroup.mit.edu/papers/isaacma…). Initial VOC (a-pinene, eg cleaners, fragrances etc, often abundant indoors) destroyed quickly. But... MANY products, more oxidized. Same in air cleaners, same oxidation that hurts the virus oxidizes VOCs.

16/ Products of VOC oxidation include formaldehyde, product of oxid. of almost all VOCs, quite toxic (euro.who.int/__data/assets/…). eg FEMA trailers after Katrina, form. from wood glue was very dangerous.

Other potential toxics: peroxides, aldehydes, nitro-compounds, etc

17/ These byproducts are not seen on the studies that the manufacturers have provided. This is because they require more advanced VOC measurements, such as chemical ionization mass spectrometers (aerodyne.com/product/chemic… or ionicon.com/series/details…).

18/ Studies can be done, many research groups have instruments. Pls reply if you know of any such study, manufacturers have not provided any upon request. Because TMK this has not been studied, we don’t know how dangerous it is.

19/ But we should follow **precautionary principle** (en.wikipedia.org/wiki/Precautio…), especially since very effective and completely safe filtering-based solutions exists.

20/ Back to germicidal UV, can photolyze O3 present indoors, although at low concentrations --> similar chemistry (see our paper doi.org/10.1039/C9CS00…). This is an additional reason to avoid UV, unless really really really the HEPA filters etc. cannot be done.

21/ BOTTOM LINE:
YES: better filters in HVAC, portable HEPA cleaners, and makeshift fan-filter cleaners
If above just not enough (e.g. some hospitals) and professionals available to install properly AND maintain: germicidal UV
Avoid: ions, oxidation, plasmas, and UV within HEPA

22/ Comments?
- Happy to update this, if peer-reviewed evidence addressing all oxidation products is provided. Not seen any so far.
- Expect dismissal from manufacturers (again). But please provide evidence.
- Comments? Anything I missed?
@ShellyMBoulder @CorsIAQ @IAQinGWN

This is the unrolled version of thread:

https://twitter.com/UnrollHelper/status/1291765883019436033

I didn't mention foggers with bleach or hydrogen peroxide. I don't like them either. Do not use when ppl present. Money better spent on HEPA, IMHO