There are three types of air cleaners:
- Subtractive: like filters, will remove the contaminant when it comes in contact with the media
- Additive: like ionizers, will emit ozone/radicals/ROS/etc.. to react with the air (will emit something to remove something)
- Hybrid
Filter media (MERV, HEPA), UVGI are proven because Standards/guidelines/>decade of testing exist to test efficiency and by-products.
NO Standard exist today to test additive/electronic air cleaners (none, nada, zero, nul, nula), therefore they are Unproven.
Efficiency can be determined using the clean air delivery rate metric (CADR) = airflow of clean air.
For more info: ahamverifide.org/ahams-air-filt…
Rule of thumb: the CADR of your air cleaner should be equal to at least two-thirds of the room’s area to get ~5 ACH.
Selection chart of some air purifiers (option 1):
-Start by CADR (y-axis)
- Consider cost (x axis) – the lower the better. This is cost of purchase of unit + replacement of filter per year.
-Consider noise (look at color of the dot).
-Consider >1 cleaner to keep noise down.
Selection chart of some air purifiers (option 2):
-Start by area of room (y-axis)
- Consider cost (x axis) – the lower the better. This is cost of purchase of unit + replacement of filter per year.
-Consider noise (look at color of the dot)
-Consider >1 cleaner to keep noise down
If you are looking for purifiers in Australia/NZ, graph in $AUD here: @RobynSchofield3
Note: Corsi-Rosenthal box data will be added soon (I am missing the noise level!)
Don’t make the mistake of Dr. Fauci, make sure you are getting enough clean air for what you are paying for (expressed by x-axis $/ft2). You want high CADR with low cost and low noise.
For example, say that you have a room 400 ft2, you need an air cleaner with CADR ~ 250 CFM. See options in red circle.
When you disable ionizer, the Blue Air purifier (fan and non-HEPA) decreases in performance. Take a look at the chart, much worse performance in terms of $/ft2 compared to other air cleaners. @smartrestartaps
(3/3)
Blue air (tm) purifier is a powerful ionizer and no-HEPA filtration. High level of ions has been shown to affect negatively cardiac markers (study done on college kids)
The impact on generating harmful gases (VOCs, formaldehyde) were not measured.
Slide by @ChemDelphine
Graph here compares performance/cost of needle point ionizer versus HEPA filters. Not a good investment.
Needle point ionization (2/3):
Recent studies showed negligible effect on particle removal (field study in office as installed, field study classroom as installed, chamber study x 3, airplane x 2).
For additive/electronic air cleaners:
Studies showed potential by-product formation.
Morale of the tweet (1/3):
Choose only air cleaners that have mechanical filtration ( particulate filter media) with MERV 13 or higher or HEPA designation.
MERV 13 and HEPA are proven technologies, most are cost effective.
Be aware of false marketing:
HEPA-like
HEPA-style
99% HEPA
HEPAsilent (tm)
HEPA Ultra Ionic
HEPAFast
HEPA Efficiency
HEPA Function
HEPA Action
Plasma HEPA
Super HEPA
HyperHEPA
HEMPA
are ALL subpar versions of what constitutes a HEPA air filter. Either HEPA or NOT HEPA
Stay away of unproven technologies
Check the open letter for write-up and resources: @MarcelHarmon1
🧵 Answers to what the different strategies will result in term of air change rate
Example used throughout the thread:
👉Classroom: 960 ft2: 30' x 32’
👉Number of occupants: 30 (this is the design number, taking into account average classroom density).
Ventilation by code, this classroom should have 415 CFM airflow of outside air or 14 CFM/person.
👉This airflow is being forced through the HVAC system (unit in classroom or mechanical room).
When we check ventilation, the answer is not “yes” or “no”, the answer should be how much?
I looked at many classrooms. Similar findings. Most do not have outside air and problems with exhaust fans of bathrooms (unless I am missing something - no reports it was fixed?).
Most schools have windows that can be open but windows are not a reliable method to get you a specific air change rate. For more information about ventilation:
COVID Safety Plan (1/2)
- People density was at 50% (by design)
- Other measures below.
Everything was followed properly, except social distancing.
Masks were enforced, even outside. One might ask how can you enforce masks if people are drunk? I saw 2 people holding a person to his room while a third person holding by two fingers his mask in place.. commitment!
🧵 about ventilation
- Mechanical versus Natural ventilation
- ASHRAE Standard 62.1 historical rates
- Why schools are under-ventilated?
- What is the cost of outside air?
- What we should do about it?
Note 1: According to ASHRAE Standard 62.1 (most prominent ventilation standard), ventilation is not only about outside air. Rather, it is the sum of outside air and return air that is cleaned.
In this thread, I will focus only on outside air.
Note 2: schools are either designed to have mechanical ventilation (intentional OA from HVAC unit) or natural ventilation (intentional OA through windows and doors).
Most schools in North America are mechanically ventilated (for many reasons mainly for comfort and safety).
1) Misleading marketing:
The filter used is called "HEPASilent". It is not a HEPA filter.
It is a common pleated filter + ionizer.
2) By-products:
The company claim to produce low ozone.
It is certified by CARB to produce < 50 ppb.
They have a carbon filter which will capture some ozone - although % will decrease with use (we don't know how much % versus age).
Indoor ozone levels should be < 5 ppb
3) As we discussed previously, with every ionizer, there is a potential production of formaldehyde (and other VOCs) and ultrafine particles. Example of one ionizer tested by Lawrence Berkeley lab: