Purifier Sizing
Air safety can seem nebulous because we can't see or sometimes even smell contamination to build intuition about how quickly pathogens are being scrubbed from the air. Luckily, simple formulas inform how many and how big of air purifier(s) will add a particular degree of safety.
Airflow per Occupant Method
As Joey Fox explains, the new ASHRAE 241-2023 guidelines for Control of Infectious Aerosols bring a better-reasoned approach to room safety calculations than ACH: clean airflow per occupant. The required clean airflow depends on expected occupancy and activity. For example, healthcare, singing and harder breathing in gym require more, but for common office, schools and residential is 30-50 CFM/person:
Luckily this range corresponds to the CADR we get per PC fan and per sq ft of Filtrete area. (Specifically ~40 DUST CADR per Arctic P12, ~45 DUST CADR per SickleFlow, ~50 POLLEN CADR per SickleFlow).
Rule of thumb: each PC fan in CleanAirKits boxes covers 1 room occupant. e.g.
- 2 person bedroom: 2-barrel Exhalaron
- 5-person breakfast nook: 5-fan Luggable
- 14-person dinner party: Two 7-fan Luggable XL
- 21-person classroom: 3x 7-fan Luggable XL, or 2x Tower of Power
Classroom requirements may be halved if HVAC already ensures ~25 CFM of outdoor/cleaned airflow per person.
Air Changes per Hour (ACH)
Air Changes per Hour (ACH) quantifies the rate at which contaminants are cleared from a room's air. It's a broad indicator of a room's air safety under typical occupancy assumptions, though less meaningful at occupancy extremes. A doubling of ACH means contaminants are scrubbed twice as fast, cutting any inhaled dosages roughly in half and halving (on average) the risk of contagion between occupants. (Contagion risk between a particular pair of individuals also depends on their separation and any airflows connecting them). Here are the latest descriptions of qualitative ACH safety from The Lancet COVID-19 Commission:
Proposed Non-infectious Air Delivery Rates (NAD) for Reducing Exposure to Airborne Respiratory Diseases; The Lancet COVID-19 Commission Task Force on Safe School, Safe Work, and Safe Travel
| Volumetric flow rate per volume | Volumetric flow rate per person | Volumetric flow rate per floor area | |||
|---|---|---|---|---|---|
| ACHe | cfm/person | L/s/person | cfm/ft2 | Us/m2 | |
| Good | 4 | 21 | L/s/10 |
0.75 + ASHRAE minimum outdoor air ventilation |
3.8 + ASHRAE minimum outdoor air ventilation |
| Better | 6 | 30 | 14 |
1.0 + ASHRAE minimum outdoor air ventilation |
5.1 + ASHRAE minimum outdoor air ventilation |
| Best | >6 | >30 | >14 |
>1.0 + ASHRAE minimum outdoor air ventilation |
>5.1 + ASHRAE minimum outdoor air ventilation |
To read the full report, visit:
covid19commission.org/safe-work-travel
Sources of ACH like HVAC systems, open windows, and air purifiers are all additive. Other notable ACH levels:
- 1 ACH: typical background air change rate from leaks and natural circulation without mechanical HVAC
- 1 ACH: typical boost from opening windows
- 2-3 ACH: typical capacity of pre-pandemic HVAC circulating to outside or retrofitted with MERV13
- 6 ACH: pre-pandemic design target for general areas of hospitals. Cuts transmission 82% compared to 1 ACH
- 12-14 ACH: hospital isolation room (minimum). Comparable to outdoors?
- 12-14 ACH: hospital isolation room (minimum). Comparable to outdoors?
To compute the ACH Boost from purifier(s) with a particular clean air delivery rate (CADR) in cubic feet per minute (CFM), we just need the room volume (length X width X ceiling height) in cubic feet:
ACH_boost = CADR * 60 / VolumeOfRoom
Since tall ceilings are inherently safer by allowing broader dispersion, Joey Fox suggests capping the ceiling height at 8' in this ACH formula. This way safer extra tall ceilings do not demand extra strong purifiers.
ACH as a Time Constant
Unfortunately we cannot simply say that the 5 ACH rate means a room is completely cleaned every 12 minutes. Due to infinite remixing of the cleaned portion with dirty room air, 12 minutes represents the exponential decay time constant for (1-exp(-1)) = 63.2% of contaminants to be removed. Within 3 time constants or 36 minutes, 95% of contaminants will have been removed. By doubling the cleaning rate to 10 ACH, the time to clear 95% of contaminants will drop to 18 minutes.
As you can see, even with 5 ACH, it can take quite awhile for a previously contaminated room to be even 95% cleared after turning on ventilation. Thus it's important to keep HVAC fans and air purifiers running continuously during occupied hours. This way they can be gradually removing infectious aerosols as they're generated, preventing buildup.
Likewise, remember to let purifiers work 20-30 minutes in e.g. a hotel room with unknown contamination state before assuming it's safe.
Doubling ACH to halve dosages & risks
If you want air quality upgrades to have a substantial impact on viral transmission rates, keep a suitably ambitious target in mind. By basic dilution principles, we can target doubling ACH over existing levels to roughly halve transmission risks. For a slightly more sophisticated model, try Dr. Corsi's SafeAirSpaces risk modeling tool. If a facilities expert is not able to provide analysis on existing room ACH, one can conservatively assume old construction is 2 ACH. The additional purifier CADR needed to double ACH is:
CADR_doubling = ACH_existing * VolumeOfRoom / 60
The number of purifiers needed of a particular CADR capacity is:
# purifiers = CADR_doubling / CADR_purifier_model
Creating a Well-Mixed Room
Risk reductions from high ACH assume strong mixing of air within a room. Corsi Rosenthal Boxes help to create room circulation but may not be sufficient to mix large spaces. Consider multiple units or pairing with a slow ceiling fan or oscillating fan to ensure even mixing and coverage in large rooms.