Purifier <-> Room 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.
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, universal indicator of any room's air safety. 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:
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?
20 ACH: operating room minimum (around operating table)
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
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. For instance, we can target doubling ACH over existing levels to roughly halve transmission risks. If a facilities expert is not able to provide analysis on existing room ACH, one can conservatively assume old construction is 2-3 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