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Short answer
Air changes per hour (ACH) is the room’s airflow rate divided by the room’s volume: Q / V = ACH, with both measured in matching units per hour. The CDC’s Environmental Infection Control Guidelines use this relationship, and note that most ventilation rates for health-care facilities are expressed as room ACH. A higher ACH clears airborne contaminants faster, but ACH alone does not describe pressure direction or air mixing.
Definition and scope
ACH expresses how many times, per hour, a volume of air equal to the room’s volume moves through the space. The CDC’s Appendix B on air sets out the formula Q / V = ACH, where Q is the air flow rate in cubic feet per hour and V is the room volume in cubic feet. The figure is a rate of airflow relative to room size, not a measurement of how clean the air is.
In practice the term appears wherever ventilation is specified for a room: patient-care areas, isolation rooms and laboratories. This guide covers the arithmetic and what the number does and does not tell you. Required values for specific room types are set by the governing standard and are covered in ASHRAE 170 ventilation requirements for healthcare.
How air changes per hour is calculated
The air changes per hour formula
Using the CDC’s definitions: ACH = Q / V, with Q in cubic feet per hour and V in cubic feet. Because the CDC defines Q per hour, airflow quoted in cubic feet per minute (cfm) must be multiplied by 60 before dividing. This unit conversion is simple arithmetic on the CDC definition, not a separate rule from the guideline.
Worked example
This is an illustrative calculation, not a measurement of any real room. A room is 20 ft by 15 ft with a 9 ft ceiling, so V = 20 × 15 × 9 = 2,700 cubic feet. Air moves through it at 450 cfm.
- Convert to hourly flow: Q = 450 × 60 = 27,000 cubic feet per hour.
- Divide by volume: ACH = 27,000 / 2,700 = 10.
Forgetting the × 60 step would give 0.17 and understate the rate by a factor of 60.
What ACH means for clearing airborne contaminants
The CDC’s guidelines go further than the ratio and give the time needed to remove airborne contaminants: t2 − t1 = −[ln(C2 / C1) / (Q / V)] × 60, with t1 = 0. Table B.1 applies it to removal of 99% and 99.9% of contaminant. Rows marked with a plus sign are, in the CDC’s words, frequently cited ACH values for patient-care areas.
| ACH | Minutes to remove 99% | Minutes to remove 99.9% |
|---|---|---|
| 2 | 138 | 207 |
| 4 | 69 | 104 |
| 6+ | 46 | 69 |
| 8 | 35 | 52 |
| 10+ | 28 | 41 |
| 12+ | 23 | 35 |
| 15+ | 18 | 28 |
| 20 | 14 | 21 |
| 50 | 6 | 8 |
Source: CDC Environmental Infection Control Guidelines, Appendix B, Table B.1 (checked 2026-10-29).
The CDC states that the times assume perfect mixing of the air in the space (mixing factor = 1) and that perfect mixing usually does not occur. Values apply to an empty room with no aerosol-generating source; with a person present and generating aerosol, the table does not apply. Removal times are longer where mixing is poor or air is stagnant.
The table also shows a pattern that the formula implies: doubling ACH halves the time, so moving from 6 to 12 ACH cuts the 99% time from 46 to 23 minutes. The CDC also notes that peak efficiency for particle removal in the air space occurs between 12 and 15 ACH.
Where it matters for labs and clinics
The CDC’s air chapter lists, for a negative-pressure airborne infection isolation (AII) room, a ventilation rate of at least 12 ACH for renovation or new construction, with airflow into the room. AII rooms are set at negative pressure to prevent airborne microorganisms in the room from entering hallways and corridors. A protective environment room is the opposite case, with positive pressure and airflow out to the adjacent area. See the airborne infection isolation room guide for how these rooms are specified.
For laboratories, room layout and airflow are covered in the laboratory design guide. Where air passes through filters before recirculation or supply, filter performance is a separate question from ACH; see MERV ratings and HEPA filter certification.
Common misunderstandings
One air change does not remove all the contaminant
Applying the CDC’s formula for a single hour at 1 ACH, with perfect mixing, gives a concentration of e−1, about 37% of the starting level, so roughly 63% is removed. This is calculated from the CDC formula, not quoted from it. Removal is gradual.
ACH is not the same as airflow in cfm
Airflow (Q) is a volume per unit time; ACH divides it by room size. The same 450 cfm gives 10 ACH in a 2,700 cubic foot room but only 5 ACH in a room twice that size.
ACH does not say which way air moves
A room can meet an ACH figure and still have the wrong pressure relationship. The CDC specifies pressure direction separately from ACH for AII and protective environment rooms.
The table is not a performance guarantee
Because the table assumes perfect mixing and an empty room, it shows a benchmark under ideal conditions, not the time a particular occupied room will take.
FAQ
What is the air changes per hour formula?
ACH = Q / V, where Q is the air flow rate in cubic feet per hour and V is the room volume in cubic feet. If airflow is in cubic feet per minute, multiply by 60 first. For a 2,700 cubic foot room at 450 cfm, ACH = (450 × 60) / 2,700 = 10.
How many air changes per hour does an airborne infection isolation room need?
The CDC’s air chapter lists at least 12 ACH for a negative-pressure AII room in renovation or new construction, with airflow into the room. Check the governing standard for your jurisdiction; see the ASHRAE 170 guide.
Sources
- CDC, Environmental Infection Control Guidelines, Appendix B: Air, cdc.gov/infection-control/hcp/environmental-control/appendix-b-air.html (checked 2026-10-29).
- CDC, Environmental Infection Control Guidelines, Air chapter, cdc.gov/infection-control/hcp/environmental-control/air.html (checked 2026-10-29).
