What ASHRAE 241 Is
Published in June 2023 as Standard 241-2023, Control of Infectious Aerosols, ASHRAE 241 is the first comprehensive consensus standard for managing airborne disease transmission in buildings. It is a voluntary standard given legal force only when adopted by a code authority, written into a project specification or lease, or applied voluntarily by an owner. Addendum a was published in October 2024, and the standard is maintained continuously, so it should always be read with its current addenda rather than the 2023 text alone.
Equivalent Clean Airflow (ECAi)
The standard is built on equivalent clean airflow (ECA): the flow rate of pathogen-free air that, if delivered uniformly to the breathing zone, would match the infection-risk reduction of the actual mix of outdoor air, filtered recirculation, and air cleaning. Each technology is credited on a common basis, outdoor air at full value, recirculated air at its filter single-pass removal efficiency, and in-room cleaners at their tested clean-air delivery rate.
Required rates come from Table 5-1 as ECAi per person. These are not outdoor-air rates:
| Space type | cfm/person | L/s/person |
|---|---|---|
| Office | 30 | 15 |
| Classroom | 40 | 20 |
| Retail | 40 | 20 |
| Food and beverage | 60 | 30 |
| Gym / exercise | 80 | 40 |
| Healthcare waiting | 90 | 45 |
| Auditorium / worship | 50 | 25 |
The target is set to achieve roughly a 0.1% hourly infection probability 96% of the time (Wells-Riley model).
Filtration, IRMM, and Heat Recovery
Compliance requires prerequisite outdoor air per ASHRAE 62.1 and a minimum filtration of MERV-A 11 (effective January 2025). Buildings operate in Infection Risk Management Mode (IRMM) during periods of elevated transmission. Crucially for efficiency, ASHRAE TC 5.5 practical guidance for epidemic operation of energy recovery ventilation systems shows that properly configured energy recovery ventilators can keep running in IRMM, so infection-risk resilience need not abandon the carbon savings of heat recovery.
Why It Matters
Because 241 credits filtered air, owners can meet targets without crushing ventilation heating and cooling loads, supporting both IAQ and energy management goals at the same time.
Frequently Asked Questions
What is ASHRAE 241 and when does it apply?
ASHRAE 241 is the 2023 consensus standard for control of infectious aerosols; it becomes binding only when adopted by a code authority, written into a spec or lease, or applied voluntarily.
What is Equivalent Clean Airflow, or ECAi?
ECAi is the pathogen-free airflow that would match the risk reduction of the actual mix of outdoor air, filtered recirculation, and air cleaning, credited on a common basis per person.
Can energy recovery ventilation run during infection risk mode?
Yes, ASHRAE TC 5.5 guidance shows properly configured ERVs can keep running in Infection Risk Management Mode, preserving both air quality and energy savings.
What filtration level does ASHRAE 241 require?
It requires prerequisite outdoor air per ASHRAE 62.1 and a minimum effective filtration of MERV-A 11 from January 2025.