Standards guide · ASHRAE 241

ASHRAE 241 for Hospitals: ECAi Explained, with a Portable HEPA Calculator

How ASHRAE 241-2023 measures equivalent clean airflow per person, how filtration and HEPA units are credited, how it fits with ASHRAE 170, and how many portable HEPA units a room needs.

Updated September 30, 2026 Free ECAi calculator For hospital engineers and facility managers

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Quick answer

ASHRAE 241-2023 sets an equivalent clean airflow per person (ECAi) that a space must receive in infection risk management mode. Outdoor air, filtration of recirculated air and tested air cleaners all count. For filtration, the credit is airflow × filter efficiency, and filters must be at least MERV-A 11 to count. A HEPA unit (≥99.97% at 0.3 µm) credits almost all of the air it treats. In hospitals, 241 is applied on top of ASHRAE 170, not instead of it. Use the calculator below to size portable HEPA units for a room.

ECAiClean airflow per person, by occupancy

Q × efficiencyHow filtration is credited

MERV-A 11+Minimum filter for credit

IRMMWhen 241 requirements apply

What ASHRAE 241 requires

ANSI/ASHRAE Standard 241, Control of Infectious Aerosols, was released in June 2023. It asks building owners to plan and verify how each space will deliver enough equivalent clean airflow to reduce airborne infection risk during infection risk management mode (IRMM), which the owner or authority having jurisdiction activates during periods of elevated risk.

ECAi targets are set per person by occupancy category (Standard 241 Table 5-1).
Contributions come from outdoor air, filtered recirculated air and tested air cleaners.
Filters must be at least MERV-A 11 (or equivalent) to receive credit. MERV-A reflects efficiency after loading.
Non-filter air cleaners must be tested under the standard's test method and pass safety testing for byproducts such as ozone and formaldehyde.
An assessment and a building readiness plan document how targets are met.

How filtration is credited

For any filtration device, the clean airflow it contributes is its airflow multiplied by its efficiency against infectious aerosols:

ECA contribution (cfm) = airflow through the filter (cfm) × filter efficiency
Required clean airflow for a space = occupants × ECAi (cfm per person).

A HEPA filter captures at least 99.97% of 0.3 µm particles, so a portable HEPA unit moving 300 cfm contributes almost 300 cfm of clean air. A MERV 13 filter in an air handler contributes less for the same airflow because its efficiency is lower. That is why portable HEPA units are a common way to close a 241 gap without rebuilding the HVAC system.

ECAi gap calculator for portable HEPA units

Occupants in the space ECAi target (cfm per person, from Table 5-1) Outdoor air delivered to the space (cfm) Other filtered recirculated air (cfm × efficiency) Portable HEPA unit airflow (cfm) Filter efficiency credited (%)

Planning aid only. Use the ECAi value for your occupancy category from ASHRAE 241 Table 5-1, measured airflows, and the tested filter efficiency. The default target of 40 cfm per person is an example, not a value for your space.

ASHRAE 241 in hospitals

In healthcare, ASHRAE 170 sets the baseline ventilation, pressure relationships and filter efficiencies for each space type. ASHRAE 241 adds an infection-risk clean-airflow target that applies in IRMM. Spaces where 241 most often needs extra clean air are those with high occupancy and ordinary ventilation: emergency department waiting areas, outpatient waiting rooms, staff areas and general patient areas. Operating rooms and isolation rooms already have high air change rates and HEPA or MERV 16 filtration under 170. Our ASHRAE 241 vs 170 comparison sets the two side by side.

Our units and ASHRAE 241

Two-stage filtration: a washable metal-foam pre-filter followed by a HEPA filter. How it works →
What we supply for your 241 assessment: measured airflow and a third-party whole-unit test: the IAS-NC500-HI-CUSTOM tested at >99.99% for particles ≥0.3 µm (report KJ2026002491, available on request).
Variable-speed fan: airflow set on a variable-frequency drive to match each room's clean-airflow gap.
No byproducts: mechanical filtration only; no ozone, ions or chemicals.
Built-in differential pressure gauges to show when the HEPA filter needs replacing.
Custom sizes through our OEM and custom manufacturing service.

Whether a space meets ASHRAE 241 is determined by the engineer's assessment of all contributions, not by any single product.

ASHRAE 241 FAQ

What is ASHRAE Standard 241?

ANSI/ASHRAE Standard 241-2023, Control of Infectious Aerosols, released in June 2023, sets requirements for reducing the risk of airborne infection in buildings. Its core metric is the equivalent clean airflow rate for infection risk mitigation (ECAi), which a space must provide per occupant when operating in infection risk management mode.

What is ECAi?

ECAi is the equivalent clean airflow per person a space needs in infection risk management mode, given in L/s or cfm per person by occupancy category. Outdoor air, filtration of recirculated air and tested air cleaners can all contribute to it.

What is infection risk management mode (IRMM)?

IRMM is the operating mode used during periods of elevated infection risk, such as outbreaks or respiratory-virus season. The building owner or authority having jurisdiction decides when it applies. Standard 241 requirements apply while the building is in IRMM.

How does filtration count toward ECAi?

The contribution of a filter or filtration unit is the airflow through it multiplied by its efficiency against infectious aerosols. Filters must be at least MERV-A 11 (or equivalent) to receive credit. A HEPA filter, at least 99.97% at 0.3 µm, gives close to full credit for the air it treats.

Do Iodine Air Systems units count toward ASHRAE 241?

Our units are HEPA filtration units: a washable metal-foam pre-filter plus a HEPA filter. We provide the measured airflow and a third-party whole-unit test (IAS-NC500-HI-CUSTOM: >99.99% for particles ≥0.3 µm, report KJ2026002491) so your engineer can calculate each unit's ECAi contribution. Airflow is adjustable on a variable-frequency drive. They use no ozone, ions or chemicals. Whether a space meets 241 is decided by the engineer's assessment, not by any single product.

Does ASHRAE 241 replace ASHRAE 170 in hospitals?

No. ASHRAE 170 sets the baseline ventilation, pressure and filtration for each healthcare space type. ASHRAE 241 adds an infection-risk clean-airflow target for IRMM. A hospital space may need to satisfy both. See our ASHRAE 241 vs 170 comparison.

Do air cleaners other than filters count?

Other air-cleaning technologies can count only if they are tested under the standard's air-cleaner test method and pass its safety testing, which covers byproducts such as ozone and formaldehyde. Mechanical HEPA filtration does not generate these byproducts.

How many portable HEPA units does a room need?

Use the calculator on this page: enter occupants, the ECAi target for your space type, the outdoor air delivered and the unit airflow. It shows the clean airflow gap and how many units close it. Your engineer should confirm the targets from Standard 241 Table 5-1.

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