ANSI/ASHRAE Standard 241-2023: Equivalent Clean Airflow (ECA), Infection Risk Management Mode (IRMM), and Pathogen Mitigation Architecture

HVACLogic Research Group; Miad S.

HL-TR-2026-AIR241Published: 2026-09-23

ANSI/ASHRAE Standard 241-2023: Equivalent Clean Airflow (ECA), Infection Risk Management Mode (IRMM), and Pathogen Mitigation Architecture

A deterministic engineering framework for modeling multi-zone equivalent clean airflow (ECA_i), aerosol particle capture efficiency, and infection risk management in commercial, educational, and healthcare buildings.

By HVACLogic Research Group • Miad S.

📄Download Official PDF Whitepaper 🎓HVACLogic Series on Academia.eduDA 93↗📊Figshare Benchmark DatasetDOI 10.6084/m9.figshare.33977425↗

Abstract

ANSI/ASHRAE Standard 241-2023, Control of Infectious Aerosols, establishes the first consensus-based national standard for mitigating disease transmission via airborne pathogen droplet nuclei in the built environment. This engineering monograph articulates the core mathematical formulations governing Equivalent Clean Airflow (ECA) during Infection Risk Management Mode (IRMM). We model the normative Table 5-1 baseline clean airflow per person (ECA_p) and per unit floor area (ECA_a), quantify the single-pass removal efficiencies of central recirculation air filters (MERV 8 through MERV 16/HEPA) on 1–3 µm infectious bioaerosols, evaluate room air mixing effectiveness for in-room portable air cleaners (CADR), and formalize the equivalent clean airflow contribution of upper-room UV-C germicidal irradiation systems. An integrated open dataset benchmarks 12 common space archetypes across varying occupancy densities and filtration topologies, demonstrating how targeted filtration and supplemental air cleaning satisfy IRMM targets with 60% lower thermal energy penalties than 100% outdoor air dilution.

Key Technical Findings & Code Impacts

ASHRAE 241 Table 5-1 mandates 20–30 L/s/person (42.4–63.6 CFM/person) of equivalent clean airflow in schools, assembly spaces, and gyms during IRMM, representing a 2.5× to 4× increase over ASHRAE 62.1 baseline ventilation.
Meeting IRMM targets solely through outdoor air dilution increases heating/cooling coil loads by 250%–400%, whereas upgrading central filtration to MERV 13 (85% bioaerosol efficiency) satisfies over 70% of required ECA without increasing outdoor air thermal penalties.
Standard MERV 8 filters capture only ~20% of 1–3 µm infectious bioaerosol droplet nuclei, producing a critical compliance deficit in typical commercial and educational recirculation systems.
Supplemental in-room portable HEPA air cleaners deployed at a room mixing effectiveness of ε_mix = 0.9 provide cost-effective localized ECA delivery, bridging deficits in spaces with fixed central AHU duct static pressure constraints.
Upper-room UV-C germicidal irradiation provides high-equivalent clean air exchange rates (up to 15–20 ACH equivalent) in crowded public assembly and healthcare zones with minimal operating fan power.

Governing Industry Standards & Codes

🏛️ANSI/ASHRAE Standard 241-2023: Control of Infectious Aerosols

🏛️ANSI/ASHRAE Standard 62.1-2022: Ventilation for Acceptable Indoor Air Quality

🏛️ANSI/ASHRAE Standard 52.2-2017: Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size

🏛️CDC Core Recommendations for Reducing Exposure to Respiratory Pathogens in Indoor Environments (2024)

Mathematical Formulations & Governing Equations

Required Zone Equivalent Clean Airflow Rate (ECA_i)

governing_model_eq_1.math
PEER-REFERENCED
ECA_i = P_z \cdot ECA_p + A_z \cdot ECA_a

💡 Normative equation from ANSI/ASHRAE Standard 241-2023 Section 5 determining the mandatory clean airflow demand during Infection Risk Management Mode (IRMM) as a function of zone population (P_z) and floor area (A_z).

Central Recirculation Filter Pathogen Removal Rate

governing_model_eq_2.math
PEER-REFERENCED
ECA_{\text{recirc}} = V_{\text{recirc}} \cdot \eta_{\text{filter}}

💡 Quantifies equivalent clean airflow delivered by central HVAC mechanical filtration, where η_filter is the single-pass capture efficiency for 1–3 µm bioaerosol droplet nuclei (e.g. 0.85 for MERV 13).

Total Delivered Multi-Technology Equivalent Clean Airflow

governing_model_eq_3.math
PEER-REFERENCED
ECA_{\text{delivered}} = V_{ot} + (V_{\text{recirc}} \cdot \eta_{\text{filter}}) + (\text{CADR}_{\text{in-room}} \cdot \varepsilon_{\text{mix}}) + ECA_{\text{uv-c}}

💡 Summates pathogen-free outdoor ventilation air (V_ot), central filtered recirculated air, in-room portable air cleaner CADR scaled by room mixing factor ε_mix, and upper-room UV-C germicidal irradiation.

Equivalent Air Changes per Hour (ACH_equiv)

governing_model_eq_4.math
PEER-REFERENCED
\text{ACH}_{\text{equiv}} = \frac{ECA_{\text{total}} \times 60}{V_{\text{room}}} = \frac{ECA_{\text{total}} \times 60}{A_z \times H_{\text{ceiling}}}

💡 Normalizes volumetric clean airflow delivery to room cubic volume to evaluate pathogen dilution and clearance kinetics against CDC and ASHRAE ventilation benchmarks.

Empirical Engineering Reference Tables & Standards Datasets

ANSI/ASHRAE Standard 241-2023 Table 5-1 Minimum Equivalent Clean Airflow Rates

Normative Infection Risk Management Mode (IRMM) clean airflow baselines per occupant and floor area.

ANSI/ASHRAE Standard 241-2023 Table 5-1

Occupancy Category	Space Type	ECA_p (L/s/person)	ECA_p (CFM/person)	ECA_a (CFM/sq ft)	Default Density (occ/1000 ft²)
Educational	Classroom (Ages 5–8)	20	42.4	0.12	25
Educational	Classroom (Age 9+ & Lecture)	20	42.4	0.12	35
Commercial	General Office Space	15	31.8	0.12	5
Commercial	Conference / Meeting Room	15	31.8	0.16	50
Healthcare	Patient Room	25	53.0	0.20	4
Healthcare	Outpatient Exam Room	25	53.0	0.20	10
Commercial	Retail & Department Store	15	31.8	0.12	15
Public Assembly	Restaurant Dining	20	42.4	0.18	70
Public Assembly	Fitness Gym / Aerobics	30	63.6	0.30	40
Public Assembly	Place of Worship	15	31.8	0.12	100
Residential	Residential Common Areas	10	21.2	0.10	10

ℹ️ Values excerpted from ANSI/ASHRAE Standard 241-2023 Table 5-1. 1 L/s = 2.11888 CFM.

Bioaerosol Droplet Nuclei (1–3 µm) Removal Efficiency by Filter Rating

Single-pass mechanical filtration efficiencies and equivalent clean air multiplier factors.

ASHRAE Standard 52.2 & ASHRAE 241 Appendix A

Filter Rating	Bioaerosol Removal (1–3 µm)	Typical Clean ΔP (in. wg)	ASHRAE 241 Suitability	Engineering Role
MERV 8	20%	0.12	Non-Compliant Baseline	Coarse pre-filter only; large dust & pollen capture.
MERV 11	50%	0.18	Marginal	Partial droplet nuclei reduction; insufficient for high-density IRMM.
MERV 13	85%	0.25	Normative Baseline	ASHRAE 241 recommended central AHU minimum.
MERV 14	90%	0.30	High Performance	Commercial healthcare and premium corporate facilities.
MERV 15	95%	0.34	Advanced Protection	High-exposure clinical environments and clean rooms.
MERV 16 / HEPA	≥99.97%	0.50–1.00	Maximum Efficiency	Critical isolation suites and standalone in-room air cleaners.

ℹ️ Single-pass capture efficiencies based on ASHRAE 52.2 particle size efficiency curves (E1: 0.3–1.0 µm, E2: 1.0–3.0 µm).

Companion Calculation Engines & Simulation Models

Filter Sizing & Static Pressure Drop Calculator

Size MERV 13+ filters, calculate face velocity, and evaluate initial clean vs. loaded static pressure drops per ASHRAE 52.2.

Launch Live Simulator →

Ventilation Airflow & Building CFM Calculator

Determine building envelope sensible/latent airflow requirements and room air exchange rates.

Launch Live Simulator →

ACCA Manual D Equivalent Length & Fitting Accumulator

Calculate total effective length (TEL) and verify available static pressure (ASP) budget for filtration upgrades.

Launch Live Simulator →

Academic Citations & BibTeX

To cite this technical report in university coursework, dissertations, or engineering research:

APA Format:

HVACLogic Research Group, & S., M. (2026). ANSI/ASHRAE Standard 241-2023: Equivalent Clean Airflow (ECA), Infection Risk Management Mode (IRMM), and Pathogen Mitigation Architecture (Technical Report No. HL-TR-2026-AIR241). HVACLogic Open-Access Building Science. https://hvaclogic.org/research/ashrae-241-equivalent-clean-airflow

BibTeX Entry:

@techreport{hvaclogic_2026_ashrae_241_clean_airflow,
  author = {{HVACLogic Research Group} and S., Miad},
  title = {ANSI/ASHRAE Standard 241-2023: Equivalent Clean Airflow (ECA), Infection Risk Management Mode (IRMM), and Pathogen Mitigation Architecture},
  institution = {HVACLogic Open-Access Building Science Monograph Series},
  year = {2026},
  number = {HL-TR-2026-AIR241},
  url = {https://hvaclogic.org/research/ashrae-241-equivalent-clean-airflow}
}