Introduction to ASHRAE Standard 241: Control of Infectious Aerosols

Precipitated by the SARS-CoV-2 pandemic, the world quickly realized that we didn’t know how much disease transmission mitigation was enough. To answer this question and at the request of the White House, The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) developed Standard 241-2023, Control of Infectious Aerosols. The primary intent of this standard is to provide building owners and operators with procedural guidance for responding to periods of increased infection risk such as during flu season and pandemics. This guidance outlines how to develop a Building Readiness Plan (BRP) which documents the engineering and non-engineering controls that the facility systems will use when they decide to enter Infection Risk Management Mode (IRMM) to achieve their infection mitigation goals.

While it is widely known ASHRAE Standard 62.1, Ventilation and Acceptable Indoor Air Quality and ASHRAE Standard 170, Ventilation of Health Care Facilities dictate ventilation airflow rates and filter efficiencies as code minimums, these airflow rates are too low to address infection risk and don’t address other types of engineering controls. Created as a complementary standard to those previously mentioned, ASHRAE Standard 241 introduced a new metric, Equivalent Clean Airflow (ECA), to quantify how much mitigation is enough. ECA is defined as “the theoretical flow rate of pathogen-free air that, if distributed uniformly within the breathing zone, would have the same effect on infectious aerosol concentration as the sum of actual outdoor airflow, filtered airflow, and inactivation of infectious aerosols.” This value in Cubic Feet per Minute (CFM) is used to compare the target airflow for the space, zone, or system to what the building systems can achieve and is meant to quantify the equivalent effectiveness of using multiple engineering and non-engineering controls to reduce contamination risk from an infectious aerosol. These controls can include reducing the amount of people allowed in a space, the quantity of ventilation air introduced, filter efficiency, ductwork-mounted UV lights, upper-room UV lights, in-room portable UV lights, and in-room fan filter air cleaners. ASHRAE does not recommend or rank any of the engineering control technologies in the Standard but acknowledges that each is effective and contributes toward reducing risk. As such, the Standard allows for the inclusion of new technologies as they come to market.

The Standard authors have taken the stance that no single engineering control will work in every situation, and in most cases, multiple controls will need to be in place to achieve the target value ECA. To simplify the implementation, they created a spreadsheet called the Equivalent Clean Air Calculator (ECAC),  which is open to public access through ASHRAE’s website. The ECAC spreadsheet has a column for the current parameters and the target ECA when the building is in Infection Risk Management Mode. Subsequent columns allow users to create combinations of various controls. These options can then be compared to facilitate the selection of the optimum solution for achieving the ECA target.  

If you would like to learn more about this innovative Standard, determine your current equivalent clean air value, or incorporate a Building Readiness Plan into your new, existing, or renovated facility, please contact Doug C. Barnhart, PE at or 717-845-7654 or Kyle J. Courtney, PE at or 814-237-2180.

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