Understanding ASHRAE Standard 241: Control of Infectious Aerosols

As the world adapts to the ongoing risk of airborne disease transmission in indoor spaces, enhancing indoor air quality (IAQ) has become a top priority for building operators and HVAC professionals. To address this, the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) developed Standard 241, offering clear guidelines for mitigating the spread of infectious aerosols in buildings through effective ventilation, filtration, and air cleaning strategies.

This article explores the key components of ASHRAE Standard 241 and its role in minimizing the spread of airborne pathogens in indoor environments.

What is ASHRAE Standard 241?

ASHRAE Standard 241: Control of Infectious Aerosols sets minimum requirements for reducing the transmission of infectious aerosols, which are tiny particles that can carry viruses, bacteria, and other pathogens through the air. This standard provides guidance to building owners, operators, and HVAC professionals to improve IAQ, prevent disease transmission, and protect building occupants.

The standard applies to various building types, including offices, schools, healthcare facilities, and residential spaces, and outlines specific requirements for ventilation, air filtration, and air cleaning technologies.

Key Components of ASHRAE Standard 241

Equivalent Clean Airflow (ECA-i)

Equivalent Clean Airflow (ECA) is a core concept in ASHRAE 241, quantifying the overall effectiveness of air cleaning methods in removing airborne contaminants, especially infectious aerosols. The ECA measures the combined impact of:

a.????? Clean outdoor air supplied through ventilation.

b.????? Cleaned indoor air processed by filtration or air cleaning technologies.

If increasing outdoor air ventilation is not feasible due to energy concerns or climate control issues, ASHRAE 241 allows for the use of filtration and air cleaning technologies (such as bipolar ionization or UVGI) to achieve the required ECA, ensuring that indoor air quality remains uncompromised while maintaining energy efficiency.

Filtration Requirements

Beginning in January 2025, ASHRAE Standard 241 introduces stricter requirements for air filtration, mandating the use of MERV-A rated filters for more effective control of infectious aerosols. Unlike standard MERV-rated filters, MERV-A filters are tested under conditions that simulate real-world use, ensuring consistent performance over time. This shift aims to enhance the reliability of filtration systems in removing harmful airborne particles, such as viruses and bacteria.

Air Cleaning Technologies

ASHRAE 241 also recognizes the importance of air cleaning technologies as a supplement to ventilation and filtration systems. These technologies help neutralize or remove pathogens from the air and include:

Bipolar Ionization: This technology emits ions that attach to particles, causing them to clump together and either fall out of the air or be more easily captured by filters.

Portable Air Cleaners: Often equipped with HEPA filters, these devices can be placed in specific areas to provide additional localized air cleaning.

Ultraviolet Germicidal Irradiation (UVGI): UVGI systems use UV-C light to deactivate microorganisms, making them unable to replicate or cause infection.

Safety Testing for Air Cleaning Technologies

While technologies like UVGI and bipolar ionization are effective at neutralizing pathogens, they can produce harmful byproducts like ozone if not properly tested. To prevent unintended negative impacts on IAQ, ASHRAE 241 mandates safety testing for all air cleaning devices. For example, products must be certified with UL 2998 to ensure ozone safety. Further details on safety testing are provided in Table A-1 of ASHRAE Standard 241 (2023).

Building Commissioning and Operation

For an air quality system to function effectively, it must be designed, installed, and maintained correctly. ASHRAE Standard 241 includes guidelines for building commissioning, ensuring that HVAC systems meet design specifications and perform as intended.

Ongoing maintenance is essential for ensuring that systems continue to function optimally over time. This includes regular filter replacement, monitoring air cleaning devices, and adjusting ventilation rates to match occupancy levels.

Risk Assessment and Building Type Considerations

ASHRAE 241 emphasizes the importance of performing a risk assessment to determine the appropriate ventilation, filtration, and air cleaning strategies for different building types. Not all buildings present the same risk of airborne infection. For example, hospitals and long-term care facilities, where vulnerable populations are present, require more stringent air quality controls than office buildings or residential spaces.

Conclusion

ASHRAE Standard 241 offers a comprehensive framework for controlling infectious aerosols in indoor environments. By setting clear guidelines for ventilation, filtration, and air cleaning technologies, it ensures that building operators can maintain high IAQ standards while mitigating the risk of airborne disease transmission.

The introduction of MERV-A rated filters and the focus on safety testing for air cleaning devices mark significant steps toward improving the overall health and safety of indoor spaces, making the standard a critical tool for protecting occupants in various building types.