Introduction to ASHRAE Standard 241: Preparing for the New Normal

Recently, the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) released Standard 241-2023 -- Control of Infectious Aerosols, proposing minimum requirements to reduce the risk of disease transmission indoors. Our previous executive summaries — focusing on core concepts inside the standard — were well-received by our clients and partners. Desiring to always remain at the forefront of client assistance, we now present an introduction to Standard 241 itself for the busy building owner.

What Is ASHRAE Standard 241?

Standard 241 comprehensively defines itself in its first section:

“The purpose of this standard is to establish minimum requirements for control of infectious aerosols to reduce risk of disease transmission in the occupiable space in new buildings, existing buildings, and major [...] renovations to existing buildings, including requirements for both outdoor air system and air cleaning system design, installation, commissioning, operation, and maintenance.”

Or, to put it quite simply, Standard 241 was created to help indoor spaces reduce the risk of infection from airborne diseases. Everything about it serves this straightforward goal.

How Does Standard 241 Affect Building Owners and Managers?

The primary responsibility of building owners and managers is to educate themselves on Standard 241. As part of this education, you have some new vocabulary and abbreviations to learn, such as:

• Authority Having Jurisdiction (AHJ): “the agency or agent responsible for determining compliance with this standard.” This is often the building owner or manager.
• Building Readiness Plan (BRP): “a plan that documents the engineering and nonengineering controls that the facility systems will use for the facility to achieve its goals.”
• Equivalent Clean Airflow: “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.” In other words, Equivalent Clean Airflow is the ideal toward's which your combined ventilation, filtration, and air cleaning efforts strive.
• Infection Risk Management Mode (IRMM): “the mode of operation in which measures to reduce infectious aerosol exposure documented in a building readiness plan are active.”
• Long-Range Transmission: “disease transmission that is due to aerosols emitted by an infector who is not in close proximity to (within approximately 3 ft [1 m] of) a susceptible occupant.”

Since building managers/owners are often the AHJs for their buildings, they are also responsible for spearheading the development of a BRP and for defining IRMM parameters. This will take time and teamwork, but — once such policies are in place — their clear documentation should help things run quite smoothly. We’ve expounded upon both these concepts for building managers in previous blogs.

What Is Standard 241 Not About?

Standard 241 is neither a replacement for previous ASHRAE guidelines nor is it a broad-reaching attempt to define indoor air quality in general. In fact, compliance with a previous air quality standard (62.1, 62.2, 170, “or equivalent”) is a prerequisite for Standard 241. However, the AHJ has a great deal of flexibility when determining “equivalent” norms.

Also, the standard only deals with long-range disease transmission from infectious aerosols. It neither aims to reduce the risk of transmission from close contact nor from diseases that are not transmitted through the air. This caveat, however, is also a revolutionary response to how COVID-19 changed our understanding of disease transmission…

Why Was Standard 241 Needed?

A modification of air quality guidance was needed for 3 important reasons:

1. Due to an extremely unfortunate misinterpretation of mid-20th-century research, most pathologists believed that airborne diseases spread exclusively through close contact. However, during the pandemic, it became apparent that indoor spaces could also be a method of long-range disease transmission. Standard 241 — focusing on long-range aerosols — is a response to this corrected understanding.
2. Per ASHRAE’s foreword to the document, “Explicit requirements for airborne infection risk management have been absent for a century from indoor air quality (IAQ) standards with the exception of those written for health care facilities and laboratories.” Instead, efforts at improving air quality focused mostly on chemical and particle contaminants. Standard 241 aims to correct this oversight.
3. Previously, both the government and ASHRAE only acknowledged filter-based and ultraviolet-based air cleaning methods. However, additional air purification technologies have been advancing and proving themselves in the field for decades. Standard 241 provides guidance for calculating the cleaning contribution of active purification methods (such as ActivePure) without endorsing any single technology over the others.

How Does Standard 241 Affect Air Purification Distributors?

Standard 241 acknowledges many ideas that ActivePure has championed for years, such as:

• Air quality requires a combination of ventilation, filtration, and air cleaning
• There are multiple proven air-cleaning technologies besides fibrous filtration
• Aerosols can be a vector of long-range disease transmission

Standard 241 also confirms that the universal surrogate for air purification testing is Bacteriophage MS2. A bacteriophage is a virus that infects bacteria; (yes bacteria can become sick). MS2 is a small, non-enveloped virus that targets E. coli bacteria, making it safer for testing than human viruses. Because small non-enveloped viruses are more difficult to neutralize than enveloped viruses (such as SARS-CoV-2), any air purification method that inactivates MS2 has a high chance of inactivating multiple types of pathogens.