ASHRAE Standard 241: Control of Infectious Aerosols – Understanding its impact for the Cleaning Industry

ASHRAE Standard 241: Control of Infectious Aerosols – Understanding its impact for the Cleaning Industry

An Article by Dr Gráinne Cunniffe, Principal Investigator, iAIR Institute


Introduction to ASHRAE Standard 241

ASHRAE 241ASHRAE, the American Society of Heating, Refrigerating, and Air-Conditioning Engineers, is a worldwide professional organization dedicated to advancing the fields of heating, ventilation, air conditioning, refrigeration, and related disciplines. ASHRAE, commissioned by the White House, and building upon the work of the ASHRAE Epidemic Task Force, has recently unveiled a ground-breaking standard, ASHRAE 241, targeting the “Control of Infectious Aerosols” in new buildings, existing buildings, and major renovations. Representing a substantial leap forward, this new standard addresses the transmission of airborne diseases, essentially providing guidance on how to operate buildings in a way to help prevent the spread of airborne infectious aerosols in various indoor spaces. While the standard does not have regulatory powers in itself, ASHRAE standards are often referenced in local building codes, and professionals involved in building and maintaining built environments typically follow voluntary standards for best-in-class in indoor air quality, and to incorporate resilience into building management systems so enhanced performance can be activated when required.


Importance of Indoor Air Quality (IAQ)

The significance of indoor air quality (IAQ) has continued to gain recognition, due to its established impact on the health and well-being of occupants in various built environments. Airborne and surface viruses and bacteria, along with pollutants, allergens and molds, can pose both short-term and long-term health risks to occupants. Infectious aerosols, which are microscopic particles exhaled by individuals, can carry disease-causing pathogens. These particles are so small they can linger in the air for extended periods and pose a risk when inhaled. Facilities management professionals and the cleaning industry in general constantly strive to incorporate new methods, products and technologies to create healthier indoor environments. An understanding of the fundamentals of ASHRAE 241 and its implementation will contribute towards these efforts.


Key Features of ASHRAE 241

ASHRAE 241 establishes minimum requirements to reduce the risk of disease transmission through exposure to infectious aerosols. A key feature of the standard is that it introduces targets for “equivalent clean airflow rates” which go beyond existing IAQ ventilation rates set by its predecessor, ASHRAE 62.1, which served as the standard for Ventilation and Acceptable Indoor Air Quality. This aspect considers that pathogen-free airflow can be provided to occupied regions of buildings either through ventilation (i.e., outdoor air) or air purification within the indoor environment through the application of filtration and air cleaning technologies. This approach is designed to regulate the presence of infectious aerosols across various occupancy categories, e.g. school, office, healthcare waiting room. This innovative standard not only establishes these rates but also provides a methodology for their calculation facilitating an evidence-based strategy for managing infectious aerosols within indoor environments.


Infection Risk Management Mode (IRMM)

 The standard considers the varying occupant density, activity levels and aerosol generation inherent in different environments, offering a nuanced and tailored approach to ventilation requirements. The standard introduces the concept of Infection Risk Management Mode (IRMM), requiring higher levels of equivalent clean airflow during periods of heightened disease transmission risk. The standard provides target values for different indoor spaces, for example, it calls for a higher ventilation rate in high-risk spaces such as gyms and healthcare waiting rooms compared to lower-risk environments like warehouses and residential homes. It’s important to remember that AHJs (Authorities Having Jurisdiction), such as health departments, determine when to implement this infectious aerosol standard. ASHRAE 241 can be applied at the discretion of a building’s management during periods of high viral loads, such as influenza or “flu” season. By adhering to the standard’s guidelines, building owners contribute to the creation of healthier environments, significantly reducing the concentration of infectious aerosols.


Air Cleaning Technologies


Importantly, the standard recognizes the positive impact air cleaning technologies can have on our indoor spaces. The standard encourages the utilization of independently tested and certified air-cleaning solutions, adding a layer of assurance regarding their efficacy and safety. The appendices within the standard include test methodologies and requirements which consider both the performance and safety of these technologies. For example, ultraviolet light (UV-C) and germicidal based air purifiers must demonstrate function and safety, including the evaluation of potentially harmful by-products like ozone (O3). Certified air-cleaning solutions bring an additional layer of assurance, providing professionals in the cleaning industry and facilities management with confidence in their efficacy in effectively cleaning the air. “The requirements for filter and air cleaner testing incorporated in this standard go well beyond what is found in current standards,” the authors wrote in the foreword. “They are a major step in the direction of creating uniform and effective technology-agnostic criteria for characterizing filter and air-cleaner performance and safety.”

It is important to note that while reducing the presence of airborne infectious pathogens certainly helps improve IAQ, additional steps such as the use of a HEPA or media-based filter and effective cleaning using certified products are still required to actively remove both the inactivated pathogens and other pollutants such as particulate matter (e.g.,  PM1, PM2.5) and allergens (e.g. pet dander, dust mite allergen) from surfaces and the breathing zone.


Building Readiness Plan (BRP)

ASHRAE 241 provides guidance on planning, commissioning, and maintenance, ensuring that buildings are prepared for periods of elevated risk. The standard also refers to a Building Readiness Plan (BRP), which should be created after the assessment, planning, and implementation phases to describe both the engineering and non-engineering controls that the facility’s systems will use to achieve its specific target equivalent clean airflow for infection control. Of particular interest to the cleaning industry, ASHRAE 241 contains a table which lays out “minimum maintenance activity and frequency for ventilation system equipment and associated components” with recommended frequency for cleaning, monitoring or replacing parts related to air cleaning.

The publication of ASHRAE 241 provides a complete framework for planning, design, operation, and maintenance of systems that reduce risk of airborne infection transmission. In the ever-evolving landscape of building standards and IAQ considerations, ASHRAE Standard 241 provides a major step forward, particularly through the introduction of the infection risk management mode which enables rapid responses to extreme circumstances, and through establishing the “equivalent clean air flow rate” approach which incorporates the contribution of clean air provided through effective and safe air purification technologies. For cleaning professionals and facilities management experts, embracing this standard alongside employing certified cleaning solutions acts as a significant commitment towards protecting public health and fostering healthier indoor environments.


About the Author

Dr. Gráinne Cunniffe

Gráinne Cunniffe

Gráinne graduated from Trinity College Dublin with a first class hons degree in Experimental Physics, before obtaining a Ph.D. in Regenerative Medicine from Queen’s University Belfast. Her postdoctoral research in Royal College of Surgeons focused on bone tissue engineering and gene therapy, before she returned to Trinity College where she managed the biomedical labs in addition to leading a suite of advanced biomaterials and translational pre-clinical research studies. Her research career has focused on purpose-driven and influential science with a health focus, through leadership roles across industry, academia and clinical research.Gráinne has extended her passion for life-long learning and development with a number of qualifications, including Diplomas in Creativity, Innovation and Entrepreneurship from the UCD Innovation Academy, Lean Six Sigma, Statistics, Design Thinking and Continuous Improvement. 

Key Words

ASHRAE 241, infectious aerosols, facilities management, cleaning industry, indoor air quality solutions, air quality, asthma, health, allergies, allergens, healthy home, third party certification, standards, indoor environment, asthma & allergy friendly® Certification Program, Allergy Standards, healthier indoor environment, ventilation, HVAC, air cleaners, air cleaning technology


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