What is ASHRAE Standard 241 Control of Infectious Aerosols?
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ASHRAE Standard 241 (also known as ASHAE 241p, ASHRAE Standard 241p, and ASHRAE indoor air quality standards 241-2023) is a framework for increasing occupant wellness in indoor spaces by minimizing the risk of human airborne transmission (infectious aerosols). This framework consists of guidelines, documentation, examples, and appendices that give mandates and guidance to municipalities who will transfer this framework into local and state building codes.
ASHRAE is the most authoritative international body on how buildings need to be built and how they should operate. The ASHRAE framework is used in over 123 countries.
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What makes ASHRAE Standard 241 a big deal?
ASHRAE Standard 241 is the first standard that aims to reduce the risk of viral transmission in all buildings.
ASHRAE Sandard 241 (also known as ASHRAE 241 or ASHRAE 241p) guidelines impact the indoor air environments of all buildings (new buildings, existing buildings, and major renovations), aimed to safeguard against pathogens such as COVID-19, Influenza, and more.
ASHRAE 241 includes requirements for ventilation, filtration, and air-cleaning system design, installation, commission, operation, documentation, and maintenance of all ventilation types and approved air cleaning methods.
ASHRAE 241 differs from previously published standards because its adherence requires onsite independent verification by approved Authority Having Jurisdiction (AHJ) to test, verify and measure system operations, functional performance testing to evaluate effectiveness.
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How does ASHRAE Standard 241 define control of infectious aerosols?
The first step in control is to define what you are controlling, and how to measure the outcome of control. This is included in the framework of ASHRAE Standard 241 as a building block for greater indoor health.
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ASHRAE 241 defines what clean air is, and how to use filtration and air-cleaning technology to achieve clean air and limit transmission across sets of diverse industries. It also outlines how to reduce risk when harmful aerosols are present and proactively creates and maintains healthy indoor environments across all indoor spaces.
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ASHRAE Standard 241 introduces Building Readiness Plans (BRP) and Infection Risk Management Mode (IRMM) documentation to increase resilience with validated plans and processes ready to address heightened transmission risk.
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What does ECA stand for, and why does it matter?
Standard 241 breaks new ground by defining and setting requirements for Equivalent Clean Airflow rate (ECA): equivalent clean airflow for infection risk mitigation per hour. When testing ECA, it is measuring how quickly non-clean air is replaced by clean air.
The use of equivalent in this term denotes not only measuring outdoor air as the source of clean air, but also factoring in filtered or disinfected air. A similar term that has been used in the past is equivalent or effective air changes per hour (eACH or ACHe).
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Digital measurement and analysis of ECA is a superior method because it measures the outcome you are looking to achieve, not the expectations of spreadsheets and spec sheets. With digital testing sensors measure and report on the diffusion of actual air particles in the room. Providing superior results and insights into where you may be wasting energy.
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What does ECAi mean?
ECAi is defined as, equivalent clean air per individual, it is expressed by the number of air changes per occupant required in units of flow (Cubic feet per min or Liters per min) within a space. The ECAi number indicates the number of occupants that room is safe to occupy the room with minimal transmission risk.
This metric considers both occupancy levels and air changes needed to mitigate risk. Clean air can be provided by outdoor air, as well as by filtered recirculated air and air disinfected, this term is denoted as ECAi. ECAi is used in IRMM to measure risk level for individuals, and keep risk within approved measures - even when risk is high.
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ECA and ECAi must be functionally tested by independently parties to validate industry applicable IRMM levels have been functionally reached and documented, as outlined in Standard 241.
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If control of infectious aerosols is defined by clean air rates, how is it measured in ASRHAE 241?
ASHRAE Standard 241 breaks new ground by defining and setting requirements for Equivalent Clean Airflow rate (ECA): equivalent clean airflow for infection risk mitigation per hour. When testing ECA, it is measuring how quickly non clean air is replaced by clean air.
The use of equivalent in this term denotes not only measuring outdoor air as the source of clean air, but also factoring in filtered or disinfected air. A similar term that has been used in the past is equivalent or effective air changes per hour (eACH or ACHe).
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A few methods of approved testing are outlined in the standard and appendixes, some are more lengthy than others and some may rely on recent data to validate air changes. While traditional methods like TAB reports can be used, new digital technology outlined in Appendix C provides a much richer and less costly data in one day, versus 6-8 weeks for the traditional TAB approach.
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What is in an ASHRAE Standard 241 Building Readiness Plan, and where do I get one?
A Building Readiness Plan (BRP), is documentation of procedures, ventilation system operating schedules and values, air cleaning technologies, filtration levels, and any changes made to the system that helps owners/operators proactively prepare for moments of higher risk.
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For new buildings BRP is created with design, CxP and project team, it is included as an appendix to the systems manual, Current Facility Requirements, and O&M manual.
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Within the published ASHRAE Standard (www.ashrae.org/241) you will find a BRP template to use, if you need one.
Part of the BRP will ask you for baseline measures for air flow, measured in ECA. If you are doing digital testing these results will likely be provided in this BRP format.
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What is IRMM (Infection Response Management Mode)?
The Infection Response Management Mode (IRMM) aspect of Standard 241 introduces the concept of resilience—the ability to respond to extreme circumstances outside normal conditions—into the realm of indoor air quality control design and operation.
IRMM outlines a proactive plan to keep indoor air safe when transmission risk for airborne infections are high. ASRHAE Standard 241 provides specific guidance on airflow and clean air rates needed in IRMM situation based on the industry, level of activity, and presence of higher risk populations, or locations.
IRMM documentation is included in BRP, and must clearly outline how to enable IRMM and testing results to validate rates, it is triggered by Authority Having Jurisdiction (AHJ) or by building or workplace owners.
IRMM and normal mode ECA measures - gives you a baseline and insight into the level of safety and energy consumption for each mode. Systems used in IRMM mode that are not active in normal mode will need to be functionally revalidated by an independent source every six months.
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IRMM can have engineering and non-engineering adjustments to meet standard.
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What do you do if your air doesn't meet ASHRAE Standard 241?
It is likely you will have areas that need to be optimized (this is normal). For most people this is the very first look at the data, as this measure is new with ASHRAE Standard 241 - and the measurement methods now leverage innovation honed over the last 5 years.
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You may find your elevators are badly ventilated, you may find over-ventilation in certain rooms, while the room next door is under-ventilated. The beauty of digital testing is the real-time results (via sensor measurements and ML algorithms that crunch the data). This real-time system allows you to optimize, adjust, and balance your air to look and feel the way you want.
Digital testing for ASHRAE Standard 241 puts you in control, and keeps you there.
Interested in learning more about digital testing for 241?
What is ASHRAE acceptable air quality?
Acceptable air quality is a multi faceted question that varies based on the type of building, function, and state of infection risk. Healthcare and Labs had min requirements prior to 241 but for general commercial and multi residential this is new.
ASHRAE Standard 241 (also known as ASHRAE 241), clearly defines needs for IRMM (Infection Risk Management Mode) for commercial and multi resident buildings with Standard 241 documentation for infectious aerosols, other air quality measures are found in Standard 90.1.
The "normal mode" as define by ASHRAE 241 (used for everyday function) is more general - to gain general understanding of acceptable air quality you can look to the CDC, which uniformly sets a minimum of 5 eACH (equivalent air changes per hour) for all buildings across the US.
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Here is a great podcast resource for ASHRAE specific guidelines.
What is the current ASHRAE standard?
There are many standards produced by ASHRAE for different purposes, objectives, and functional areas. ASHRAE standards evolve and adjust over time to meet the ongoing needs of buildings and building occupants. ​
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ASHRAE does not create building code, they (as an organization) create standards that can be transferred to building codes, but that is decided upon by the jurisdiction and can be edited locally when incorperated into building code.
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ASHRAE standards are used in 123 countries and serves as a major thought leader and innovator in building operations, processes, maintenance, and safety.
The list of current standards is too large to list but you can find the list of current standards here, and also if you are looking for proposals and drafts for public comment look here.
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