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ASHRAE has published a draft of ASHRAE Standard 241 Control of Infectious Aerosols.

It is open for public comment until May 26, 2023.

Publication is expected in July.

Comments on the proposed Standard can be made here


ASHRAE is the a professional society of building mechanical engineers that sets building energy and ventilation standards, among other standards and guidelines.

Most commercial buildings in North America follow ASHRAE standards. Residential buildings tend to follow the IECC standards, which borrow heavily or defer to ASHRAE standards. Many other countries and jurisdictions globally model their codes off of ASHRAE standards and borrow directly from it. I suspect many jurisdictions will adopt Standard 241 as they do other ASHRAE energy and ventilation standards.

The ASHRAE Standard Project Committee 241P is chaired by William Bahnfleth, Ph.D., P.E., whose name should be familiar to those who were following the research on COVID-19 aerosol transfer.

Ventilation standards have always considered the health of occupants, but with a focus on chronic air pollutants, not exposure to airborne infectious diseases. This a new standard aimed to given guidance to buildings specifically with airborne infectious disease transfer in mind.

ASHRAE Standard 241 makes several improvements beyond outdoor air ventilation:

  • effective ventilation rates are roughly double for key spaces like offices and classrooms
  • allows air cleaners to contribute to the ventilation requirement
  • sets minimum requirements for maintenance and filter replacement, which has been perhaps the main challenge to mitigation systems

I suspect many buildings will comply with the standard with permanent or semi-permanent air cleaners, rather than increasing the outdoor air ventilation rate.

Increasing the outdoor air ventilation rate is costly, as heating and cooling outdoor air is the primary contributor to heating and cooling loads in commercial buildings (see this report by my colleagues).

What does this mean for those working on building-level infectious disease transfer mitigation?

I doubt EAs can contribute to the science of air cleaning devices or research around infectious disease transfer as it relates to the standard unless they've been working directly on that for some time in an academic or industry capacity. EAs have tended to bring optimism but show a significant lack of expertise, as evidenced by the misplaced fervor around UV-C as a mitigation technology.

There are several things that EAs can help with:

  • Ensure ASHRAE Standard 241 gets adopted by jurisdictions. This could look like writing to your city, county, or state code office and requesting that new or existing buildings to follow Standard 241.
  • Help schools apply for grants to comply with Standard 241 There are billions of dollars available for schools, colleges, and universities to improve their buildings to reduce airborne infectious disease transfer. However, most schools lack the staff or expertise to apply for funding and carry out these projects. Unfortunately, there are a lot of hucksters looking to take advantage of the funds and sell systems that simply don't work, or don't adequately reduce airborne transfer compared to the best mitigation options.





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Hey Matthew, thanks for sharing this. Can you provide some more information (or link to your thoughts elsewhere) on why fervor around UV-C is misplaced? As you know, ASHRAE Standards 185.1 and 185.2 concern testing of UV devices for germicidal irradiation, so I'd be particularly interested to know if this was an area that ASHRAE itself had concluded was unpromising.

Here's a related comment from last year: https://forum.effectivealtruism.org/posts/nopFhTtoiyGX8Bs7G/uvc-air-purifier-design-and-testing-strategy?commentId=ZywtAzPB2Ci5PLfPC

UV systems have been around for ~100 years. They work great in some specific applications. Newer UV-C technology is a marginal improvement, but doesn't significantly address the cost, design expertise, and maintenance challenges that have kept UV systems from widespread use. Air filters are generally better for most applications. I do expect we will see more UV-C systems in particular applications, but it is far from the one-technology-to-rule-them-all solution that the EA community seems to think it is. This follows a historic pattern of the EA community generally over-hyping singular technology solutions to major problems in other cause areas, probably because of the techno-optimist worldview that many EAs have.

See ASHRAE's guidance on filtration and air cleaning technologies for more details and comparisons: https://www.ashrae.org/technical-resources/filtration-disinfection

Could you elaborate on what you consider to be the specific marginal improvements of newer UV-C technologies?

Possibly cheaper, and less ozone production.

Hi Matt - Have you had any experience with testing based on the ECA or ECAi metrics outlined in 241? I would prefer not to do a full TAB, but looking at solutions like BreatheScore, I think this meets the Appredix C option in the standard. Anyone have suggestions?

Woudl also love to hear any feedback if anyone has used https://www.poppy.com/measure-certify-air-health-cre-real-estate-building 

Your help is much appreciated:)


I suggest you consult with a licensed professional mechanical engineer who specializes in air quality testing. I can't make any recommendations without knowing the specifics of the system and the building occupancy, which a consultant can analyze.

ok, thanks. Any general experience you can share about measuring to 241 standards would be greatly appreciated. 

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