Understanding ASHRAE Standard 241: Control of Infectious Aerosols

ASHRAE, the American Society of Heating, Refrigerating and Air-Conditioning Engineers, is a global professional organization committed to advancing the arts and sciences of heating, ventilation, air conditioning, refrigeration and their allied fields. They have recently introduced a new standard, ASHRAE 241-2023, which focuses on the ‘Control of Infectious Aerosols.’ This new standard is a significant step forward in addressing the transmission of airborne diseases in non-healthcare settings.

The Genesis of ASHRAE 241

The genesis of ASHRAE 241 was during the COVID-19 pandemic, where there was a widespread realization that ventilation rates in non-healthcare settings were not designed to mitigate airborne disease transmission. The pandemic highlighted the inadequacies of our buildings’ ventilation systems, and organizations scrambled to upgrade their systems without a clear understanding of ‘how much’ ventilation was needed.

ASHRAE 241 was developed in response to this gap. It introduces targets for equivalent clean airflow rates, which buildings were not originally designed to achieve. These rates go above and beyond existing indoor air quality ventilation rates and recommendations provided by various organizations during the pandemic.

Equivalent Clean Airflow Rates

One of the key features of ASHRAE 241 is the introduction of equivalent clean airflow rates. These rates are provided for different occupancy categories to control infectious aerosols. The standard also offers a methodology to calculate these values.

Before ASHRAE 241, ASHRAE 62.1, the standard for Ventilation and Acceptable Indoor Air Quality, was used as a basis for determining ventilation rates. However, ASHRAE 241’s rates significantly increase from current ventilation requirements, indicating that the existing standards were not sufficient for airborne disease mitigation.

Occupancy Category	241 Equivalent Clean Airflow (lps/person)	Calculated Equivalent Air Changes per Hour	Calculated Equivalent CO2 (ppm)	62.1 Outdoor Air Ventilation Rate (Ips/person)
Correctional Cell	15	5	710	4.9
Correctional Dayroom	20	8	660	3.5
Restaurant	30	28	600	5.1
Cafeteria	30	40	600	4.7
Gym	40	3.7	770	22.9
Office	15	1	790	8.5
Call Center	15	12	790	3.5
Retail	20	4	850	7.8
Transportation Waiting	30	40	600	4.1
Daycare	20	6.7	620	8.6
Elementary School	20	6.7	600	7.4
High School	20	9.3	660	6.7
Lecture Hall	25	50	620	4
Manufacturing	25	2.3	770	17.9
Sorting, packaging, light assembly	10	0.9	1300	12.4
Warehouse	10	0.1	1300	35
Health Care Exam Room	20	5.3	700	–
Health Care Group Treatment Area	35	9.3	580	–
Health Care Patient Room	35	9.3	550	–
Health Care Resident Room	25	5.3	600	–
Health Care Waiting Room	45	30	540	–
Auditorium	25	50	620	2.7
Place of Religious Worship	25	40	620	2.8
Museum	30	16	700	4.6
Convention	30	60	600	2.8
Spectator Area	25	50	640	4
Lobbies	25	50	760	2.7
Residential Common Space	25	0.7-5	620	–
Residential Dwelling Unit	15	0.4-3	710	–

Benefits of ASHRAE 241

The primary benefit of ASHRAE 241 is that it provides a more appropriate and evidence-based approach to controlling infectious aerosols in non-healthcare settings. By focusing on equivalent clean airflow rates, it offers a more accurate metric for ventilation than existing standards.

ASHRAE 241 is not just a set of guidelines, but a proper standard applicable to all relevant spaces instead of general guidance. It takes into account the occupant density and aerosol generation of different spaces, offering a more nuanced approach to ventilation requirements.

For instance, the standard recognizes that a high-risk space like an exercise class would require a higher ventilation rate than a lower risk space like a library. It also highlights the importance of occupant density in determining the appropriate airflow rate, addressing the limitations of the Air Changes per Hour (ACH) metric, which does not consider room occupancy or density.

Personal Thoughts

I still feel there is confusion even among experts when we talk about the 241 standard (Control of Infectious Aerosols) and Indoor Air Quality (IAQ). There are two different things and we shouldn’t confuse them. IAQ shouldn’t be associated with infectious control.

Moreover, the ASHRAE Standard 241 states that UV-C densification and similar germicidal technologies are allowed as long as they don’t create by-products like ozone (O3), but although these technologies may kill some pathogens they won’t remove aerosols (aka PM1, PM2.5, etc) from the air unless we use a media like HEPA filters. Such solutions will not improve indoor air quality from a pollution standpoint but from an infection perspective and it may create public confusion as people will assume that they are safe from wildfire smoke or similar situations just because the HVAC system has a UV-C lamp inside.

Conclusion

ASHRAE 241 represents a significant step forward in our understanding and control of infectious aerosols in indoor environments. It recognizes the gaps in our current ventilation standards and provides a more nuanced, evidence-based approach to controlling airborne disease transmission.

The discrepancies between the current indoor air quality standard (ASHRAE 62.1) and the control of infectious aerosols (ASHRAE 241) highlight the necessary changes required to provide safer and healthier indoor environments. By adopting ASHRAE 241, building owners and operators can make their spaces safer for occupants and contribute to the broader fight against airborne diseases.

While the introduction of ASHRAE 241 is a significant step forward, it is just the beginning. It is now up to building owners, operators, and the wider industry to adopt these new standards and contribute to creating safer indoor environments for all.