Analysis of Indoor Airborne Infectious Disease Transmission Risk Based on Current Design Standards
Open Access
- Author:
- Fortner, Kyle
- Graduate Program:
- Architectural Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- July 29, 2024
- Committee Members:
- William P Bahnfleth, Thesis Advisor/Co-Advisor
James Freihaut, Committee Member
Julian Wang, Professor in Charge/Director of Graduate Studies
Greg Pavlak, Committee Member - Keywords:
- Indoor Air Quality
Infectious Aerosols
Risk Management
Equivalent Clean Airflow - Abstract:
- On January 30th, 2020, the World Health Organization (WHO) declared the coronavirus disease 2019 (COVID-19) a Public Health Emergency of International Concern (PHEIC), prompting an international response to curb its spread to minimize human suffering. COVID-19, caused by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), resulted in 766 million reported cases and more than seven million deaths worldwide as of May 10th, 2023 (World Health Organization, 2020). Ventilation of buildings is the most established engineering control to prevent the spread of airborne diseases, thus the pandemic quickly highlighted a critical need to reevaluate the way existing indoor air quality (IAQ) standards define “acceptable air quality” with respect to controlling indoor airborne infectious disease transmission; legacy indoor air quality standards’ minimum ventilation rates were never intended to control infectious airborne disease transmission. It is generally accepted that established minimum standards insufficiently protect occupants from infectious aerosols and improvements are needed. One of the positive outcomes from the pandemic was the decoupling of clean air and outdoor air, resulting in a greater appreciation for the concept of equivalent clean air. Equivalent clean airflow (ECA) is quickly becoming the standard for controlling infectious aerosols. ASHRAE recently debuted a groundbreaking standard, ASHRAE Standard 241-2023, to establish minimum ECA per person, known as ECAi, with the sole purpose to control infectious aerosols. The standard is intended to be implemented during times of increased risk, and it is not a proposed change to the minimum standards. No consensus has been reached yet regarding what specific changes are necessary to the minimum IAQ standards. Two of the three IAQ standards, ASHRAE 62.1, and 170, specify ventilation rates for more than 291 unique occupancies. It would be useful to establish an estimated baseline risk of the current standards prior to determining what, if any, changes should be made. The goals of this research were to investigate the evolution of IAQ standards, develop a model to estimate the baseline long-range transmission risk of indoor infectious diseases posed by existing standards, and to evaluate the relative benefits of emerging standards such as ASHRAE 241. The estimated equivalent clean airflow per person for occupancies covered by ASHRAE 62.1 ranged from 7 to 107 cfm per person, with an average of 30 cfm per person. The equivalent clean airflow range for ASHRAE 62.2, the residential occupancy standard, was 11 to 79 cfm per person for a representative sample of hypothetical dwelling units. For ASHRAE 170, the healthcare ventilation standard, the average equivalent clean air changes per hour was 6.1, ranging from 1 to 19.5 hr-1. Ranges provided are due primarily due to variation of space type, but are influenced by a multitude of other factors to include HVAC system type, filter efficiency, climate zone, etc. Once equivalent clean airflow rates were estimated for all spaces, a subset of spaces was run through a Monte Carlo risk analysis simulation using the Wells-Riley equation to model the most likely infection probability for the most current ASHRAE ventilation design standards. Among the modeled spaces, the space with the highest median infection probability was 0.369 for a booking/waiting room in a correctional facility. The highest level of risk, as measured by median expected cases, was 48.05 for a spectator area. While these simulation results are limited by the data fidelity and assumptions used, they provide a comprehensive assessment of the level of protection existing buildings provide if they meet the minimum ventilation requirements that does not exist elsewhere in the literature. The tool developed to establish baseline risk of existing standards has potential for additional applications that require modeling the risk of infectious aerosol as part of a larger problem set, such as pairing with an energy or economic model to evaluate the energy or cost effectiveness of different engineering controls.