March 17, 2022 / Abby Roberts

Controlling Airborne COVID-19 Transmission

This post is based on a presentation given at AIHce EXP 2021 by Lisa Brosseau, James Chang, and Tom Smith. It is the fifth in the “Essentials of Pandemic Response” series based on AIHA's recently publishedebook. The mention of specific products or companies does not constitute endorsement by AIHA.

According to Lisa Brosseau, ScD, CIH, the COVID-19 pandemic has revealed the importance of involving OEHS professionals in pandemic response, protecting the health of essential workers, and relying on other workplace controls in addition to the use of face coverings. It has also demonstrated the need to develop controls for aerosolized and airborne virus transmission. For OEHS purposes, aerosol virus transmission occurs through inhalation of a virus-containing particle by an uninfected person within a distance of 6 feet from the source, while airborne transmission occurs via inhalation at distances greater than 6 feet.

Along with James Chang, CIH, and Tom Smith, Brosseau presented the educational session “Key Topics in Addressing the COVID-19 Pandemic, Part 2: Scientific Issues and the Hierarchy of Controls” at AIHce EXP 2021. This session was part of a series that began with “Key Topics in Addressing the COVID-19 Pandemic, Part 1: The Varied International Experience,” covered in the February 22 blog post.

Airborne Transmission of SARS-CoV-2

Prior to CDC’s acknowledgement of the importance of airborne SARS-CoV-2 transmission, Brosseau recognized the significance of viruses’ ability to spread through aerosol droplets. In 2015, Brosseau, as a professor at the University of Illinois Chicago, coauthored an article in the Journal of Occupational and Environmental Medicine, “Aerosol Transmission of Infectious Disease,” which recommends criteria for establishing the possibility that a virus may be transmitted through aerosols. These criteria take into account potential sources of aerosol transmission, the virus’s viability in the environment as it travels to an uninfected person, and the airborne virus’s ability to access receptor tissue. According to Brosseau, data indicating the aerosol and airborne transmissibility of SARS-CoV-2 was available from very early in the pandemic by analogy to similar viruses, such as other human coronaviruses and influenza viruses. For viruses similar to SARS-CoV-2, it had been shown that patients generate virus-laden aerosols through coughing, breathing, and talking. Similar viruses had also been found to remain viable in the air for hours at a time and to access receptor tissue in the respiratory systems of uninfected individuals via inhalation.

As part of ACGIH’s Pandemic Response Task Force, Brosseau, now a part-time consultant with the University of Minnesota Center for Infectious Disease Research and Policy, helped produce several free, publicly available factsheets on workplace COVID-19 prevention. One of these factsheets concisely outlines the mechanism and risk factors for airborne SARS-CoV-2 transmission. Risk factors include enclosed spaces, poor ventilation, higher numbers of infected people within a space, and longer periods of time spent in the space. This factsheet does not count proximity to infected people as a significant risk factor under the reasoning that given enough time spent in a space with an infected person, airborne viral particles will become distributed throughout the space’s air.

Other factsheets cover control banding for the purpose of identifying aerosol infection risk, the use of building ventilation systems to limit concentrations of airborne infectious particles, and workers’ use of respirators, not face coverings, in occupational environments. This factsheet stresses that workers often must remain in a space for enough time that an infectious dose of airborne viral particles can penetrate the barrier of the face covering. ACGIH’s factsheet asserts that a respirator is the only personal protective equipment option for protecting workers from infectious aerosols emitted by coworkers and the public.

Use of Elastomeric Respirators in Healthcare Environments

After Brosseau, Chang spoke on the use of elastomeric respirators for protection against COVID-19. In the late 1990s, global concerns over a possible avian influenza pandemic prompted the creation of pandemic models for PPE consumption and advocated for the establishment of PPE stockpiles. However, stockpiles based on disposable PPE were by necessity very large and created inventory management problems. Chang, the director of safety at the University of Maryland Medical Center (UMMC) since 2006, worked with his colleagues to identify alternative controls to N95 respirators. As engineering controls are not always feasible in a healthcare environment, they evaluated reusable elastomeric respirators for criteria such as fit, comfort, cleanability, and durability. UMMC purchased 1,500 elastomeric respirators of 3M’s 7500 series and kept them in storage in preparation for a possible future pandemic.

The swine flu (H1N1) pandemic arrived in 2008, and the lessons that Chang and his colleagues learned then would prepare them for the onset of the COVID-19 pandemic in 2020. First of all, supply lines were not able to meet the sudden demand for PPE under pandemic circumstances. Hospitals found themselves competing with other hospitals, state governments, and even the federal government for PPE. PPE suppliers allocated respirators based off the number that was ordered in the previous year, which proved inadequate during a sudden pandemic. The Strategic National Stockpile did not take facilities’ preferred brands of respirators into account when distributing PPE, requiring users to perform fit testing every time a new shipment arrived.

Dr. Stella Hines, MD, a pulmonologist with the UMMC, conducted a study on user perceptions of respirators that was published online in early 2019. Dr. Hines’ study found that, while healthcare staff generally preferred disposable N95 respirators due to increased comfort and the convenience of not having to clean a reusable respirator, experienced users nevertheless preferred the reusable respirators in high-risk situations.

By the end of 2019, healthcare workers were beginning to hear of cases of undiagnosed pneumonia and possible airborne respiratory disease occurring in Wuhan, China. Chang’s colleagues at the University of Maryland School of Medicine warned UMMC healthcare staff to take these reports seriously, and UMMC was successfully able to buy additional stocks of reusable respirators in preparation for what would become known as COVID-19. Attempts to purchase additional disposable N95s, however, were not successful.

UMMC prepared for COVID-19 differently than for the 2008 H1N1 pandemic. In 2008, the facility issued each staff member a reusable respirator that they were responsible for taking home to clean and bringing to work to use. In 2020, however, respirators were issued to each staff member at the start of their shift and collected for cleaning at each shift’s end: this strategy effectively doubled UMMC’s respirator supply. This required a vigorous cleaning procedure, and a detailed, stepwise cleaning procedure was developed by UMMC’s sterile processing unit, which would also be responsible for decontaminating N95s for reuse during the oncoming PPE shortages. UMMC’s nursing staff directed a mass fit-testing program. Biocontainment areas were established for cohorting patients with COVID-19, allowing UMMC to further conserve PPE and limit potential staff exposures.

UMMC found that large orders for PPE would be automatically canceled by suppliers but that placing multiple smaller orders for PPE was a more productive approach. The facility also used N95s donated by hardware stores, other companies, and the University of Maryland’s research labs, in addition to drawing from its stockpile of reusable respirators.

Other lessons learned by UMMC during the early days of the COVID-19 pandemic included:

  • the importance of buying additional spare respirator parts as well as respirators
  • the need for UMMC’s centralized PPE distribution center to track returns of reusable respirators in addition to respirator assignments
  • the need to improve fit-test recordkeeping to accommodate an increased number of fit tests being performed
  • the relatively limited use of the N95 decontamination program—even during a PPE shortage, most staff members still preferred to discard N95s after use
  • the need for guidelines for replacing filter cartridges in reusable respirators

User concerns were not assuaged by the traditional criteria for changing elastomeric respirator filter cartridges when increased breathing resistance is detected or when the filter became damaged or contaminated. Consequently, UMMC determined a guideline of replacing cartridges after one year of use.

The difficulty of speaking while wearing an elastomeric respirator was another significant source of dissatisfaction for UMMC staff. Chang suggested that future research should focus on improving the audibility of wearer’s voices through the respirator. He also recommended that elastomeric respirators be introduced to the Strategic National Stockpile.

A Stepwise Process for Improving Building Ventilation

Most commercial building heating, ventilation, and air conditioning (HVAC) systems are not designed to protect people from aerosolized pathogens. Smith is the president and CEO of 3Flow, a firm specializing in laboratory ventilation solutions. After Chang’s presentation, he spoke on various tools that OEHS professionals can use to improve the performance of building ventilation systems in ways that reduce occupants’ risk of airborne SARS-CoV-2 transmission.

Building ventilation can be thought of as divided in two parts: the communal air space located “below the ceiling,” and the air supply and exhaust or return systems “above the ceiling.” Primary SARS-CoV-2 exposure occurs in the communal air space, as opposed to within the inner workings of the HVAC system. While the infectious dose of SARS-CoV-2 is currently not known, the airflow patterns within a room can increase or reduce exposure dose by moving aerosolized particles toward or away from uninfected occupants and diluting the concentration of particles within the space. In a well-mixed room, viral particles may accumulate in the air until all occupants and surfaces are contaminated.

Furthermore, ventilation systems often serve multiple spaces with different functions. Aerosol transmission in a room may be affected by occupant type, density, and proximity; activities performed by occupants; the duration of occupancy; airflow quantity and distribution; the potential for aerosol generation; the presence of additional exposure control devices; the design and operation of the HVAC system; and the types of filtration, recirculation, and reentrainment devices or mechanisms in place.

OEHS professionals can perform a building ventilation investigation to identify opportunities to reduce occupants’ risk of exposure. The first step of the investigation, as with many OEHS interventions, is risk evaluation. At his facility, Smith started by determining which rooms people congregated in and for how long. He used a scorecard to numerically rank each room by occupant information and ventilation factors and assigned an aerosol exposure risk band to identify the spaces of most concern. For his facility, he found only five rooms in which occupants were at moderate or high risk for aerosol exposure.

The next step of Smith’s ventilation investigation was to inspect and measure the operation of the HVAC system. He mapped the air supply and exhaust systems within the building to get an idea of where the air was going and measured air change rates for each room. But this did not tell him whether the current rate of air change provided adequate ventilation to reduce concentrations of airborne viral particles.

Therefore, the third step of the investigation involved the conduction of airflow visualization tests to determine where, within a room, an uninfected occupant would most likely be exposed to particles emitted by an infected occupant. A visualization test using manikins designed to emit smoke revealed that the room’s current rate of air change was not enough to reduce aerosol concentrations.

Fourth, Smith’s investigation performed a tracer test to determine the dispersal of aerosols within the room, the accumulation decay of airborne particle concentrations, and the migration of aerosols to spaces outside the room. The tracer test used an aerosol generator located in the center of the room and an array of particle counter devices and photoionization detectors collecting samples throughout the space. Theoretically, a well-mixed space would show the same aerosol concentrations at every sample collection point at every point in time. However, Smith’s tracer test showed that this room’s air was not perfectly mixed, so aerosolized particles were not evenly distributed through the space over time. In fact, the test showed that ventilation in this room was overall 3.5 times less effective, and 10–20 times less effective at some sample locations, than in a theoretical well-mixed room. At these points, occupants would be at much higher risk of exposure than predicted using the general dilution equation.

Other tests conducted in this room compared the dispersion of gas to that of particulates and assessed the effects of high-efficiency particulate air (HEPA) filters—which were found to significantly improve room ventilation and reduce occupants’ dose of aerosolized particles.

From this point, Smith could apply appropriate safety measures throughout the facility based on the level of risk in each room. Resources could be directed towards rooms where the need for prevention was most critical. Following the hierarchy of controls, infected individuals were first discouraged from entering the building. The facility’s airflow management program provided an additional level of engineering controls. This program determined the amount of airflow required in each room and at what times, based on both the building’s design and on use factors for each space. Importantly, not all spaces required the use of special measures.

Additional Resources:

Brosseau, Lisa; Chang, James; and Smith, Tom: “Key Topics in Addressing the COVID-19 Pandemic, Part 2: Scientific Issues and the Hierarchy of Controls,” AIHce EXP Virtual Conference Presentation (May 26, 2021).

The Journal of Occupational and Environmental Medicine: “Aerosol Transmission of Infectious Disease,” (May 2015).

National Academies of Science, Engineering, and Medicine: Reusable Elastomeric Respirators in Health Care: Considerations for Routine and Surge Use (2019).

Abby Roberts

Abby Roberts is the editorial assistant at The Synergist.


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