As we seek to return to normalcy without a vaccine, COVID-19 confronts us with a troubling reality. We spend 90% of our time indoors in the U.S. and Europe, and scientific evidence indicates we are nearly 20 times more likely to be infected by the virus indoors than outdoors. COVID-19 has impacted many indoor settings such as schools, offices, churches, restaurants and bars, with prisons, meatpacking plants, and long-term care facilities being most affected due to high occupancy, poor ventilation and vulnerable populations.

Increasingly, scientists believe airborne transmission is a major route for the spread of COVID-19. Viral respiratory droplets released from coughing, sneezing, talking, and breathing can aerosolize into smaller particles, stay suspended in the air for hours, and travel significantly farther than six feet. A key scientific debate has been whether the virus is infectious in aerosols.

Though the virus is clearly detectable in aerosols, no one had been able to provide evidence  that it is “live” until the University of Florida. Skeptics of airborne transmission have been using this lack of evidence to challenge the importance of this mechanism. As the prominent aerosol scientist Linsey Marr said about the UF study, “If this isn’t a smoking gun, then I don’t know what is.” Even those who acknowledge aerosol’s infectiousness debate the relative importance of different viral transmission routes, including airborne or fomite, droplet or aerosol, direct or indirect contact, or a combination of mechanisms.

The role of airborne transmission of COVID-19 has a huge bearing on infection control in the built environment in two important respects. First, common approaches characterized by deep cleaning are incomplete and possibly misguided altogether. Second, masking and social distancing by themselves might be insufficient for mitigating airborne transmission.

Lisa Brosseau, a retired professor of public health, says that masks can limit larger particles’ spread, but they are less helpful for smaller particles. Aerosol mobility of over 30 feet, and suspension in air for hours can reduce the efficacy of six-foot social distancing mandates.

The fundamental question is what can and should we do to mitigate airborne transmission and create “safe” indoor environments amid COVID-19? One critically important and often overlooked area is engineering and Heating, Ventilation, and Air Conditioning (HVAC) controls. The American Industrial Hygiene Association (AIHA) states, “Engineering controls that can keep infectious aerosols at very low levels indoors offer the greatest promise to protect non-healthcare workers and other vulnerable populations as we reopen our businesses and workplace.”

Similarly, the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) states, “Changes in building operations, including the operation of heating, ventilating, and air conditioning systems can reduce airborne exposures.” It highlights the following HVAC strategies based on evidence-based literature:

• Enhanced filtration that includes higher Minimum Efficiency Reporting Value(MERV) filters over code minimums in occupant-dense and/or higher-risk spaces
• Upper-room UltraViolet Germicidal Irradiation (UVGI), with possible in-room fans, as a supplement to supply airflow
• Local exhaust ventilation for source control
• Personalized ventilation systems for certain high-risk tasks
• Portable, free-standing High-Efficiency Particulate Air (HEPA) filters
• Temperature and humidity control

ASHRAE is careful to qualify its recommendations with the caveat that the system’s impact will depend on the source location, strength, and distribution of the released aerosol, droplet size, temperature, air distribution, humidity, and filtration. Each indoor environment is unique; conditions within each indoor environment are dynamic, and there is not a one-size-fits-all strategy for infection control.