Q: How much is virus transmission connected to indoor environments?
Indoor transmission is much higher than outdoor transmission, indicating a concentration of virus indoors compared to outdoors. In one study from China of 300 outbreaks (3+ people infected), all of the outbreaks occurred indoors (80% at home, 20% elsewhere). Studies reviewed by WHO found that transmission is much more likely in enclosed spaces with up to one hour of contact time. Outdoor transmission is rare, but possible with extended close contact.

Q: Can COVID-19 be transmitted through HVAC systems? Are there any documented cases of COVID-19 transmission through HVAC systems? 
A: While there are no documented cases of COVID-19 transmission through HVAC systems, we know that it can happen. Here's why.

  • The virus has a "half-life" in the air of ~60-80 minutes, more than enough time to transport from one region of a building to another through an HVAC system [1]. Viral laden droplets travel in "cloud" patterns much further than 6 feet and large droplets take several minutes to "fall." [2]
  • We know the virus can spread via intra-space airflows [3].
  • We know that viral matter can move efficiently through an indoor space and be transported through an HVAC system. For instance, in a study from University of Nebraska, more than twice as much viral matter was present on air exhaust duct grills than on any other surface, including cell phones and bathroom surfaces. Viral matter under beds in isolation rooms was as concentrated as that on cell phones and bathroom surfaces.

Q: Is 6 ft enough distance between people?
A: The 6 ft distancing rule was based on the assumption that the dominant transmission path for the virus is large droplets expired during sneezes and coughs, as is the case with other viruses such as MERS, SARS and influenza. However, recent studies have shown transmission can occur from finer particles that spread farther from the source. Virus laden droplets seem to travel farther than bacteria laden droplets. According to one study, bacteria laden droplets fell within 6 ft of the respiration/cough emission source, while virus laden droplets fell much farther away than 6 ft from the source.

Though COVID-19 can spread farther than 6 ft, the viral load in the air is highest near an infected person. Santarpia and colleagues found a 50% reduction in viral load at a 6 ft distance from an infected person.  Best practices: Maintain at least 6 ft distance, wear face masks, and use proper hygiene.

Q: How does contact time impact virus transmission?
Infection also relies on viral load which is impacted by contact time with droplets or aerosols containing the virus, ventilation/dilution rates, humidity, and even temperature. Momentary contact outdoors or indoors is unlikely to result in infection. Usually transmission is associated with a close-contact exposure with an infected individual of 5 minutes or more.

Q: What's the difference between airborne and droplet transmission?
A: Airborne and droplet transmission are the same thing (respiratory emitted droplets). Respiratory droplets range from microns (so-called "big" droplets of 10 microns that take 10 minutes to "fall" to the ground), to sub-micron (airborne). The good news is that viral-laden droplets across the spectrum can be filtered as well as killed with UVGI (ultraviolet germicidal radiation).

Q: What is the COVID-19 decay limit? 
A: The "Infection Parameter" is a ratio of persons infected by an infectious person over a 2-week period. It has a mathematical boundary value of 2.72 (the mathematical constant "e"). IP values greater than 2.72 is the infection accelerating region and values less than this is a region of decay. Dr. Ty Newell has posted two papers on MedRxiv that provide more background on IP, and its dependence on human behavior through social distancing and disease transmission efficiency (paper 1paper 2).

Q: Does the wide variety of the virus affect the accuracy of test results for current or past infections? 
A: The tests recommended by the CDC are tracking the conserve regions of the genome. For this reason, the infection will still be detected.

Other Questions

Overall recommendations

Increasing airflow

Filtration and air cleaning

Ultraviolet irradiation

Risk assessment

Have a question about building modifications during COVID-19? Contact SEDAC at info@sedac.org or 800.214.7954.

Interested in receiving a COVID-19 Readiness Building Assessment? SEDAC's COVID-19 Readiness Assessments give you the independent, expert advice you need to identify medium to high risk spaces and make building modifications to reduce the risk of virus transmission.