Is 6 Feet Enough?

man sneezing into his elbow

How COVID Spreads through the Air

Doctors and scientists will often simplify real-world complexities in order to present an easy to understand and clear public health message. Simplifying these complexities may leave out crucial details that have life and death consequences. The controversy surrounding airborne transmission of COVID-19 is one such example. The simple message from pre-COVID times was that airborne transmission of respiratory diseases was either via short-distance droplet or long-distance airborne. That ended up not being true.

Short-Distance Droplet

According to the CDC, “respiratory droplets carrying infectious pathogens transmit infection when they travel directly from the respiratory tract of the infectious individual to susceptible mucosal surfaces of the recipient, generally over short distances, necessitating facial protection.”  Large droplets from a cough or sneeze eject out of the mouth like cannonballs and fall to the floor in a downward trajectory due to gravity. These large droplets typically fall to the floor within 3-6 feet of the infected individual. But if the spray of a cough hits a person’s face before it reaches the ground, “drop spray” transmission can occur. 

Long-Distance Airborne

Conversely, “airborne” was the term widely used pre-COVID to describe diseases that were spread over large distances across a building.  Very few diseases were put into this category, but it included tuberculosis (TB), measles, and Legionnaires’ disease. These diseases can possibly travel through a heating, ventilating, and air-conditioning (HVAC) system and infect an individual that never came in direct contact with the infectious individual.

Where does COVID-19 Fit In?

This is where things get interesting. Early reports from health officials were that droplet and contact transmissions were the main routes. There wasn’t any evidence of long-distance airborne transmission happening from one corner of a building to another via HVAC systems. Therefore, many health agencies quickly dismissed airborne transmission and focused on droplets and contact. 

But wait… super-spreader events started happening in early 2020 that could not be explained by droplets or contact. COVID was spreading beyond the theoretical barrier of 6 feet, but there wasn’t evidence of it traveling long-distances across a building. What was the problem? The legacy transmission categories of “droplet” and “airborne” weren’t taking into consideration the real world complexities of viruses. In essence, we needed a new category!

New Categories

Early in the pandemic, the world’s foremost expert on airborne transmission, Professor Yuguo Li of the University of Hong Kong, recommended we divide airborne transmission into sub-categories to better explain how infectious droplets travel. Very small respiratory droplets known as “aerosols” or “droplet nuclei” are in high concentration in the breath of a contagious person.  If you are in close proximity to this infected individual for a long time, you will have an elevated exposure and a high likelihood of transmission. Professor Li calls this “short-range airborne” and emphasizes that this involves small aerosols that come from breathing and talking rather than large droplets, which are more likely from coughs and sneezes. 

As you move away from the infected individual, the aerosol concentration generally decreases, yet there could still be an infectious dose. Professor Li introduced a crucial, new category he calls “extended short-range”. This refers to the build-up of infectious aerosols in a poorly-ventilated, enclosed room. In the past, this category wasn’t recognized because it didn’t match either short-range droplets or long-range airborne transmission seen with TB. This new category is likely where the majority of COVID-19 transmission occurs. Unfortunately, the term extended short-range transmission doesn’t easily roll of the tongue and hasn’t gained popularity in public health messaging.

How do I Prevent COVID Transmission?

Property managers and companies don’t have time to research aerosol science—they want to get people back to work and to that end they have devoted untold hours to establishing risk-reduction plans. Most of these plans were developed internally and have yet to be reviewed by 3rd party experts. Many internal plans have great strategies for social distancing, cleaning surfaces, and face coverings, but lack a plan of action for outdoor air ventilation and filtration, which are crucial for extended short-range transmission. 

Indoor Science can audit your back-to-work plan to ensure that all modes of disease transmission are being properly mitigated. For something as important as your employees’ health and safety, hiring an expert to evaluate your plan is prudent. If your building has inadequate ventilation, then no… 6 feet is not enough.


Indoor Science is now part of Aftermath Environmental Quality Services and can provide nationwide coverage. Please contact us at 877-695-7054 to discuss how we can partner with you and your organization.