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How Ventilation Affects the Spread of COVID-19 Indoors

Key points:

  • COVID-19 spreads through the air via close contact from person-to-person.

  • The risk of contagion is high in indoor spaces with poor ventilation. It’s crucial not to recirculate air and ensure fresh air gets into the room.

  • Indoor quality air monitoring systems, like the one designed by Zeptive, can help you monitor your building’s air health and make the most effective improvements to your space.


As COVID-19 cases continue to rise globally, everyone is searching for ways to slow the spread. Ensuring quality indoor ventilation – in conjunction with masking and social distancing – is crucial to reduce COVID-19 transmission.

How does ventilation indoors affect the spread of COVID-19?

The quality of the indoor air we breathe affects our health now more than ever as we navigate how to bring people back together during this pandemic.

That’s because COVID-19 spreads through respiratory droplets made when an infected person coughs, sneezes, or talks. These droplets can stay in the air for minutes or hours – and travel tens of meters on air currents – depending on their size and weight.

Quality indoor air ventilation can reduce the virus’s spread. Fresh air helps to push out virus-filled air from a room, reducing the risk of COVID-19 as long as other safety measures like masking and social distancing are followed.

Putting the importance of ventilation into perspective. An example:

Imagine a poorly-ventilated room with six people, one of whom has COVID-19. Even if they are wearing masks, four people are at risk of infection because of virus-containing droplets in the air, according to a new scientific tool developed by José-Luis Jiménez, an expert in the chemistry and dynamics of air particles at the University of Colorado Boulder, which was tested by El País, a daily newspaper in Spain.

However, the risk of infection drops to less than one person - when the space is properly ventilated and the amount of time people spend in the room is cut by half - along with wearing face masks.

How can I reduce the risk of COVID-19 transmission indoors?

  • Assess the quality of your indoor air: Installation of air quality monitors, such as the one produced by Zeptive, can give you a continuous pulse on the health of your ventilation. The goal is to minimize air recirculation in a room and ensure you have enough outdoor air coming in. Zeptive’s system measures key indicators of good ventilation, like carbon dioxide (CO2) levels and air particle filtration, and provides real-time metrics to users. Monitors can also help optimize the deployment of expensive ventilation and filtration equipment, like portable air filters, and confirm the equipment continues to operate effectively.

  • Consult with Heating, Ventilation, and Air Conditioning (HVAC) professionals: These professionals can inspect your ventilation systems to ensure they deliver clean air to your space. HVAC professionals can recommend improvements to existing systems that ensure better ventilation and filtration.

  • Avoid overcrowding: Because COVID-19 spreads through the air, it’s important to keep people at least six feet apart, particularly on public transportation and in public areas. Wearing masks reduces the transmission of aerosolized respiratory droplets that contain virus, especially when social distancing cannot be achieved.

  • Use other airborne infection control measures: The Centers for Disease Control and Prevention (CDC) and further research recommend using local exhaust, high-efficiency air filtration, and germicidal ultraviolet lights to provide localized ventilation, filtration, and air cleaning when upgrading or replacing existing HVAC systems is not practical or cost effective.

By Kristin Erekson Barton, MA, CHES Reviewed by Cindy Bistany, DHSc

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