Proper ventilation is an important part of reducing the risk of spreading COVID-19 indoors. COVID-19 spreads through respiratory droplets that infected individuals expel when they breathe, speak, cough or sneeze. These droplets can then land on surfaces or be inhaled by others. Ventilation helps reduce transmission risk by diluting and removing these potentially infectious respiratory droplets from the air. There are several strategies you can use to optimize ventilation in any room.
Why is ventilation important for COVID-19?
Ventilation is important because COVID-19 spreads through exposure to respiratory droplets expelled into the air by infected individuals. These droplets can remain suspended in the air for hours and accumulate if there is insufficient fresh air exchange. Good ventilation brings in fresh outdoor air and removes old stale air that may contain respiratory droplets, helping reduce potential viral concentration in the indoor space. By diluting and filtering out contaminants, proper ventilation decreases the likelihood that others in the room will inhale enough viral particles to become infected. Along with vaccination, masking and distancing, ventilation is a key component of mitigating airborne transmission of COVID-19.
How does proper ventilation work to reduce COVID-19 spread?
Ventilation works through two mechanisms to reduce the spread of COVID-19:
Dilution of infectious particles
Bringing in outdoor air dilutes the concentration of any potential viral particles in the room air. The more fresh air that is brought inside, the more any lingering respiratory droplets will be diluted. Higher ventilation rates enhance dilution.
Removal of infectious particles
Ventilation also removes and exchanges old stale air with new fresh air. This active replacement helps remove concentrates of viral particles from the room before they can be breathed in by others. Higher air exchange rates enhance removal.
Between dilution and removal, proper ventilation significantly cuts down on the amount of viable viral particles occupants are exposed to, lowering COVID transmission risk.
What are the recommended ventilation rates for COVID?
Authorities such as the CDC, ASHRAE and WHO recommend targeting higher ventilation and filtration rates to reduce COVID transmission indoors.
Target air exchange rates
The CDC recommends targeting 5-6 air changes per hour (ACH) as a minimum for indoor spaces:
| Authority | Recommended air exchanges per hour |
|---|---|
| CDC | At least 5-6 ACH |
| ASHRAE | 6 or more ACH |
| WHO | At least 6 ACH |
For high risk spaces like gyms or crowded areas, experts recommend exceeding 6 ACH to enhance dilution and removal of respiratory droplets.
Target minimum outdoor air supply
Increasing the fraction of outdoor air circulated inside is also recommended:
| Authority | Minimum outdoor air supply |
|---|---|
| CDC | As high as 100% |
| ASHRAE | At least 30% |
Maximizing the outdoor air percentage further reduces recirculation of potentially contaminated air.
Target minimum filter ratings
Higher efficiency air filters such as MERV-13 or greater are also suggested to remove fine respiratory particles from recirculated air:
| Authority | Minimum filter rating |
|---|---|
| CDC | MERV-13 or higher |
| ASHRAE | MERV-13 or higher |
Overall, targeting higher air exchange rates, increased outdoor air supply and better filtration helps reduce COVID exposure indoors.
How can you achieve optimal COVID ventilation rates?
There are several practical strategies to achieve the recommended air exchange rates in rooms:
Use portable HEPA air cleaners
Portable HEPA filter units are an easy way to boost air cleaning and ventilation. Size the HEPA unit appropriately for the room size and aim for 5-6 air changes per hour. Place air cleaners near high traffic areas.
Adjust HVAC settings
Increase central HVAC ventilation rates to target 5-6 ACH or higher. Maximize outdoor air percentage and use MERV-13 or better filters. Consult an HVAC professional if needed.
Open windows
Cracking open windows brings in fresh outdoor air through natural ventilation. This dilutes and replaces potentially contaminated room air. Open opposite windows to create cross-ventilation air currents.
Use fans
Strategically place fans in exterior-facing windows to draw fresh air into a room while another fan exhausts indoor air outside. This pushes out old air and pulls in new air.
Limit room occupancy
Lower occupancy means each person receives a higher ventilation rate per person. This further reduces COVID risk if enhancing other ventilation is difficult.
Monitor air quality
Use CO2 monitors to track indoor air quality and ensure adequate ventilation rates are maintained. Keep CO2 under 800 ppm.
Run ventilation longer
Extend ventilation before and after occupancy to flush out lingering respiratory droplets and achieve air change targets.
Hire an expert
Consult qualified ventilation professionals to inspect, adjust and upgrade ventilation systems as needed.
Prioritizing these practical ventilation improvements creates a safer indoor COVID environment.
What are the minimum COVID ventilation requirements for different spaces?
Authorities provide the following minimum COVID ventilation recommendations for higher-risk indoor spaces:
Schools
- 5-6 total air changes per hour
- Maximize outdoor air supply
- MERV-13 or higher filters
- Portable HEPA units as needed
Offices
- 6+ total air changes per hour
- 30%+ outdoor air supply
- MERV-13 or higher filters
- Monitor CO2 levels
Bars & Restaurants
- 6-12 air changes per hour
- Air curtains for entranceways
- Outdoor seating where possible
- HEPA filtration
Gyms & Fitness Centers
- 10+ air changes per hour
- 100% outdoor air supply
- CO2 monitoring
- Portable HEPA units in workout areas
Retail Stores
- 6+ air changes per hour
- MERV-13 filtration
- CO2 monitoring
- Occupancy limits
Optimizing ventilation using these evidence-based recommendations creates healthier indoor spaces.
What are other ways to improve COVID ventilation?
In addition to meeting minimum air exchange rates, other strategies can further enhance ventilation:
Direct air flow for targeted removal
Use fans or ductwork to direct clean air into high occupancy areas and contaminated air out. This focused air current removes respiratory droplets more efficiently.
Use upper-room UVGI disinfection
Install UVGI lamps high up on walls or ceilings to inactivate airborne viruses and prevent recirculation. This provides supplementary disinfection.
Adopt portable air cleaners
Install portable HEPA filter units to boost local ventilation and cleaning in high density zones or areas far from windows.
Keep humidity between 40-60%
Maintaining mid-range humidity facilitates virus removal and reduces transmission risk compared to dry air.
Generate clean air zones
Use downward fans, plexiglass barriers and air curtains to create clean air zones around susceptible individuals for added protection.
Monitor and confirm ventilation rates
Use CO2 monitoring, air flow testing and other validation methods to ensure ventilation systems are performing as designed.
Layering these extra ventilation controls further reduces COVID exposure.
What are the limits of ventilation for reducing COVID-19 spread?
While ventilation is helpful, it has limitations:
Doesn’t replace other precautions
Ventilation should be used alongside vaccination, masking, distancing and other measures. Ventilation alone cannot prevent all transmission.
Not effective everywhere
Ventilation is less effective in large, crowded indoor spaces where maintaining high air exchange rates is difficult.
Hard to implement universally
Many buildings have outdated HVAC systems without capacity to ventilate to modern standards. Fixing these issues takes time and investment.
Variable effectiveness
Ventilation relies on proper design, sizing, installation, maintenance and operation to work correctly. Poor implementation reduces benefits.
Insufficient data
More research is still needed to refine ventilation recommendations for differing conditions like climate, activity levels and variants.
Element of diminishing returns
While more ventilation reduces risk, the benefits plateau as rates exceed 10-12 air changes per hour.
Ventilation helps create healthier spaces but works best as part of a comprehensive infection control approach.
Conclusion
Ventilation is an important evidence-based strategy to reduce the airborne spread of COVID-19 indoors. Targeting ventilation rates of 5-6 air changes per hour, maximizing outdoor air supply, installing MERV-13 or better filters and using supplemental portable HEPA units can significantly reduce transmission risks. While not a standalone solution, optimizing ventilation will be a key long-term component of living with COVID. With proper design, sizing and implementation informed by emerging data, we can create healthier indoor spaces as we move into the next phase of the pandemic. Going forward, greater investment in upgraded ventilation represents an opportunity to build more resilient buildings.