Ventilation is the most neglected and misunderstood of COVID mitigation methods; however, many epidemiologists and experts feel it may be a critical mitigation method, particularly as more virulent COVID variants are spreading around the globe. Before going much further, here is a quick review about how COVID is transmitted.

From the beginning of the pandemic, it was obvious that COVID was transmitted via close contact with infected people who expelled small COVID-19 respiratory droplets when they coughed, sneezed, spoke, or sang. These small droplets are heavy, so they typically don’t travel farther than 6 feet, but if these droplets land in the mouth or nose and/or are inhaled, then a person would be exposed to COVID. 

Initially, close contact and surface transmission were considered to be the primary methods of COVID transmission; however, data has shown that surface transmission of COVID is limited. Of greater concern is airborne transmission where COVID is transmitted by aerosolized particles that are much smaller than respiratory droplets, <5 μm versus 60–100 μm, respectively. These tiny droplets evaporate faster; therefore, they can travel much farther than 6 feet and remain in the air for longer, particularly in areas with poor ventilation. 

It took until July 2020 for the airborne transmission method to be fully acknowledged by the World Health Organization after a letter from 239 scientists from 32 countries demanded revised guidance. In October 2020, the CDC reticently updated their COVID webpages to include this method of transmission.

 “It’s time to stop pussyfooting around the fact that the virus is transmitted mostly through the air…If we properly acknowledge this, and get the right recommendations and guidance into place, this is our chance to end the pandemic in the next six months…If we don’t do this, it could very well drag on.” Linsey Marr, an expert on aerosols at Virginia Tech.

Why does ventilation matter?

Ventilation involves forcing outdoor air into indoor spaces. Along with masks and social distancing, it should be part of any strategy to prevent COVID transmission among children and adults in school buildings.

The most likely sources of COVID transmission are indoor spaces with close contact, because the virus concentration in the air can increase to such a level that a person is more likely to inhale the virus. In comparison, outdoor locations allow the air to circulate, which ensures lower concentration of any particle, including the COVID virus. Reducing airborne concentration of COVID is considered to be key to reducing transmission of COVID in indoor spaces, such as schools. 

BMJ 2020

Does opening a window help? 

Opening windows can help, but windows are not the solution for several reasons: 

  1. Not all classrooms have windows or windows that open. 

  2. Classroom windows and doors in school buildings may not improve ventilation. In other words, no one has tested the fresh air ventilation rates of the rooms or building to see if enough fresh air is circulating to push out the older stale air.

  3. Opening outside building doors reduces the security of school buildings.

What are other ventilation options?

Ventilation involves bringing outdoor air into the indoors to reduce the concentration of COVID in the air. This can be accomplished by: 

  • Opening windows and doors to create cross-ventilation.

  • Careful placement of fans to increase effective ventilation of open windows and doors.

  • Air-handling units that remove stale air from a room and release it outside.

  • Opening up outdoor dampers to reduce or eliminate indoor air recirculation.

  • Properly maintained and functioning heating and air conditioning (HVAC) systems that continually move air. This may include disabling the demand-controlled ventilation (DCV), which is the equivalent of a household setting the HVAC system fan to the “on” position instead of “auto,” so that the fan will operate continuously, even if the desired temperature is reached.

  • Improving filtration of central air as high as possible without overly reducing airflow.

  • Use of portable high-efficiency particulate air (HEPA) fan/filtration systems.

  • Ultraviolet (UV) germicidal irradiation systems, especially if increasing room ventilation proves to be difficult.

  • Many more options that can be found at CDC’s Ventilation in Buildings.

The CDC recommends layered approaches to COVID mitigation and ventilation, which means that a combination of ventilation approaches may be necessary to reduce COVID viral load in schools and individual classrooms. For example, one study in the Netherlands found that aerosols can build up in gym spaces even with ventilation or mobile air filters alone, but the study found that ventilation combined with mobile air filters could reduce aerosol buildup by 80-90%

How expensive is it to improve ventilation in schools?

Improving ventilation will stress school district budgets. Unfortunately, based on historic renovation and school repair practices, ventilation improvements will likely be implemented unevenly across schools and school districts.

A 2016 study in Education Week found that it would cost about $4.5 million per school for approximately 84,000 public schools to get the schools in “good overall condition.” Obviously, ventilation would be only a portion of that, but HVAC system improvement and replacement would be a significant investment when most school districts struggle to fund capital improvements, particularly after an economic disruption like the one caused by the pandemic. 

The CDC estimates that costs per room for ventilation methods to range from no cost by opening windows and disabling DCV controls, to about $500 for portable HEPA systems or $1,500 for UV systems. 

The U.S. Government Accountability Office (GAO) estimated that about 41% of school districts need to update or replace HVAC or ventilation systems in at least half their schools. 

This same GAO report found that capital improvement funding varies significantly by state and school district between property taxes and state funding, which would contribute to a likely uneven distribution of ventilation improvements. Maintenance and updates of school buildings have historically been unequitable among high- and low-poverty schools. The GAO estimates that capital expenditures are $300 less per student in high-poverty districts than in low-poverty districts: $719 per student versus $1,016 per student, respectively.

How much ventilation is enough?

This is unknown, because there are no U.S. governmental standards for ventilation. Nevertheless, ventilation is deemed so critical that 13 well-respected scientists, including Dr. David Michaels, an epidemiologist who led the Occupational Safety and Health Administration (OSHA) during the Obama administration, wrote a letter on February 15, 2021 asking President Biden’s administration to set air standards for a variety of workplaces like grocery stores, schools, buses, and prisons in order to save lives. At the same time, President Biden has directed OSHA to issue emergency temporary standards for Covid-19 (e.g., for ventilation and masks) by March 15, 2020.

What are the next steps for schools?

In the meantime, schools need to improve ventilation using the best available ventilation methods suggested by the CDC. Also, school districts must plan to improve ventilation across their school system. Where ventilation cannot be immediately improved, schools should do what they can to reduce the risk of infection based on CDC mitigation strategies:

  • Reduce the number of people in the classroom in order to reduce the COVID viral load.

  • Maintain 6-foot distancing, as much as possible, to reduce the likelihood of person-to-person transmission of COVID.

  • Plan for outdoor activities and meals, since the risk of infection is known to be much lower outside where ventilation is better.

  • Continue universal in-school masking and follow recent mask recommendations

Clearly, overhauling ventilation will be hard for cash-strapped school districts. If historic flu outbreaks in schools are any gauge of school ventilation, schools have an uphill battle to improve ventilation in order to prevent COVID outbreaks. They will need to rely heavily on other recommended CDC mitigation methods until ventilation can be improved. 

V is for Ventilation. Increase ventilation to reduce COVID risk in classrooms. 

Mask up and keep safe, Your @safeFCPS Communications Lead

Sources and Additional Resources:

Blockena, B., et al. Ventilation and air cleaning to limit aerosol particle concentrations in a gym during the COVID-19 pandemic. Building and Environment. 193: 107659. (Apr 15, 2021).

Burnette II, Daarel. Aging Buildings. Poor Ventilation. What Will It Take to Keep Coronavirus Out of Schools? Education Week. (Aug 12, 2020).

CDC. Ventilation in Buildings. (Updated Feb. 9, 2021).

Data: U.S. School Buildings: Age, Condition, and Spending. Education Week. (Nov 28, 2017).

GAO. K-12 EDUCATION: School Districts Frequently Identified Multiple Building Systems Needing Updates or Replacement. (June 2020).

Editorial: COVID-19 transmission—up in the air. The Lancet, Respiratory Medicine. 8 (12):  1159. (Dec 1, 2020).

Jones, Nicholas R, et al. Two metres or one: what is the evidence for physical distancing in covid-19? BMJ 2020. 370: 3223. (Aug 25, 2020).

Mandavilli, Apoorva. 239 Experts With One Big Claim: The Coronavirus Is Airborne. New York Times. (Jul 4, 2020, updated Nov 19, 2020).

Mandavilli, Apoorva. Scientists Call on C.D.C. to Set Air Standards for Workplaces, Now. New York Times. (Feb 17, 2021).

Sparks, Sarah D. What the CDC Guidelines Don’t Say About Classroom Ventilation and COVID-19 Spread. Education Week. (Feb 17, 2021).