The virus that causes COVID-19, SARS-CoV-2, is primarily transmitted through tiny, virus-containing particles called aerosols. These aerosols are released when an infected person breathes, speaks, coughs, or sneezes, and they can remain suspended in the air for extended periods, especially indoors. Because transmission is airborne, the quality and movement of indoor air directly affect the risk of infection. Opening windows is one of the simplest and most accessible ways to improve indoor air quality using natural ventilation.
How Diluting Air Reduces Risk
Ventilation addresses airborne COVID-19 transmission through the principle of dilution. When an infected person releases aerosols into a closed space, viral particles build up, creating a higher concentration of the infectious agent. The goal of ventilation is to replace this contaminated indoor air with fresh, pathogen-free air from outside.
This process increases the Air Change Rate (ACH), which measures how many times the total volume of air in a room is replaced in an hour. Increasing the ACH significantly lowers the concentration of viral aerosols, effectively diluting the overall viral load. This reduction means a susceptible person is less likely to inhale a sufficient dose of the virus to cause infection. Dilution is an effective engineering control that reduces the risk of transmission across the entire room.
Maximizing Airflow with Open Windows
Opening a single window offers some benefit, but maximizing airflow requires creating a clear path for air to enter and exit the space. The most effective strategy for natural ventilation is establishing a cross-breeze by opening windows on opposite sides of the room or building. This setup creates a pressure difference that draws fresh air in from one side while pushing stale air out from the other.
If only one window is available, opening it slightly at both the top and bottom can still improve circulation, especially with double-hung windows. The top opening allows warmer, potentially contaminated air to escape, while the bottom opening draws in cooler, denser fresh air. When a cross-breeze is not feasible, the strategic use of fans can simulate the effect.
A box fan placed in a window and facing out will pull contaminated air from the room and exhaust it outside; this is the preferred method for removing aerosols. Conversely, placing a fan facing in forces fresh air into the room, but this can push contaminated air into other areas of the building. Kitchen or bathroom exhaust fans can also supplement natural ventilation by actively pulling air out. In colder weather, windows do not need to be fully open; even partially opening them for a few minutes each hour helps flush out accumulated aerosols without causing excessive cold.
Layering Protection Beyond Ventilation
While opening windows is a powerful tool, it is not a standalone solution, and its effectiveness is limited by factors like outdoor air quality, extreme weather, and insufficient airflow. Improving ventilation should be part of a broader, multi-layered approach to infection mitigation. These layers work together to provide a greater level of protection than any single measure.
One important layer is mechanical air cleaning, which involves using High-Efficiency Particulate Air (HEPA) filtration units. HEPA filters capture extremely small particles, including viral aerosols, effectively scrubbing the air clean when natural airflow is inadequate.
Another element is source control, which means wearing high-quality masks, such as N95 or KN95 respirators. These masks prevent aerosols from being released by an infected person and reduce the amount inhaled by a susceptible person.
Combining better ventilation with high-quality masking and maintaining physical distance significantly reduces the overall risk of infection. This layered strategy provides a robust defense against airborne transmission.