The scientific understanding of how SARS-CoV-2, the virus that causes COVID-19, spreads has evolved significantly since the pandemic began. Initially, the focus was on surface and large droplet contamination, but a growing body of evidence has shifted the consensus to the virus being predominantly airborne. Understanding what “airborne” means is necessary for grasping how the virus travels and how to reduce transmission risk.
Understanding Respiratory Virus Transmission
When people breathe, talk, cough, or sneeze, they release respiratory particles from their mouth and nose. These virus-carrying particles vary in size, leading to different modes of transmission. The two primary modes are droplet and airborne transmission, with the distinction based on the size and aerodynamic properties of the particles.
Droplet transmission involves larger respiratory particles, greater than 5 micrometers in diameter. Due to their size and weight, these droplets are pulled down by gravity quickly, traveling less than six feet before falling onto surfaces. Think of them like a fine spray; they travel a short distance and then settle. Infection occurs if these droplets land directly on the mucous membranes—eyes, nose, or mouth—of a nearby person.
Airborne transmission involves smaller particles called aerosols, which are smaller than 5 micrometers. These are light enough to remain suspended in the air for minutes to hours, much like smoke, and can be produced in high numbers by talking or singing. Because they linger and travel on air currents, they can be inhaled by people far from the infected individual, posing a risk beyond close contact.
The Case for Airborne SARS-CoV-2
The consensus that SARS-CoV-2 is airborne grew from analyzing transmission events where other explanations were insufficient. A primary piece of evidence came from superspreader events, where one infected person transmitted the virus to many others. For example, at a choir practice in Skagit County, Washington, one symptomatic person infected 52 others despite attendees avoiding physical contact. This pattern of infection could not be explained by droplet or surface contact, pointing toward airborne transmission in a poorly ventilated indoor space.
Further evidence is the role of asymptomatic and presymptomatic transmission. Studies estimate that at least 40 percent of all SARS-CoV-2 transmission comes from people who are not actively coughing or sneezing. These individuals primarily release smaller aerosol particles through breathing and talking. This “silent” transmission was a major driver of the global spread, a feature that supports a predominantly airborne route.
Laboratory studies have solidified the case for airborne transmission. Research showed that SARS-CoV-2 can remain viable and infectious in experimentally created aerosols for up to 16 hours. Other studies detected viral RNA in air samples from the rooms of COVID-19 patients, confirming that virus-laden particles are present in real-world settings. This combination of outbreak analysis and lab findings provides a robust foundation for the airborne conclusion.
How Environment Affects Airborne Spread
The environment plays a role in the risk of airborne virus transmission, with a major difference between indoor and outdoor settings. Outdoors, the vast volume of air rapidly dilutes virus-containing aerosols, and ultraviolet (UV) radiation from the sun helps inactivate the virus, making transmission much less likely. In contrast, poorly ventilated indoor spaces allow aerosols to accumulate, increasing the concentration of the virus and the risk of exposure for everyone inside.
Ventilation is a determinant of risk indoors. Good ventilation involves bringing in fresh outdoor air to dilute the concentration of airborne contaminants, including viruses. In buildings with mechanical systems, this is measured in air changes per hour (ACH), which describes how many times the room’s air is replaced with fresh air. In poorly ventilated spaces, virus-laden aerosols can linger for hours and move throughout a building.
Two other interconnected factors are crowding and the duration of exposure. The more people who occupy an indoor space, the higher the potential concentration of aerosols if one or more of them are infected. The length of time spent in that space is also directly related to risk; a brief exposure in a crowded room is less risky than spending several hours there. The combination of poor ventilation, crowding, and long duration creates the highest-risk environment.
Effective Strategies for Mitigation
Given the evidence for airborne transmission, mitigation strategies focus on reducing the inhalation of infectious aerosols. High-quality, well-fitting masks are recommended for this purpose. Respirators like N95s, KN95s, and KF94s are designed to filter out very small airborne particles. An N95 respirator, for instance, filters at least 95% of airborne particles, making it highly effective at capturing virus-containing aerosols. The fit of the mask is also important, as a snug fit prevents air from leaking around the edges.
Improving indoor ventilation is another effective strategy. For the average person, this can be as simple as opening windows and doors to create cross-ventilation, allowing fresh outdoor air to replace contaminated indoor air. Using exhaust fans in kitchens and bathrooms can also help pull indoor air outside. In buildings with HVAC systems, upgrading the filters to a higher MERV (Minimum Efficiency Reporting Value) rating can capture more airborne particles.
When improving ventilation is not feasible, air filtration can effectively remove aerosols from the air. Portable air purifiers equipped with High-Efficiency Particulate Air (HEPA) filters are a good option. HEPA filters are designed to capture at least 99.97% of airborne particles with a size of 0.3 microns, which includes the respiratory aerosols that carry viruses. Placing a HEPA filter in a room can reduce the concentration of infectious particles, adding a layer of protection in crowded or poorly ventilated settings.