The World Health Organization officially recognized that SARS-CoV-2, the virus causing COVID-19, spreads through the air on December 23, 2021, nearly two years after the pandemic began. That acknowledgment came after prolonged scientific debate and criticism from aerosol scientists who argued the evidence had been clear much earlier. The delay shaped global public health guidance in ways that are still being untangled today.
Why It Took WHO Two Years
Early in the pandemic, WHO maintained that COVID-19 spread primarily through large respiratory droplets. Under the traditional framework, these droplets were defined as particles larger than 5 microns in diameter that fall to the ground within about one to two meters of the person who expelled them. This classification drove the familiar guidance: stay six feet apart, wash your hands, and clean surfaces.
Aerosol scientists pushed back almost immediately. They pointed out that infected people also exhale much smaller particles (under 5 microns) that can linger in the air for minutes to hours and travel well beyond two meters, especially in poorly ventilated indoor spaces. By mid-2020, hundreds of researchers had signed open letters urging WHO to update its position. The organization was slow to move, partly because of institutional inertia around how respiratory diseases had been classified for decades, and partly because it set a high evidence bar for changing guidance that affected every country’s pandemic response.
When WHO finally used the word “airborne” in December 2021, it validated what many scientists had been saying since early 2020: that breathing shared indoor air with an infectious person is a primary route of transmission, not just an exceptional risk during medical procedures.
How WHO Now Describes Transmission “Through the Air”
In April 2024, WHO and other leading health agencies introduced a new framework that retired the old droplet-versus-aerosol divide altogether. The updated terminology replaces both “droplets” and “aerosols” with a single term: infectious respiratory particles, or IRPs. The key insight is that these particles exist on a continuous spectrum of sizes, with no clean cutoff separating large from small. The old 5-micron boundary, which had defined infection control policy for decades, is no longer treated as a meaningful dividing line.
Under this framework, the broad category is “through the air” transmission, which covers two routes. The first is airborne transmission (also called inhalation), where infectious particles are expelled into the air and inhaled by another person. This can happen at short or long range, depending on airflow, humidity, temperature, and ventilation. The second is direct deposition, where particles land directly on someone’s mouth, nose, or eyes at close range. Both routes involve particles traveling through the air, but they differ in distance and mechanism of entry.
This unified language matters because it shifts attention toward ventilation, air filtration, and respiratory protection as core prevention tools, rather than treating them as extras reserved for special circumstances.
What This Means for Protection
The airborne recognition reshaped how we think about staying safe indoors. When a virus spreads mainly through large droplets, hand washing and surface cleaning are central defenses. When it also spreads through smaller particles that float in shared air, ventilation and mask quality become far more important.
Despite acknowledging airborne spread, WHO’s guidance on respirators has remained cautious. A review of its COVID prevention recommendations found that the organization did not clearly recommend N95 or FFP2 respirators over standard surgical masks for most settings. Its guideline development group concluded that the evidence was “insufficient to recommend one mask over another except in some specific conditions.” Some infection control experts criticized this position, arguing that the physics of airborne particles makes higher-filtration respirators a straightforward upgrade in protection.
For everyday situations, the practical takeaway is that a well-fitting respirator (N95, KN95, or FFP2) filters out a much higher percentage of small airborne particles than a loose surgical mask. In crowded or poorly ventilated indoor spaces, that difference matters. Opening windows, using portable air purifiers with HEPA filters, and spending time outdoors when possible all reduce the concentration of infectious particles you breathe.
Higher-Risk Settings
Certain medical procedures generate unusually high concentrations of infectious aerosols. WHO maintains a specific list of these aerosol-generating procedures, which includes intubation, CPR, manual ventilation, bronchoscopy, non-invasive positive pressure ventilation, tracheotomy, sputum induction, dental procedures, and autopsies. Healthcare workers performing these procedures face elevated exposure and are advised to use the highest level of respiratory protection available.
Outside of hospitals, the environments that carry the most risk share common features: indoor spaces, poor ventilation, crowds, prolonged exposure time, and activities like singing or shouting that increase particle output. Restaurants, bars, gyms, choir rehearsals, and crowded public transit have all been linked to documented transmission clusters, consistent with airborne spread rather than droplet-only transmission. The common thread is shared air in an enclosed space, not close physical contact with contaminated surfaces.
Why the Classification Matters Beyond COVID
The debate over airborne transmission was never just about one virus. The old framework, with its rigid 5-micron cutoff between droplets and aerosols, had governed infection control for diseases like influenza and tuberculosis for years. By adopting the new continuous-spectrum model for infectious respiratory particles, WHO has set the stage for how future respiratory outbreaks will be classified and managed from the start.
The practical implication is that ventilation and air quality are now recognized as infection control tools on par with hand hygiene. Buildings with better air filtration and fresh air exchange reduce transmission of respiratory viruses broadly, not just SARS-CoV-2. For individuals, understanding that respiratory viruses travel through shared air makes it easier to assess risk in any indoor environment: how big is the space, how many people are in it, how long will you be there, and is the air moving.