The duration that SARS-CoV-2 remains infectious in the air is highly variable, depending on the size of the respiratory particles and the surrounding environmental conditions. Airborne transmission occurs when a person inhales virus-laden particles suspended in the air. This airborne route is now recognized as a major way the virus spreads, especially in poorly ventilated indoor settings. The time the virus can linger and remain viable is directly tied to the physics of the particles that carry it.
Understanding Droplets and Aerosols
The respiratory particles expelled when a person breathes, speaks, coughs, or sneezes are traditionally categorized into two groups based on their size and behavior. Large respiratory droplets are heavier particles, generally larger than 100 micrometers, which are pulled quickly to the ground by gravity, typically settling within seconds and traveling only a short distance, usually less than two meters. These larger particles are the basis for traditional social distancing guidelines.
Smaller particles, known as aerosols, are less affected by gravity and remain suspended in the air for extended periods. When a large droplet is expelled, its water content can evaporate rapidly, shrinking it into a droplet nucleus that behaves like an aerosol. These tiny, buoyant particles drift on air currents, allowing them to travel much farther than two meters and accumulate in enclosed spaces. Aerosols are the primary mechanism for long-range airborne transmission.
Measured Duration of Airborne Viability
Scientific studies conducted under controlled laboratory settings, such as bioaerosol chambers, provide a baseline for how long the virus can remain infectious in the air. These experiments show that SARS-CoV-2 can remain viable in aerosol form for a duration ranging from minutes to several hours. One widely cited study found that the virus remained infectious for at least three hours, with the infectious concentration only gradually decreasing over that time.
The concept of “viability” means the virus is still capable of infecting a host cell, not simply that its genetic material is detectable. Researchers often report the virus’s half-life, which is the time required for half of the infectious particles to lose their ability to cause infection. The median half-life for SARS-CoV-2 has been estimated to be approximately 1.1 to 1.2 hours under specific laboratory conditions (around 21–23°C and 65% relative humidity). Although the virus may remain infective for many hours in a sealed chamber, the infectious concentration rapidly decreases, meaning the risk is highest immediately following the particle’s release.
Environmental Conditions That Impact Air Time
The duration a viral aerosol remains suspended and viable is influenced by the environment in a real-world setting. Temperature and humidity levels are factors that affect the survival rate of the virus. Lower temperatures generally help preserve the virus, allowing it to maintain its infectiousness for longer periods.
Humidity plays a complex role, but maintaining relative humidity between 40% and 60% often limits the survival of airborne pathogens. High humidity can cause aerosols to grow larger and settle faster, while very low humidity can cause them to shrink and remain airborne. The most significant factor indoors is the air exchange rate, which is the speed at which indoor air is replaced with fresh outdoor air. Rapid air movement and replacement drastically reduces the time the virus stays in the room air by diluting and removing the viral particles.
Practical Steps for Air Quality Improvement
Since the highest risk of transmission is in indoor settings with poor air quality, improving ventilation and filtration is a direct way to reduce the time the virus lingers. Increasing natural ventilation by opening windows and doors brings fresh outdoor air inside, which dilutes the concentration of airborne viral particles. For mechanical systems, the goal is to increase the air changes per hour (ACH), which is the number of times the total air volume in a room is replaced in one hour.
Air filtration is a highly effective strategy for removing viral aerosols. Portable air purifiers equipped with High-Efficiency Particulate Air (HEPA) filters are effective because they capture particles the size of viral aerosols. In larger buildings, upgrading the Heating, Ventilation, and Air Conditioning (HVAC) system’s filters to a higher Minimum Efficiency Reporting Value (MERV) rating, such as MERV 13, improves particle capture. Monitoring carbon dioxide (CO2) levels can also serve as a proxy for ventilation effectiveness, as high CO2 readings often indicate a lack of fresh air exchange.