The SARS-CoV-2 virus, responsible for COVID-19, spreads primarily through airborne transmission. This occurs when infectious particles released by an infected individual are inhaled by others. Understanding how long these particles can remain suspended in the air is important for mitigating transmission risk.
How the Virus Travels in Air
The SARS-CoV-2 virus travels through the air in respiratory fluid particles expelled when an infected person breathes, talks, coughs, or sneezes. Larger respiratory droplets, greater than 5-10 micrometers in diameter, tend to fall quickly to surfaces within a few feet of the source.
Smaller airborne particles, known as aerosols or droplet nuclei, are less than 5 micrometers in diameter. These tiny aerosols can remain suspended in the air for extended periods, from minutes to hours, and can travel over distances greater than 1 meter. Their ability to stay aloft longer makes them a significant factor in airborne transmission, particularly in indoor environments.
Factors Influencing Airborne Time
Several factors influence how long SARS-CoV-2 particles remain viable and suspended in the air. Ventilation plays an important role, as effective air movement and exchange dilute and remove virus-laden aerosols from a space. Poor ventilation allows these particles to accumulate, increasing their concentration over time.
Humidity also affects particle stability. The virus’s deactivation rate can be high at intermediate humidity levels (40-60%), where the virus is more exposed to salts and solutes as droplets evaporate. Temperature also impacts viral viability, with increasing temperatures reducing the virus’s half-life, while low temperatures allow for prolonged survival. Smaller particle sizes remain airborne longer than larger droplets, as they are less affected by gravity and air resistance.
Understanding Risk in Indoor Settings
The combination of environmental factors and particle behavior creates varying levels of risk in different indoor settings. In poorly ventilated rooms, virus-laden aerosols can build up, leading to higher concentrations of infectious particles in the air. Crowded spaces increase risk due to a greater number of potential emitters and susceptible individuals in close proximity.
Prolonged exposure in such environments allows individuals to inhale a greater number of infectious particles. The concentration of airborne virus particles, influenced by emission rates from activities like speaking loudly or singing, and the duration of occupancy, influences the risk of transmission. Consequently, indoor spaces are where the majority of SARS-CoV-2 infections occur, distinguishing them from outdoor environments where dilution is greater.
Steps to Reduce Airborne Spread
To mitigate the risk of airborne SARS-CoV-2 spread, several actionable steps can be taken. Improving ventilation is a key strategy, which can involve opening windows and doors to bring in more outdoor air or running mechanical ventilation systems at higher speeds. Upgrading air filters in HVAC systems to higher efficiency ratings can also help capture more virus-carrying particles.
Using portable air purifiers with HEPA filters can further clean the air. Wearing well-fitting masks reduces the emission of virus particles and lowers the risk of inhalation. Maintaining physical distance, especially indoors, and avoiding crowded or poorly ventilated spaces also helps reduce exposure to airborne virus concentrations.