Physical distancing is a public health measure, sometimes called social distancing, intended to reduce the transmission of infectious respiratory diseases like COVID-19 by increasing the space between people. This non-pharmaceutical intervention (NPI) slows the spread of the SARS-CoV-2 virus. The goal is to minimize the probability of an uninfected person encountering infectious particles emitted by an infected individual, thereby suppressing the disease transmission rate.
Understanding Droplet and Aerosol Transmission
The reason distance works as a mitigation strategy is rooted in the physics of how respiratory particles travel after they are expelled from the body. When an infected person coughs, sneezes, sings, or even talks, they release a spectrum of particles that can carry the virus. These particles are broadly categorized by size: larger respiratory droplets and much smaller aerosols.
Large respiratory droplets are typically defined as being greater than 5 to 10 micrometers in diameter. Due to their size and mass, gravity acts on these droplets quickly, causing them to fall out of the air and settle onto surfaces or the ground within a relatively short distance, often a few feet. The risk of transmission from these larger particles is greatest when people are in close proximity to one another.
Aerosols, or droplet nuclei, are the much smaller particles, generally less than 5 micrometers. Because they are lightweight, aerosols defy gravity and can remain suspended in the air for extended periods, sometimes hours. Air currents and ventilation patterns can carry these tiny infectious particles further than large droplets, potentially infecting people well beyond a few feet, especially in indoor settings.
The Standard Distance and Its Limitations
The recommendation to maintain a specific distance, most notably six feet or two meters, became a widely adopted guideline during the pandemic. This figure was established based on older research suggesting that most heavy respiratory droplets would fall to the ground within this range. The six-foot rule was intended to create a safe zone against the primary transmission mechanism understood at the pandemic’s start.
However, the specific measurement of six feet was not the result of controlled clinical trials comparing six feet to three feet or ten feet. Some countries, like the World Health Organization (WHO), recommended a shorter distance of one meter, or about three feet. This discrepancy highlighted that the six-foot rule was a practical policy guideline rather than a definitive, scientifically proven hard barrier.
The understanding that smaller, aerosolized particles play a significant role in COVID-19 spread revealed the limitations of a fixed distance rule. Since aerosols can drift across a room and accumulate in shared air, simply standing six feet away is not a guarantee of safety, particularly in poorly ventilated indoor environments. For airborne transmission, the concentration of virus in the shared air becomes a more relevant factor than the exact distance between individuals.
Mitigating Risk Beyond Physical Separation
Since distance alone does not fully prevent aerosol transmission, a layered strategy of mitigation measures is necessary to substantially reduce risk in shared spaces. One of the most effective controls is improving air exchange through ventilation. Bringing fresh outdoor air into a space or using high-efficiency particulate air (HEPA) filters helps to dilute and remove virus-laden aerosols, lowering the overall concentration of infectious particles in the air.
The total duration of exposure is another major factor, as the risk of infection increases significantly over time, even when maintaining a distance. Brief encounters carry a much lower risk than prolonged interactions, such as spending hours in the same room with an infected person. Public health guidelines often recognized this by recommending time limits on interactions to minimize the dose of inhaled virus.
Masking serves as a dual-purpose intervention that significantly enhances the effectiveness of distance. A mask acts as source control, reducing the amount of virus-containing droplets and aerosols an infected person releases into the air. It also provides a degree of protection for the wearer by filtering inhaled particles. When worn universally, masks limit the viral load in the shared environment.