The transmission of respiratory illnesses involves the expulsion of infectious particles through activities like coughing, sneezing, talking, or breathing. The behavior of these expelled particles determines the mode of transmission, broadly categorized into two distinct routes: droplet and airborne. The physical characteristics of the particles, particularly their size and mass, dictate how far they travel and how long they remain a threat, impacting protective strategies.
Mechanics of Droplet Transmission
Droplet transmission involves larger, heavier respiratory particles, typically defined as 5 to 10 micrometers (\(\mu\text{m}\)) in diameter or larger. Due to their mass, they are quickly pulled down by gravity, following a short, ballistic trajectory. Droplets fall out of the air rapidly, generally settling within 3 to 6 feet (1 to 2 meters) from the source. The primary mode of infection is the direct spray of these particles onto the mucous membranes of another person’s eyes, nose, or mouth. This short-range travel limits the risk of transmission to close-range interaction.
Mechanics of Airborne Transmission
Airborne transmission, often referred to as aerosol transmission, involves much smaller particles, generally defined as less than 5 micrometers in diameter. Their tiny size means they are not subject to the same gravitational forces as larger droplets. Instead of falling, these micro-droplets are light enough to remain suspended in the air, sometimes for hours, moving with air currents. This allows infectious particles to travel beyond the typical 6-foot boundary and distribute throughout an indoor space. Aerosolization allows micro-droplets to be created even through quiet breathing and speaking, leading to potential exposure after the infected person has left the area.
Practical Impact on Disease Control
The fundamental differences in particle behavior necessitate distinct public health and personal protective strategies. Because droplet transmission is a short-range event, measures like physical distancing—the 6-foot rule—are effective at limiting direct exposure to large, expelled particles. For this route, standard surgical masks are generally sufficient to catch the majority of larger particles expelled by the wearer, protecting others from direct spray.
In contrast, controlling airborne transmission requires managing the air itself, as the particles remain suspended and spread through a room’s volume. Infections like measles and tuberculosis are classic examples of primarily airborne diseases. For these pathogens, distance alone is not enough; specialized respiratory protection, such as N95 respirators, is needed to filter out the small aerosols before they can be inhaled. Ventilation and air filtration systems, such as HEPA filters, become paramount in mitigating airborne risk because they actively remove infectious aerosols from the shared air. While diseases like the flu and the common cold are often spread via droplets, many respiratory viruses, including SARS-CoV-2, can utilize both droplet and airborne routes.