A cough is a common human reflex, and its expelled particles embark on an invisible journey. This article explores the scientific principles governing the travel distance of cough particles and the factors that influence their spread.
The Physics of a Cough
A cough is a forceful, rapid expulsion of air from the lungs, designed to clear respiratory passages. This powerful burst of air creates a high-velocity jet that propels thousands of fluid particles into the surrounding environment. These particles originate from the fluid lining of the respiratory tract and consist mainly of water, along with salts, mucus, and potentially microorganisms. The initial velocity of this expelled air can be quite high, sometimes reaching speeds around 10 meters per second (approximately 22 miles per hour).
The particles produced by a cough vary widely in size, which dictates their behavior in the air. Scientists categorize them into two types: larger respiratory droplets and smaller aerosols. Larger droplets behave like tiny projectiles, similar to the coarse spray from a spray bottle. Conversely, smaller aerosols are more akin to the fine mist from a humidifier, remaining suspended in the air for longer periods.
Measuring the Spread of Cough Particles
The trajectory of cough particles is largely determined by their size and weight. Larger respiratory droplets, due to their greater mass, are significantly affected by gravity and tend to fall out of the air within a short distance. These larger particles typically settle to the ground within about 1 to 2 meters (approximately 3 to 6 feet) from the source. This rapid descent forms the basis for the commonly referenced “6-foot rule” for physical separation.
Smaller aerosol particles behave differently, defying gravity for extended periods. These tiny particles, often less than 60 micrometers, do not fall quickly and can remain suspended in the air for minutes to hours. They are readily carried by air currents and can travel much further than larger droplets within a room, sometimes spreading up to 6 to 8 meters (about 20 to 26 feet) or more.
External Influences on Travel Distance
Several external factors alter how far cough particles can travel. Indoor air movement and humidity play a role in their dispersion. Good ventilation can dilute the concentration of airborne particles and remove them from an enclosed space, reducing their lingering time and spread. In poorly ventilated areas, smaller particles can accumulate and remain suspended longer, increasing their potential travel distance.
Humidity also affects particle behavior. In dry air, the water content of respiratory droplets evaporates more rapidly, causing them to shrink into smaller, lighter aerosol particles. This reduction in size allows them to remain airborne for longer durations and travel greater distances. In contrast, higher humidity helps droplets retain their size, causing them to settle faster and reducing their airborne travel.
Face coverings also modify the spread of cough particles. Masks work by disrupting the initial high-velocity plume of a cough, trapping many particles close to the source. This reduces the initial momentum and the distance the remaining particles can travel. Studies show that surgical masks and N95 respirators are highly effective, reducing the outward spread of droplets and limiting their travel. Even multi-layered cloth masks can offer substantial reduction, serving as a barrier to limit the dispersion of respiratory droplets.