Respiratory droplets are minute fluid particles expelled from the mouth and nose during various human activities. They consist primarily of water, saliva, mucus, and electrolytes. These droplets can also carry biological materials, including human cells and infectious agents like bacteria, fungi, and viruses. They serve as a primary vehicle for the transmission of many respiratory illnesses.
How Respiratory Droplets Are Formed
Respiratory droplets are generated through everyday actions involving exhalation. These include breathing, talking, singing, coughing, and sneezing. The force of these actions directly influences the quantity and size of droplets released. For instance, a forceful sneeze or cough produces more and larger droplets than quiet breathing or speaking.
Exhaled droplets originate from the oral cavity and upper respiratory tract. During loud speaking, thousands of fluid droplets can be expelled per second from the mouth. The size distribution of these droplets varies depending on factors like the velocity of exhaled air, fluid viscosity, and the pathway of expulsion, whether through the nose, mouth, or both.
How Droplets Spread Through the Air
Once expelled, larger respiratory droplets follow a predictable path through the air. Due to their greater size and weight, these droplets are subject to gravitational pull, causing them to follow a ballistic trajectory. They behave like tiny projectiles, quickly falling out of the air rather than remaining suspended for long periods.
This rapid settling limits their travel distance, resulting in short-range transmission. Most larger droplets fall to the ground or onto nearby surfaces within a few feet of the source. For example, large droplets from speaking might travel approximately 0.16 to 0.68 meters, while those from coughing could reach 0.58 to 1.09 meters. Sneezes might propel droplets 1.34 to 2.76 meters indoors. This limited airborne range forms the basis for physical distancing recommendations.
The Difference Between Droplets and Aerosols
The distinction between respiratory droplets and aerosols is rooted in their size and behavior in the air. Respiratory droplets are larger particles, with some definitions classifying them as greater than 5 or 10 micrometers in diameter, or even over 100 micrometers. These larger particles fall quickly due to gravity; for example, a 100-micrometer droplet may settle in 1.5 seconds.
Aerosols, sometimes called droplet nuclei, are much smaller particles. They form when the water content of respiratory droplets evaporates, leaving behind a lighter, residual particle. These smaller particles, often considered less than 5 or 10 micrometers in diameter, are light enough to remain suspended in the air for extended periods, from minutes to several hours. This allows aerosols to travel much further on air currents, explaining airborne transmission over longer distances, which differs from the short-range spread of larger droplets.
Methods for Reducing Droplet Transmission
Controlling the spread of respiratory droplets involves a combination of strategies. One primary method is source control, which prevents infected individuals from expelling droplets into the environment. Wearing face masks acts as a barrier, blocking many droplets at their point of origin and reducing their release into the air. This measure is helpful even for individuals who may be infected but show no symptoms.
Maintaining physical distance from others is another effective strategy, as it places individuals outside the ballistic trajectory of larger droplets. Guidelines suggest keeping a distance of at least 1 to 2 meters to avoid direct exposure to these quickly settling particles. This reduces the likelihood of droplets landing on a person’s mucous membranes, such as the eyes, nose, or mouth.
Environmental controls also mitigate droplet transmission. Improving ventilation in indoor spaces helps dilute and disperse airborne droplets and aerosols, preventing their concentration from building up. Regularly cleaning and disinfecting frequently touched surfaces addresses the risk of indirect transmission, where settled droplets may transfer pathogens through hand contact.