A respiratory droplet is a small aqueous droplet produced by exhalation, consisting of saliva
and other matter derived from respiratory tract
surfaces. Droplet sizes range from < 5 µm to 1000 µm, however the distinction between respiratory droplets and aerosols
, with this arbitrary cutoff has never been supported experimentally or theoretically.
Large droplets (larger than about 100 µm, but depending on conditions) fall to the ground or another surface before drying, but smaller ones fall slowly and dry so quickly that they usually become aerosolized
particles. Respiratory droplets can be produced naturally as a result of breathing, talking, sneezing, coughing, or vomiting, or can be artificially generated through aerosol-generating medical procedures
Since droplets can contain infectious bacteria
particles they are important factors in the transmission of respiratory diseases
Respiratory droplets from humans include various cells types (e.g. epithelial cells
and cells of the immune system), physiological electrolyte
s contained in mucous
), and, potentially, various pathogens
Droplets that dry in the air become droplet nuclei
which float as aerosol
s and can remain suspended in air for considerable periods of time.
The traditional hard size cutoff of 5 μm
between airborne and respiratory droplets has been criticized as a false dichotomy
not grounded in science, as exhaled particles form a continuum of sizes whose fates depend on environmental conditions in addition to their initial sizes. However, it has informed hospital based transmission based precautions for decades.
Respiratory droplets can be produced in many ways. They can be produced naturally as a result of breathing
, or singing
. They can also be artificially generated in a healthcare setting through aerosol-generating procedures such as intubation
, cardiopulmonary resuscitation
, and autopsy
Similar droplets may be formed through vomiting
, flushing toilets
, wet-cleaning surfaces, showering
or using tap water
, or spraying graywater
for agricultural purposes.
Depending on the method of formation, respiratory droplets may also contain salts
, and virus
In the case of naturally produced droplets, they can originate from different locations in the respiratory tract, which may affect their content.
There may also be differences between healthy and diseased individuals in their mucus content, quantity, and viscosity
that affects droplet formation.
Different methods of formation create droplets of different size and initial speed, which affect their transport and fate in the air. As described by the Wells curve
, the largest droplets fall sufficiently fast that they usually settle to the ground or another surface before drying out, and droplets smaller than 100 μm will rapidly dry out, before settling on a surface.
Once dry, they become solid droplet nuclei consisting of the non-volatile matter initially in the droplet. Respiratory droplets can also interact with other particles of non-biological origin in the air, which are more numerous than them.
When people are in close contact, liquid droplets produced by one person may be inhaled by another person; droplets larger than 10 μm tend to remain trapped in the nose and throat while smaller droplets will penetrate to the lower respiratory system
Advanced Computational Fluid Dynamics
(CFD) showed that at wind speeds varying from 4 to 15 km/h, respiratory droplets may travel up to 6 meters.
Role in disease transmission
A common form of disease transmission
is by way of respiratory droplets, generated by coughing
, or talking. Respiratory droplet transmission is the usual route for respiratory infections. Transmission can occur when respiratory droplets reach susceptible mucosal surfaces, such as in the eyes, nose or mouth. This can also happen indirectly via contact with contaminated surfaces
when hands then touch the face. Respiratory droplets are large and cannot remain suspended in the air for long, and are usually dispersed over short distances.
Viruses spread by droplet transmission include influenza virus
, respiratory syncytial virus
, and norovirus
such as SARS coronavirus (SARS-CoV-1)
that causes COVID-19
Bacterial and fungal infection agents may also be transmitted by respiratory droplets.
By contrast, a limited number of diseases can be spread through airborne transmission
after the respiratory droplet dries out.
and humidity affect the survivability of bioaerosol
s because as the droplet evaporates and becomes smaller, it provides less protection for the infectious agents it may contain. In general, viruses with a lipid envelope
are more stable in dry air, while those without an envelope are more stable in moist air. Viruses are also generally more stable at low air temperatures.
In a healthcare setting, droplet precautions include housing a patient in an individual room, limiting their transport outside the room and using proper personal protective equipment
Droplet precautions are one of three categories of transmission-based precautions
that are used in addition to standard precautions
based on the type of infection a patient has; the other two are contact precautions and airborne precautions.
However, aerosol-generating procedures may produce smaller droplets that travel farther, and so droplet precautions may be insufficient when such procedures are performed.
In general, higher ventilation
rates can be used as a hazard control
to dilute and remove respiratory particles. However, if unfiltered or insufficiently filtered air is exhausted to another location, it can lead to spreading of an infection.
can be used to prevent droplet transmission, both for infected patients
and healthcare personnel.
It has been noted that during the 2002–2004 SARS outbreak
, use of surgical masks and N95 respirators
tended to decrease infections of healthcare workers.
While surgical masks create a physical barrier between the mouth and nose of the wearer and potential contaminants such as splashes and respiratory droplets, they are not designed to filter or block very small particles such as those that transmit airborne diseases
because of the loose fit between the face mask and the face.
UK public-health-education poster.
German bacteriologist Carl Flügge
in 1899 was the first to show that microorganisms in droplets expelled from the respiratory tract are a means of disease transmission. In the early 20th century, the term Flügge droplet was sometimes used for particles that are large enough to not completely dry out, roughly those larger than 100 μm.
Flügge's concept of droplets as primary source and vector for respiratory transmission of diseases prevailed into the 1930s until William F. Wells
differentiated between large and small droplets.
He developed the Wells curve
, which describes how the size of respiratory droplets influences their fate and thus their ability to transmit disease.
*Basic reproduction number
*Source control (respiratory disease)