COVID-19: What is droplet transmission?

Respiratory droplets are tiny fluid droplets that your body releases into the air every time you talk, sneeze or cough. These droplets can be of different sizes. Larger droplets - those with a diameter of 5 to 10 micrometres - are called respiratory droplets and smaller droplets - those with a diameter less than 5 micrometres - are called droplet nuclei. 

The size of a droplet depends on the viscosity (thickness) of the fluid and the speed at which it is exhaled. Studies show that a single sneeze may produce about 40,000 droplets of 0.5 to 12 micrometres. These droplets can travel at a speed of 100 meters per second. Coughing, on the other hand, produces 3,000 droplet nuclei at once and talking for 1 minute can produce about 600 droplet nuclei. 

The volume of droplets and their composition varies from person to person and between various phases of the disease. For example, those who don’t yet have antibodies against a said virus have a higher viral load in their respiratory droplets than those who have generated antibodies already. 

Read more: How long does COVID-19 survive on surfaces

Studies show that respiratory droplets greater than 5 micrometres in diameter do not travel large distances and may fall on the ground before covering a distance of 1 meter. However, droplet nuclei remain in the air for prolonged periods of time and can travel large distances depending on the air pressure in a room. This makes droplet nuclei the major mode of transmission of the microbe through the air.

An article published in the New England Journal of Medicine in 2020 suggests that droplet nuclei from a sneeze, when trapped in the air, can travel up to a distance of 8 meters.

Equipment like fans may change the air current and could spread the air from the patient to even farther distances.

Read more: What is physical distancing and how is it different from social distancing

According to one study, the COVID-19 virus can stay in the air for up to 3 hours. However, the WHO had said that the findings of this study were obtained in lab settings and not from hospitals where aerosol-generating procedures are done.

Aerosols refer to suspensions of fine particles (sold/liquid) in the air - respiratory droplets or droplet nuclei are a type of aerosols when they are suspended in the air. In clinical settings, a lot of procedures increase the risk of aerosol generation and hence spread of infection. This includes tracheal intubation (putting tubes in the trachea/windpipe for oxygen therapy), manual ventilation and non-invasive ventilation (using face masks).

Now, a recent study published in the journal Emerging Infectious Diseases indicated that SARS-COV-2 may travel up to 4 meters in aerosols (in the air and as droplets on surfaces) in hospitals. The virus was found in ar vents and in all the areas downstream of the patients. However, it was also found in the doctor's office too, an upstream area. The study further suggested that the virus was found on floors and could travel through the shoes of healthcare practitioners. It is also present on computer mice and trash cans and on the bed rails of the patients. However, since the study was done in a small setting, the authors concluded that the scenario might be a bit different in bigger settings with more COVID-19 patients and more studies are needed to confirm the findings of this study.

The Centers for Disease Control and Prevention, US, recommends healthcare practitioners to be in personal protective equipment (PPE) when performing any aerosol-generating procedure or perform them in a negative pressure room. Negative pressure rooms are specialised rooms where the pressure is maintained so that the aerosols do not travel to adjacent rooms. The ventilation and exhaust in the room can also be adjusted so the air moves from the healthcare practitioner to the patient and not the other way round. (Read more: 10 steps to put on PPE for medical professionals)

A recent study suggested that hand hygiene along with masks and other PPEs can reduce the risk of transmission of COVID-19 to healthcare practitioners even when exposed to aerosol-generating procedures for at least 10 minutes. However, more studies are needed to understand how much of a risk aerosols pose when it comes to COVID-19 transmission. 

Read more: COVID-19 prevention tips for healthcare practitioners