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A photograph from 1940, taken for infectious research purposes at the Massachusetts Institute of Technology, shows respiratory droplets released through sneezing.  This proves how COVID-19 spreads through respiratory droplets.


Anyone making conclusive claims about how COVID-19 spreads is very responsible.  This global pandemic cannot be talked about lightly. 

COVID-19 is such a new disease that none of the virologists, research centres and other health professionals are 100% confident of the science behind it:

  • how easily it spreads,
  • what routes it takes,
  • what long term impacts it has on victims, and the list goes on.

So, the purpose of this article is to cover what the research centres and medical professionals DO know and what this POTENTIALLY means.


How does COVID-19 spread?


Scientists agree respiratory droplets are the main route of transmission. 

Respiratory droplets are things such as mucus and saliva, that are expelled when an infected person sneezes or coughs. These droplets travel through the air for a short time, before settling on another human or surface. You are likely to be infected by these droplets if you are within 2 metres of the sneeze or cough. 



Anatomy of airways and where droplets can end up, depending to their size and what droplets are blocked by what masks.  This shows how COVID-19 spreads.


If the droplets do fall to a surface, these can remain stable for up to 72 hours (The New England Journal of Medicine, 2020).  Another person can pick them up during this time and then infect themselves by touching their face.  This transmission is referred to as contact routes.

Airborne transmission is the route not everyone agrees on.  This is when the virus is aerosolised and these small particles are breathed in by others, infecting them.  Aerosolised means tiny particles are dispersed through the air.  And I mean tiny.  The ultramicroscopic kind.  Aerosols come from breathing; they are much smaller than the spit that comes from a cough. 

Health professionals suspected COVID-19 to be able to spread by air right from the start.  This is because the COVID-19 strain is very similar to SARS-Cov-1, which had major outbreaks from airborne transmission.


So, the big controversy. Is it airborne?


Short answer.  YES.

The NEJM’s research shows COVID-19 aerosols remaining stable and infectious in the air for up to 3 hours.

Aerosolisation is not the most main way COVID-19 spreads. However, research suggests that it could be the most dangerous; aerosols are much smaller than droplets and therefore travel further into the lungs, causing a more severe infection in the respiratory system.


Droplet larger than aerosols, when exhaled (at velocity of <1m/s), evaporate or fall to the ground less than 1.5 m away. When expelled at high velocity through coughing or sneezing, especially larger droplets (> 0.1 mm), can be carried by the jet more than 2m or 6m, respectively, away.


This transmission route is unlikely to occur outside of health care settings because a sick person needs to breathe right on you to infect you.  However, in health care facilities, where the victims are already very infectious, airborne transmission is likely.  This is due to activities that could aerosolise the virus, such as:

  • intubation
  • changing bed sheets
  • connecting/disconnecting ventilators

A study of COVID-19 hospital rooms found that “swabs taken from the air exhaust outlets tested positive, suggesting that small virus-laden droplets may be displaced by airflows and deposited on equipment such as vents.”   This evidence supports the NEJM’s research.

A question yet to answer is ‘what quantity of virus is required to infect another human?’ This will help further understand how valid airborne transmission is.


What is required to stop the spread of COVID-19?


3 actions should be taken: social distancing, disinfecting, and air purification.

  1. Social Distancing: This is MOST important in public settings as it stops the easiest way of spreading COVID-19: respiratory droplets from human to human.
  2. Regular Disinfecting: Keeping surfaces clean of the respiratory droplets is very important as COVID-19 can remain stable on surfaces for up to 3 days. Disinfecting is important as this is the only process that kills ALL organisms on the surface, including virus droplets.
  3. Air Purification: This is especially important in health care facilities, where the virus is likely to be aerosolised. There is little point have 100% pure surfaces, if the air is infectious.


So, what do we know?


  • COVID-19 is most likely to be spread between humans one of three ways (Respiratory Droplets, Contact routes and Airborne transmission).
  • The simplest route of transmission is human to human via respiratory droplets.
  • The most dangerous route is through the breathing in of aerosolised virus particles.
  • To combat this virus most effectively, a 3-part action plan is required:
    1. Social Distancing wherever possible
    2. Disinfecting all surfaces regularly
    3. Air purification (the most commonly overlooked)



About the Author:

Denver Prestidge is a fresh Industrial Hygiene and Air Quality apprentice for Presco Environmental.

Focused on sourcing and developing innovative, industry leading products for New Zealand’s food producers, Denver is always looking for ways to challenge and improve the status quo.

If you would like to get in touch with Denver, feel free to reach out at any time – denver@presco.co.nz



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