Covid-19 aerosol issues to solve…

Understanding how Covid-19 airborne contamination or any other airborne tranmission of respiratory virus airborne remains infectious and floats for hours or days in air is vital to increase the biosafety of public facilities.

Understanding the mechanism of diffusion of respiratory viruses in air, and receiving know-how to collect and detect them, will help reduce the risk of airborne transmission through ventilation system and/or aeraulics of premises.

What serious event in 2003 revealed that risk of airborne contamination of respiratory viruses and coronavirus in particular in buildings was not to be taken lightly?

SARS Coronavirus aerosols vectors of contamination in 2003.

Amoy gardens, where SARS airborne transmission by dominant winds to neighbouring buildings was discovered.

Respiratory emissions source of Covid-19 airborne contamination.

It took the Covid-19 pandemic to understand the role played by respiratory emissions from symptomatic or asymptomatic Sars-Cov-2 infected persons.

A significant part of the respiratory particles, emitted during breathing, speaking, singing, sneezing, coughing, and screaming produce micron or sub-micron emissions, named that remain suspended in the air for a very long time. But for how long?

A video from Japanese television NHK takes us through these airborne particles and answers this question.

Covid-19 airborne contamination floating in air for hours in confined environment.

This video Japanese television NHK video with English subtitles highlights the microparticles emitted during a conversation or during a sneeze that can be contaminated by Covid-19 (source video NHK Japan)

The aerodynamic size of most particles emitted during breathing varies between 0.1 microns and 10 microns (1). Micro-droplets present in an airborne Covid-19 contamination, contain water molecules that evaporates very quickly. This evaporation rate of water considerably reduces the size of these micro-droplets transforming them into “droplets nucleus“. These particles can float for hours or even days in the air stream and become important vectors of contamination.

But can these respiratory emissions remain contagious for long? Let’s see what some of the scientific studies reveal.

Covid-19 virus surviving in aerosols from several hours up to several days in a confined environment.
Covid-19 aerosol survival tested up to 3 hours
Even after 3 hours Covid-19 (Sars-Cov-2) remains infectious and survives even better than Sras (Sars-Cov-1) in aerosols.
les coronavirus peuvent rester infectieux pendant des jours dans l'air
Sars-Cov-2 can remain infectious for several days in a confined environment.

We lacked data on the survival rate of Covid-19 in aerosols, but a US study published by virologists confirms that Sars-Cov-2 coronavirus retains viability in air. However, tests for survival of Covid-19 virions in aerosols were stopped after 3 hours, but coronaviruses are able to survive much longer (2).
For more details read our LinkedIn analysis of results.

Respiratory viruses have been minimized for a long time, yet their monitoring is now important. Which respiratory viruses besides Sars-Cov-2 variants should also be monitored?

Other respiratory viruses in biological aerosols.
virus influenza
Influenzae virus
Rubella virus
varicella virus
Varicella zoster virus
Human metapneumovirus
Measles virus
Respiratory syncytial virus
Mumps virus
Human para-influenza virus

But what allows Covid-19 and other respiratory viruses to remain suspended in air for so long without sedimentation? 3 factors will influence the buoyancy of respiratory emissions.

3 factors allowing viral respiratory emissions to float in air.

First factor: airflow velocity: may counteract the sedimentation rate of the respiratory particle and prolong its residence time in air.

Second factor: the aerodynamic diameter of respiratory particles: the smaller the diameter, the longer particles remain suspended in air or even forever.

Third factor: hygrometry and temperature can accelerate the evaporation of water molecules inside the respiratory particles. This evaporation considerably reduces the size and weight of these emitted particles transformed into “droplets nucleus.

But what are these “droplet nucleus”, and what are their role in respiratory aerosols?

Droplet nuclei main vectors of Covid-19 airborne contamination.
What is a droplet nuclei?

Droplet nuclei are micro droplets which due to evaporation have dried up and reduced its original volume to become a very tiny particles (ranging usually between 0.1–5 µm diameter).

How are respiratory droplet nuclei contributing to airborne contamination?

Human respiratory emissions released by breathing, talking,  sneezing, coughing, shouting and singing  are microdroplets  containing water molecules, saliva proteins, salts and other components, as surfactants.

Depending on environmental conditions these respiratory droplets loose very quickly their water molecules, reducing drastically their aerodynamic diameter and their weight …

These shrinking due to evaporation contributes very much to maintain these droplet nuclei in suspension in air, and can be spreading at long distance, depending on the indoor air flow speed or on the outside wind speed, as shown on the left.


TalibDbouk - Own work human cough: saliva droplets disease-carrier particles may travel in air medium to unexpected considerable distances depending on the environmental conditions. CC BY-SA 4.0Created: 18 May 2020
What are the mean droplet nuclei diameters measured during Sars-Cov-2 airborne contamination in Wuhan hospitals?
submicron droplets nuclei size distribution of Covid-19
The aerodynamic size of Covid-19 (Sars-Cov-2) in some contaminated areas of a Wuhan hospital was mostly less than a micron diameter.

 Aerodynamic diameter of Sars-Cov-2 droplet nucleus are smaller than 5 microns. More precisely, most of these droplet nucleus may be smaller in diameter than one micron, and others around 0.1 micron. Collecting these small sizes in the air is a real challenge!

But what will contribute to the survival of viruses and Sars-Cov-2 in particular, inside a “droplet nucleus”?

4 factors increase or reduce the survival of respiratory viruses in air.

The wet gas puff emitted with respiratory tract emissions.

Hygrometry of ambient air.

Air temperature

The organic components inside the respiratory particle

If these factors promote the survival and buoyancy of respiratory viruses in air, what is the point of being able to collect them in aerosols?

Why respiratory viruses air sampling makes sense...

In confined environments, such as hospitals, nursing homes, schools, office buildings, airplanes or cruise shipsair sampling of Covid-19 aerosols makes sense, saving days for prophylactic measures and preventing the spread of contamination clusters.

However, which air sampling technology is fitted for the collection of viral bioaerosols in indoor air? How to find out?


We can only detect what we are able to collect...

This is particularly true, when it comes to collect Covid-19 coronavirus and other respiratory viruses from air.

Respiratory viruses cannot be detected in aerosols if we do not know which air sampling technology to use to efficiently sample them.

Therefore, it is vital to understand which technical requirements aerosol samplers must meet, so as to efficiently collect Sars-Cov-2 (Covid-19) and other airborne respiratory viruses present in air.

Let us take a look at some of these air sampler specifications.

5 requirements to be met by air samplers to collect respiratory viruses.

Which biological air samplers meet these requirements?

Find out also

Scientific bibliography

  1. Documentary Research of Human Respiratory Droplet Characteristics, Hualing Zhanga et al, Procedia Engineering 121 ( 2015 ) 1365 – 1374
  2. Aerosol and surface stability of HCoV-19 (SARS-CoV-2) compared to SARS-CoV-1 Neeltje van Doremalen et al. medRxiv 2020.03.09.20033217
  3. Survival Characteristics of Airborne Human Coronavirus 229E,  K. IJAZ et al, J. gen. Virol. (1985), 66, 2743-2748
  4. Aerodynamic characteristics and RNa concentration of SARs-CoV-2 aerosol in Wuhan Hospitals during Covid-19 outbreak , Liu. Y et al, bioRxiv, 2020.