The size of the droplet / aerosol is critical
The airborne virus is not naked and the size of the carrier drop or aerosol defines transport: how long it stays afloat, how far it can travel, how quickly it falls to surfaces, where it deposits in the respiratory system and how efficiently it is removed by masks and filters. Physics is the same for all viruses.
Many visualisations of the of coronavirus are incorrect. Fallacies include the aerosols looking too small relative to the virus, looking like water and virus only and the mass fraction of the virus being very high. More correct is few microns per aerosol, resembling mucin, NaCI, water and a sprinkle of virus, and the mass fraction of the virus being very low.
Droplets versus aerosols
Droplets have traditionally been defined as >5 µm in size. Regarding particles settling in still air, a particle of 0,5 µm takes 41 hours, while a particle of 100 µm only takes 5,8 seconds. Speaking produces 100x more aerosol than droplets. Droplets / aerosols inside the body and outside the body are different sizes, which is important for ventilation.
- Inside the body: Respiratory vs non-respiratory > 5 µm URT < 5 µm LRT
- Outside the body: Droplets, physics-based cut off 60-100 µm
- Sprayed: Ballistic drops > 100 µm, direct hit on eye nostril or mouth
Large droplets are sprayed onto the body, and are a form of contact transmission. Aerosols are inhaled into the respiratory system. The primary transmission mechanism of Covid-19 is through aerosol, not large droplets. Measles has an R0 (the reproduction number – how contagious an infectious disease is) of 15; Covid-19 has an R0 of 5,7 and flu has an R0 of 1.3. Covid-19 does not spread between floors on multistorey buildings.
Measles is a high-contagiousness aerosol-driven disease. Covid-19 is likely a lower-contagiousness aerosol driven disease. It infects best at close proximity, also at the room scale if we “help it along” (indoors, low ventilation, long time, no masks). And it has trouble infecting at long range.
Virus concentration and ventilation
Ventilation referred to here can be defined as the designed supply and removal of air to and from a treated space. The particles generated by respiratory activities are small enough to stay suspended in the air for a long time, unless they are removed from the air by ventilation (and other processes).
The ventilation standard in SA is SANS 10400 O (2011): Buildings either naturally ventilated (4.3.1) or artificially ventilated (4.3.2). Two air changes per hour in buildings allows comfort and prevents body odour – the building interior doesn’t smell stuffy. Twelve air changes per hour in buildings prevents the transmission of HBA influenza and Covid-19.
There are three fundamental approaches to natural ventilation:
- Wind-driven cross ventilation (preferred)
- Buoyancy-driven stack ventilation, and
- Single-sided ventilation
With no artificial ventilation system present, the only ventilation possible is through opening windows / doors. This tends to present cross-contamination and no dilution of the virus. With a split of a unit air-conditioner and mechanical ventilation, there is no cross-contamination and positive dilution of the virus. Here, the supply of outdoor air is provided to each room, and is extracted and expelled outdoors. Virus concentration is reduced through dilution with provided outdoor air.
Regarding atmospheric pollution, the is no one size fits all solution: with higher atmospheric pollution, there is more reliance on filters and with lower atmospheric pollution, there is more reliance on outdoor air.