How far can SARS-CoV-2 travel into the airways?

SARS-CoV-2 is transmitted by aerosols – tiny particles delivered through coughing, sneezing and talking that remain suspended into the air. When we inhale, these aerosols are ingested into our airways, carrying COVID in with them.

A team from the University of Technology (UTS) Sydney has for the first time applied a mathematical model to understand how far into the lungs SARS-CoV2 can travel. The study was recently published in Physics of Fluids.

They found that when we inhale isolated coronavirus particles, more than 65% reach our lungs’ deepest region. Here, a significant amount of virus, along with inflammation caused by our immune system response, can cause severe damage to cells, leading to low blood oxygen levels and increasing chances of death.

The team also found that more of these aerosols reach the right lung than the left. This difference can be explained by the asymmetrical anatomical structure of the lungs, and the way air flows through the different lobes. This result is in line with what has been observed in the clinic. Chest CT scans of COVID-19 patients show more significant infection in the regions predicted by the model.

“Our lungs resemble tree branches that divide up to 23 times into smaller and smaller branches,” says the study’s lead author, Dr Saidul Islam of UTS Sydney. “Due to the complexity of this geometry, it is difficult to develop a computer simulation. However, we were able to model what happens in the first 17 generations, or branches, of the airways.”

The researchers used a mathematical model that simulated three different airflow rates – sleeping (7.5 litres/minute), light activity (15 l/min) and heavy activity (30 l/min). The model revealed that a higher percentage of the virus particles are trapped in the upper airways at lower flow rates. If you inhale SARS-CoV2 during heavy activity, more of it would travel further into the deepest part of the lungs.

“Depending on our breathing rate, between 32% and 35% of viral particles are deposited in these first 17 branches. This means around 65% of virus particles escape to the deepest regions of our lungs,” says Islam.

These findings can inform the development of targeted drug delivery devices, which can deliver antivirals to the areas of the respiratory system most affected by the virus.

“Normally, when we inhale drugs from a drug delivery device, most of it is deposited in the upper airways, and only a minimum amount of drugs can reach the targeted position of the lower airways,” Islam says. “However, with diseases like COVID-19, we need to target the areas most affected.”

The group is now working on building age- and patient-specific lung models and developing drug delivery devices that can target specific regions of the lungs.

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