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Early research suggests COVID-19 virus loses 90 per cent of its infectivity after 20 minutes in air

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A new pre-print study has offered some preliminary evidence suggesting the virus that causes COVID-19 loses most of its infectiousness after 20 minutes in air.

The not-yet-peer-reviewed study out of the United Kingdom measured the stability of SARS-CoV-2 in aerosol droplets between five seconds and 20 minutes.

After 20 minutes, the researchers say the infectivity of SARS-CoV-2 dropped to about 10 per cent from where it began, with a large portion of that loss occurring within the first five minutes of being aerosolized.

The article is a pre-print and has not been peer-reviewed, meaning it hasn't been evaluated by the medical community for any potential errors or inaccuracies.

The researchers say the data suggests dry air may help limit overall exposure to SARS-CoV-2, although further research is required to confirm this, as well as the possible effect of pH and CO2 levels.

The study also based most of its measurements on versions of SARS-CoV-2 isolated early in the pandemic, including Alpha. However, it found no significant difference between the three variants that were used.

"While the current general consensus is that the half-life of SARS-CoV-2 in the aerosol phase is between 1-2 hours, if not longer, we report an initial rapid decline in infectivity within a few seconds to minutes of aerosol generation," the study's authors write.

"Under all conditions measured, the majority of SARS-CoV-2 is inactivated within 10 minutes of aerosolization. Further research is required to determine for how long the remaining fraction persists and how this may depend on the viral load in the aerosol."

The transmission of droplets containing SARS-CoV-2 has led to continued discussions around ventilation, masking and social distancing, with reports varying on how long the virus can remain aerosolized or suspended in the air.

The amount of time SARS-CoV-2 can survive has been examined since the early days of the pandemic.

The Public Health Agency of Canada says SARS-CoV-2 spreads from an infected person to others through respiratory droplets, which can range in size from large droplets that fall to the ground within seconds or minutes to small ones, sometimes called aerosols, which linger in the air especially indoors.

Information published by U.K. Research and Innovation in May 2020 states that SARS-CoV-2, like many respiratory viruses, mostly spreads between people through small droplets released from the nose or mouth of an infected person.

It said at the time that the half-life, or amount of time it takes for 50 per cent of the virus to no longer be infectious, in droplet aerosols was just over an hour, with some surviving for three hours or more.

By spring of 2021, scientists and doctors had increasingly come to the conclusion that aerosols are the primary mode of transmission for SARS-CoV-2.

The latest U.K. study used a technique called CELEBS, or Controlled Electrodynamic Levitation and Extraction of Bioaerosols onto a Substrate, which involves suspending particles containing the virus under controlled conditions. The particles are later plated onto a cell culture.

The researchers found the infectivity of SARS-CoV-2 at low relative humidity, or 40 per cent, fell almost immediately to an average of 54 per cent within five seconds.

This was followed by a period of relative stability, decreasing to an average of 19 per cent after five minutes.

Under high relative humidity conditions of 90 per cent, infectivity after aerosolization fell more gradually to 48 per cent within the first five minutes.

In both scenarios, infectivity appeared to plateau after 10 minutes before reaching similar points by the 20-minute mark.

The researchers say the data is consistent with the view that the virus primarily spreads over short distances.

"The rapid loss of infectivity demonstrated in these measurements provides an alternative explanation for a short transmission distance, with rapid airborne losses of viral infectivity making transmission decreasingly likely as distance from the particle source is increased, even if the particles that contain the virus are small and able to travel long distances," the study says.

On CO2, the researchers say elevated levels are a sign that a space is densely occupied and poorly ventilated, possibly creating conditions where SARS-CoV-2 is more stable in the air. They suggest that CO2 monitors could be valuable in assessing the relative risk of an indoor environment.

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