Bigger risk of contracting COVID-19 indoors with high carbon dioxide levels

Bigger risk of contracting COVID-19 indoors with high carbon dioxide levels
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New research has highlighted the importance of tracking carbon dioxide levels indoors for monitoring the risk of contracting COVID-19.

New research from the Cooperative Institute for Research in Environmental Sciences (CIRES) and the University of Colorado Boulder has shown how tracking the levels of carbon dioxide indoors is an inexpensive and effective way of monitoring the risk of people contracting COVID-19. The team says that, in any given indoor environment, when excess carbon dioxide levels double, the risk of transmission also roughly doubles.

The research has been published in Environmental Science & Technology Letters.

Indoor CO2 levels

It has been long known that infectious people exhale airborne viruses at the same time as they exhale carbon dioxide, meaning CO2 can serve as a “proxy” for the number of viruses in the air.

For many months, researchers around the world have been searching for a way to continually monitor COVID-19 infection risk indoors, whether in churches or bars, buses, or hospitals. Some are developing instruments that can detect viruses in the air continually, to warn of a spike or to indicate relative safety. Others tested existing laboratory-grade equipment that costs tens of thousands of dollars.

Jose-Luis Jimenez, co-author of the new assessment, a CIRES Fellow and professor of chemistry at the University of Colorado Boulder, and colleagues confirmed in the laboratory that, commercially available carbon dioxide monitors, which can cost just a few hundred dollars, were accurate. The team then created a mathematical “box model” of how an infected person exhales viruses and CO2, how others in the room inhaled and exhaled, and how the viruses and gas accumulate in the air of a room or are removed by ventilation.

The model takes into consideration infection numbers in the local community, but it does not detail airflow through rooms, which would require expensive, custom analysis for each room.

It is important to understand that there is no single CO2 level at which a person can assume a shared indoor space is “safe,” emphasised Zhe Peng, a CIRES and chemistry researcher, and lead author of the new paper, because activity, such as singing or exercise, for example, matters.

However, in each indoor space, the model can illuminate “relative” risk. If CO2 levels in a gym drop from 2,800 to 1,000 ppm (~2,400 above background levels to 600), the risk of COVID-19 transmission drops to one-quarter of the original risk. In the library, if an influx of people makes CO2 jump from 800 to 1,600 (400 to 1,200 above background), COVID transmission risk triples.

In the new paper, the researchers have shared a set of mathematical formulae and tools that experts in building systems and public health can use to pin down actual, not just relative, risk.

“Wherever you are sharing air, the lower the CO2, the lower risk of infection,” Jimenez said. The most important conclusion is that to minimise risk, keep the CO2 levels in all the spaces where the air is shared low as practically possible.


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