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Belgium has mandated carbon dioxide monitors in certain venues to help fight COVID – but how useful are they?

The Belgian authorities recently made it mandatory for people who manage hotels, restaurants, bars, banquet halls and fitness centres to monitor carbon dioxide levels at their venues. You might think this has something to do with climate change, but carbon dioxide is a proxy measure for how well-ventilated a room is and hence for how much coronavirus is floating in the air.

People emit carbon dioxide when they breathe and, all other things being equal, the more people there are in a room, the higher the carbon dioxide concentration. If the windows and doors in an occupied room are closed, carbon dioxide concentrations steadily rise. And if someone in the room has COVID, so will the concentration of coronavirus particles (virions).

Outdoor carbon dioxide concentrations now exceed 415 parts per million (ppm) but are often higher in urban areas with lots of traffic. Indoor carbon dioxide concentrations are similar to those outdoors in a well-ventilated, empty room, but tend to increase with occupancy.

Research shows that cognitive ability – the ability to think and reason – is affected by increases in carbon dioxide concentrations. Therefore, an indoor carbon dioxide concentration of around 1,000 ppm is often used as a proxy to ensure that ventilation in a room or building is sufficient. So there is more than one reason to keep the windows open.

Twitter is rife with images of carbon dioxide concentrations posted by people as they eat in restaurants, board planes and so on.

It’s not that straightforward

However, a recent report highlighted problems with using carbon dioxide concentrations to judge how good the ventilation in a room is and hence the risk of coronavirus transmission. Low carbon dioxide concentrations in large volume or low occupancy spaces do not necessarily mean that ventilation is enough to mitigate the risk of coronavirus transmission.

People emit different amounts of carbon dioxide depending on their age, sex, health and what they’re doing (talking, singing, doing a Zumba class). The amount of virus an infected person sheds can also vary from person to person by several orders of magnitude. With more than 20 occupants in a room, carbon dioxide concentrations correlate reasonably well with ventilation rates. But with fewer people in a space, individual differences in terms of carbon dioxide or viral emission might become more important.

If occupants are singing, shouting or exercising, lower carbon dioxide concentrations might be needed to reduce the risk of transmission compared with a space with more sedate activities. For instance, a well-documented case of a choir practice in the US led to 45 out of 60 singers contracting coronavirus, indicating significant transmission of the virus. This particular outbreak was fairly early on in the pandemic when aerosol transmission of the virus was less well recognised.

The choir members avoided hugging and contact, bought their own sheet music and used hand sanitiser. However, singing can release 20 times more aerosols than more sedentary activities, and the virus spread very efficiently during the 2.5-hour session.

Cooking releases carbon dioxide, but obviously not the virus. And filtration may remove the virus but not carbon dioxide. There also needs to be careful consideration of where to measure carbon dioxide concentrations, especially in a large space. Such measurements need to be representative for all the occupants, so that sensible assessments of ventilation requirements can be made. So there is no reliable carbon dioxide cut-off point that can encompass all situations – people need to consider individual conditions.

The pandemic has highlighted the topic of indoor air quality and the need for adequate ventilation in buildings. There is no doubt that checking indoor carbon dioxide concentrations is a useful guide for whether the ventilation is effective enough to make transmission of coronavirus less likely. But it is important to consider the issues mentioned above, such as the number of people in the room, the room volume, other activities within the space (such as cooking) and how representative the carbon dioxide measurements are.

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