Ventilation a key to controlling Covid spread, expert says

Increasing airflow in buildings and using CO2 monitors recommended to curb spread

TCD microbiologist Dr Kim Roberts shows a carbon dioxide monitoring device which can help people assess ventilation quality in a space. Photograph: Enda O’Dowd

TCD microbiologist Dr Kim Roberts shows a carbon dioxide monitoring device which can help people assess ventilation quality in a space. Photograph: Enda O’Dowd


As opening up for indoor dining and hospitality in some form is in the offing, ventilation becomes a key infection control weapon, according to air pollution expert Prof John Wenger.

And deploying a simple carbon dioxide (CO2) monitor while increasing airflow through buildings is an effective way of curbing Covid-19, he believes.

“Ventilation reduces the risk of spreading Covid-19 because it flushes the virus out of the indoor air,” said Prof Wenger, who is based at UCC school of chemistry. In a poorly-ventilated area the virus can build up, just like cigarette smoke can linger, so ventilation clears the amount of airborne virus and is an effective way to reduce transmission.

While it’s not a foolproof system to remove the virus, it is an effective tool, especially in poorly-ventilated indoor spaces where people gather, where “you don’t know [the risk] if you don’t measure it”.

As reopening of businesses and workplaces moves forward, employers, employees and customers should make sure all occupied indoor spaces are well ventilated, he added – and, ideally, they should be able to avail of a basic CO2 monitor with a clear display of ongoing levels.

“CO2 is a good indicator of how fresh the air is inside a room because we all breathe it out,” he said.

The successful completion of one month’s air monitoring at Nora Cronin’s Head to Toe salon on Princes Street, Cork, illustrates the benefits. The monitor was placed in the middle of an unused station, one to two metres from salon chairs in his friend’s premises. He gave Cronin simple advice: keep a window open and increase ventilation if the CO2 level rose above 800 parts per million (ppm).

Results from day one showed levels rose to about 800 ppm even with a window open. “When Nora noticed this she opened the door slightly and the CO2 dropped rapidly. This shows the importance of cross-ventilation for clearing the air. Open doors and windows that are opposite to each other to ensure a flow of air.” Preferably the open window opposite another window or door should be through the middle of a room where people are located.

“This simple demonstration shows how CO2 monitors can be used to inform occupants when they should increase ventilation by opening more windows/doors.”

He used an Aranet4 monitor in these tests, which costs €200 directly from the manufacturer. “I like it because it uses a reliable nondispersive infrared sensor (NDIR), has a clear display, decent app and is portable. There are many other NDIR sensors on the market,” he outlined on Twitter.

A monitor that uses a NDIR sensor is best. They are more expensive “but the cheaper ones just don’t do a good job because they do not measure CO2 directly”.

Delta variant

As the more transmissible Delta variant becomes dominant, it is “important to use as many different methods of reducing transmission as possible, including CO2 monitors”, said Dr Kim Roberts, assistant professor of virology at Trinity College Dublin.

“As we reopen places like restaurants we need to think really carefully about how we reduce risk, because this variant is not the same as the virus we had last summer.

“Increasing ventilation and being confident of that by using CO2 monitors can be part of that but it’s not a magic bullet and doesn’t replace the need to wear masks, limit numbers and keep tables two metres apart,” she added.

While monitors play a helpful role, studies indicate there is continuing risk where transmission occurs via short-range aerosols, for example, over the one-metre “garlic breath” conversational distance. This is because the virus is exhaled/inhaled very quickly – much too quickly for environmental parameters to impact much on the airborne virus survival.

So measures to reduce interpersonal contact and crowding (face masks and social distancing) continue to be important. This is especially the case in a scenario where the Delta variant is expected to account for 90 per cent of all coronavirus cases in the European Union by the end of August.

This is also partly why, despite being on well-ventilated aircraft (with ventilation rates of as much as 30 air changes per hour), interpersonal aerosol transmission can still – though more rarely – occur over short conversational distances of less than a metre, because exhalation/inhalation of these airborne viruses occurs too quickly for the ventilation to remove them completely from people’s breathing zones.

With air conditioning, the general advice has been to set air-conditioning systems to 100 per cent fresh air – not easy in a scenario where modern buildings are increasingly sealed more tightly to avoid energy/heating losses. In this scenario, Prof Wenger noted, “recycling air is a ‘no no’ as recirculation is not ventilation”.

Where there is mechanical ventilation with intake of air, effective circulation is possible, and there are good guidelines for facilities/building managers on what to do.

In reducing airborne transmission in spaces with insufficient ventilation, use of high-efficiency particulate air (HEPA) filters are an easy-to-apply and cost-effective short-term mitigation measure, he said.

The Department of Education said in May it would arrange for the provision to schools of CO2 monitors “over the coming months”.

Spain requires mandatory monitoring and display of CO2 in bars, gyms and other buildings, while Belgian authorities recently followed suit. This provides reassurance to users, staff and employers, Prof Wenger underlined. It is “a very simple and actionable thing to do”. The case for Ireland doing likewise is clear-cut.

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