Covid-19: What epidemiologist’s CO2 readings reveal about public transport risk

Otago University epidemiologist Dr Amanda Kvalsvig took this CO2 reading on a Wellington Metlink bus - showing levels to be more than three times what would be considered suitable. Photo / Amanda Kvalsvig

An epidemiologist tracking CO2 levels around Wellington has found some readings within often-packed city buses to be three times over a basic threshold for risk of breathing other people’s air.

Otago University’s Dr Amanda Kvalsvig said what she’d recorded backed a “strong argument” to reinstate mask mandates on public transport – a measure health bosses had also asked to be retained before September’s Covid-19 roll-backs.

In crowded, stuffy places where infectious diseases were more likely to spread, Kvalsvig said CO2 – breathed out by people – gave us a good idea of how much air we were sharing with others.

“It’s what we call a proxy measure: it’s an indicator rather than a direct measure of infection risk,” she said.

“Air quality is much more complex than that single measure, but measuring CO2 can give you useful information to guide practical action.”

With Covid-19 infections having risen since the beginning of October – and fresh warnings that daily cases could peak at more than 11,000 a day over summer – there was all the more need to be thinking about indoor air quality.

“Measuring CO2 is also important in a different and more direct way: high CO2 levels can reduce alertness and focus, so that’s another reason to keep an eye on it.”

Kvalsvig, who’s been taking an Aranet4 monitor wherever she goes in the capital, said outdoor air generally had CO2 levels around 400 parts per million (ppm), while indoors, readings of up to 800ppm were considered good.

“Above that, you want to be taking action to refresh the air: the quickest way to do that is to bring outdoor air in, such as by opening windows or doors.”

In public transport like planes, trains and buses, however, this wasn’t so easy.

“Planes need good mechanical ventilation systems with filters to remove viruses and other particles,” she said.

“Buildings and buses can use mechanical ventilation too and in cold, wet weather or when air pollution is high it can be useful not to have to keep windows open.

“So, there’s no one-size-fits-all solution, it’s all about what works in a specific situation.”

What she’d found in some settings was worrying.

“I’m a Wellington bus user and when the bus is full the CO2 levels can get very high, easily above 2000ppm,” she said.

“That’s a concern both from the respiratory infection point of view and it’s also a concern for driver alertness.”

In new buses, most windows didn’t open, so there was little passengers could do but wear a mask.

“The mask won’t change the CO2 concentration but it will filter out viruses in that rebreathed air,” she said.

“There is clearly an urgent project of work to monitor, resource, and improve ventilation on buses.”

University of Auckland aerosol chemist Dr Joel Rindelaub agreed Kvalsvig’s CO2 readings were “really high” - and recommended passengers wear N95 masks to help lower their infection risk.

Wellington operators Metlink followed NZTA-set ventilation requirements for urban buses and trains, and was currently investigating CO2 levels across its fleet, general manager Samantha Gain said.

Metlink buses were fitted with climate air conditioning systems that recirculated cabin air from a filtered air intake designed to remove dust particles, she said.

“Some double decker EV buses have passenger windows because they do not have air conditioning due to the amount of power this would require.”

On aircraft, Kvalsvig said levels could be moderately high, with readings of about 1000ppm to 2000ppm.

“That’s not too much of a concern in-flight because the air in the cabin is well-filtered during that time,” she said.

“To me, the big concern is boarding and disembarking – lots of people close together and high CO2 levels.

“So, getting on and off the plane is probably the most important time to wear a mask.”

In cars, CO2 levels rose quickly if windows were closed and air was being recirculated.

As for other indoor spaces, she’d been pleasantly surprised to find many Wellington workplace and hospitality venues often had low CO2 readings.

“That’s another reason to monitor the quality: it tells you when things are going well, as well as identifying problems,” she said.

“Several places now, Belgium for instance, are introducing requirements for venues to display a CO2 reading on the wall in commercial venues like bars and gyms so people can make a decision about whether they’re happy to stay.”

In schools, which remained a high-transmission setting for Covid-19 a large variety of infections, she wanted to see these displays introduced in every classroom.

“It’s great science communication for kids to be able to see the CO2 level drop when they take appropriate action,” she said.

“It puts them in control of their environment and that’s reassuring for them.”

For teachers particularly, but also transport and healthcare workers, air quality was an occupational safety issue – and those three groups had faced the highest risk throughout the pandemic for infection, death and the myriad lingering symptoms that can come with Long Covid.

“That’s not surprising when you think that a common thread in these different occupations is being exposed to large numbers of infectious people in crowded, enclosed spaces,” she said.

"Clean indoor air is going to be a key protection for us, allowing us to continue our daily lives while staying safe,” Otago University epidemiologist Dr Amanda Kvalsvig said. Photo / Supplied

“A bus is not just a key service for moving people around, it’s also a workplace; and drivers are spending many hours in sub-optimal conditions.”

With reinfections now making up two in 10 reported cases – the true proportion is likely higher – she said Kiwis should be aiming to get infected as few times as possible.

“Clean indoor air is going to be a key protection for us, allowing us to continue our daily lives while staying safe,” she said.

“I’d like to see the Government providing guidelines and resources for clean indoor air, with specific protections for high-risk settings and occupations.

“The consistently high CO2 measurements I’ve recorded on buses further support the already very strong argument for reinstating a mask mandate on public transport.

“After healthcare, public transport is probably the next most critical setting where masks are most needed but least used.”

Asked about the need for masks on public transport yesterday, deputy director general of health Dr Andrew Old said official advice on the matter had been “consistent”.

“We still believe that there is good evidence that masks reduce transmission - they protect you from catching Covid-19 - but also, they protect those around you.”

Old said that when people were in crowded, poorly-ventilated settings – buses, particularly - “then yes, we think masks are a good idea”.

Earlier this week, it emerged that Old had recommended masks be retained on public transport – with a risk that removing requirements could increase hospital admissions – before Cabinet decided in September to roll back mandates.

Covid-19 Response Minister Ayesha Verrall told Newsroom that Cabinet had to consider all advice, and that fatigue with mandates could jeopardise efficacy of mask requirements if they had to be introduced.

Yesterday, she told Parliament’s Health Select Committee that mask wearing would be a “logical next step” if the Government had to scale up health measures in response to rising case numbers.

While the Government has scrapped many of its Covid-19 legal powers – including the ability to implement lockdowns and vaccine mandates – it’s retained mandatory seven-day isolation periods and mask orders in certain settings like healthcare.

“We have been clear that the measures used to contain the spread of Covid-19 need to be proportionate to the risk that the virus currently poses,” Verrall told the committee.