Cram lots of people into a small space and CO2 will rise. Buses, trains and ferries rated poorly in RNZ's tests. The number of passengers made a difference; the lowest reading of 884ppm was taken in a bus with only five passengers.
The worst reading RNZ recorded, of 5737ppm, was "absurdly high", says Rindelaub. This was taken on that standing-room only double-decker bus.
"That is ridiculous and would be high risk for COVID transmission."
Masks can reduce the particles you spread into a space and reduce the particles you breathe in. Legally, they must be worn on public transport at the orange level, but transport companies are generally not enforcing this rule.
Rindelaub says if he was getting on a bus he would be opting for the best possible mask, such as an N95, and wearing it correctly. Wearing a mask beneath your nose is "pretty silly," he says.
"We have decent evidence that Covid actually attacks you in your nasal passages. That's where Covid starts to infect you. So if you don't have it over your nose, you're not really helping anybody out."
In comparison to the buses, the highest CO2 reading recorded on a train was 2430ppm over three trips. The highest on a ferry was 1514ppm taken on a single trip.
Number of readings: 21. Median: 3160ppm. Percentage of readings above 800ppm: 100 percent.
Personal responsibility and public health interventions
On the standing-room only bus, my risk of getting Covid-19 from the soup of particles in the air was reduced by a good quality mask snugly fitted to my face.
Rindelaub encourages everyone to take personal responsibility and wear the best possible mask to reduce risk.
But he's also keen to see priority given to a longer-term public health measure: better indoor ventilation.
"We spend 90 percent of our time indoors whether it's at the office, at the home, in a car or transport between the two. It's really important to have fresh air all the time."
He worries even though we now know the dangers of letting breath particles linger inside, not enough attention is given to improving ventilation.
His views are echoed by experts in other fields. Dr Julie Bennett is a senior research fellow in the University of Otago's Department of Public Health.
"I would like to see a government department or organisation take responsibility for indoor air quality. We have the Ministry for the Environment that has responsibility for outdoor air quality, but we haven't anyone responsible for indoor air quality."
The Ministry of Education has set guidelines for indoor air quality in learning spaces, and the building code has ventilation standards for new builds. But Bennett says there are no standards or guidelines for existing spaces.
There's certainly no organisation wandering around with CO2 monitors slapping fines on public spaces with high CO2.
Bennett says an immediate fix is putting prominent carbon dioxide monitors in public spaces.
These can be purchased for about $400.
She says New Zealand is behind countries such as France, which has been measuring indoor air quality for several years, and other European countries which have higher building standards than New Zealand.
But our country is not alone in overlooking indoor air quality. Ontario Society of Professional Engineers indoor air quality group chairperson Joey Fox uses Twitter to share the importance of ventilation in fighting Covid-19.
HEPA (high efficiency particulate air) filters are key in this mission, but their acceptance is patchy across Canada. Fox has been kept busy fighting online misinformation about these. He's also helped fill a vacuum of expert knowledge about how ventilation systems in buildings work.
Many of his tweets contain complex formulas, but his message is simple. Good ventilation can reduce the transmission of Covid-19 and other illnesses.
He's already seeing mask use drop off in Canada, and doubts masks alone as a way to reduce Covid-19 will be a sustainable public health measure.
"Distancing, lockdowns; those aren't practical solutions going forward, but making buildings safe, improving ventilation and filtration everywhere, that's something that we can do," he says.
He adds that it shouldn't be up to individuals to suffer the burden of filtering every particle of air they breathe so they're less likely to get sick.
"It's an obligation on society [and] on the building to provide safe spaces for people."