The places where you are breathing the highest amount of other people's 'breath backwash'

CO2 levels can be used as an indicator of the risk of catching COVID-19.
CO2 levels can be used as an indicator of the risk of catching COVID-19. Photo credit: Getty Images

Farah Hancock for RNZ

How much of the air you're breathing is air someone else exhaled? And in the midst of a pandemic caused by an airborne virus, where are the riskiest places to be? In the first in a series of five stories Farah Hancock reports on hot spots of hazardous air.

Shuffling my way onto the bus I had no idea it might be the most dangerous thing I would do all day.

It was 8.30am and the double-decker bus was standing room only, filled with morning commuters riding into the city. As we stood, jammed shoulder to shoulder and swaying with the bus's motion, I held my palm-sized CO2 monitor close and watched as the number on its digital display rose rapidly.

With no opening windows, the bus had quickly filled with carbon dioxide exhaled from our collective lungs. From a level of 516 parts per million at the bus stop, it took only 20 minutes for CO2 inside the bus to peak at a whopping 5737ppm.

Although high levels of carbon dioxide can be dangerous, there was something I was more worried about. CO2 levels can be used as an indicator of the risk of catching Covid-19; we breathe in air and release CO2 when we exhale. Covid-19 is spread via aerosol particles breathed, coughed or sneezed out by infected people. These can drift several metres and linger in the air for hours.

The higher the CO2 readings, the more virus particles are potentially in the air. I became aware of every cough and sneeze. Feeling decidedly unsafe, I glared at a passenger who pulled down their mask to burp.

At 5737ppm, the equivalent of one in every seven breaths I took on the bus was air other people had breathed out. I texted a friend: "OMG, the readings are so high I may as well let the other passengers lick my face!"

I was being a little gross, because even according to a scientist, it is a little gross.

"You can think of it as spit particles, tiny spit particles are what you are breathing in," says University of Auckland aerosol chemist Dr Joel Rindelaub. "It's breath backwash that gets people infected."

He doesn't endorse passenger face-licking, but when CO2 inside the bus is 5737ppm he jokes, "it probably wouldn't even hurt, right?"

Dr Joel Rindelaub.
Dr Joel Rindelaub. Photo credit: RNZ/Cole Eastham-Farrelly

The level of CO2 outdoors is about 420ppm. Rindelaub says a good indoor reading would be anything below 800ppm. This is also the level suggested by the United States Centers for Disease Control for indoor spaces as a benchmark for good ventilation. When readings get above 1000ppm there could be a high risk of Covid-19 transmission if someone in the space is infected.

"If you're above 2000, then that's a huge red flag."

High CO2 levels don't automatically mean you're going to catch Covid-19 - there has to be infected particles in the space, but they can indicate poor ventilation and a likelihood of high particle levels, if no filtration is used.

The places where you are breathing the highest amount of other people's 'breath backwash'
Photo credit: RNZ/Cole Eastham-Farrelly

Most people will have no way of knowing the level of risk they face from lurking breath particles too small for the human eye to see, says Rindelaub.

"CO2 is going to have no taste, no flavour, no smell. You would never know if you were in a high CO2 environment."

What's changing people's obliviousness to CO2 levels are portable carbon dioxide monitors. They've become a favourite pandemic item for gadget geeks. The small units, about the size of a deck of cards, tell you how fresh the air you're breathing is.

Rindelaub thinks it would be good if CO2 monitors were more than a BYO device. He thinks businesses should have them permanently on display, especially in places where people take their masks off for eating or drinking.

"It's a great quantitative number where you can say, all right, maybe I'm above 800, we can open a window or something, or just to help freshen the air a little bit."

In an unscientific survey, RNZ used a portable CO2 monitor to record readings from a variety of places around Auckland. Here are the results - and Rindelaub's reaction to the highest reading taken in each location.


The CO2 monitor smuggled into one school on three different days gave a top reading of 1373ppm.

"For a school learning environment, I would say that is too high; we want to reduce that ideally below 800 parts per million," says Rindelaub.

At 1373ppm, one in every 33 breaths the class of 30 children were inhaling would be breath backwash.

Masks are no longer mandatory in schools, although some schools have opted to retain them.

Government advice is to open all doors and windows if classroom readings are over 800ppm. If CO2 is higher than 1251ppm, then teachers are told to regularly vacate the room for short periods, reduce the number of students in a room, or do less vigorous activity.

However, teachers would need a CO2 monitor in their class to know when levels are high, and not enough monitors have been supplied to have one in every class.

Number of readings: 3. Median: 930ppm. Percentage of readings above 800ppm: 66 percent.


Every office is different, and every seat can be different too. Readings taken by RNZ show desks by open doors have lower readings than desks tucked into corners.

Readings ranged from 587ppm to 1167ppm.

In closed meeting rooms, levels can rise over time. Three people in a meeting room for one hour saw the level rise from around 700 to 1000ppm.

Mask wearing is not mandated in office settings at the orange traffic light setting, but some companies have chosen to adopt it.

Rindelaub's advice to avoid Covid-19 is to wear a good quality mask at work. He also suggested keeping closed-door meetings short, and to periodically open the door to refresh the air.

Number of readings: 10. Median: 801ppm. Percentage of readings above 800ppm: 50 percent.


In cafes the lowest CO2 reading of 419ppm came from an outdoor table. However, indoor tables at cafes recorded low readings of 500ppm when doors were open and there was a cross breeze.

When doors are closed, readings get higher, especially if the cafes are crowded. Cooking may also increase CO2 levels without increasing breath particles, due to combustion.

Number of readings: 4. Median: 614ppm. Percentage of readings above 800ppm: 0 percent.


Levels in shops were a mixed bag, ranging from 504 to 980ppm. The lowest reading was taken in a large format retailer and the highest in a small grocery store. The median number recorded was 628ppm, which is better than the 800ppm cut-off point for good indoor air quality.

"I think what helps a lot is people are constantly coming in and out and that door is opening so you're getting some good airflow," says Rindelaub.

At the orange traffic light setting face masks must be worn inside shops and unlike in bars, where people socialise, shoppers may be talking less, so theoretically there may be fewer particles in the air.

Number of readings: 13. Median: 628ppm. Percentage of readings above 800ppm: 31 percent.


Readings RNZ recorded in bars were lower than in public transport but this doesn't automatically equate to a lower risk, says Rindelaub.

"When you're at a bar or restaurant, you're socialising, you're going to be talking a lot. When you're talking, you're actually expelling way more particles, thousands of more particles, than just breathing regularly."

That coupled with no masks means there's an increased risk of catching an airborne disease if someone in the same space as you is sick.

The highest reading recorded was 2624ppm, from a bar with only 22 people present. The equivalent of every one in 17 breaths here is air that other people have exhaled.

Opting for bars with outdoor tables is the safest bet, says Rindelaub.

"If you can find a spot like that, I would definitely zero in on it."

Number of readings: 11. Median: 1233ppm. Percentage of readings above 800ppm: 91 percent.


Readings taken in exercise venues ranged from 630 to 4228ppm. Gyms generally don't require mask wearing.

The highest reading of 4288ppm was taken in a yoga class with 30 people in the room. This means the equivalent of one in every 10 breaths taken was backwash.

"I would definitely wear a mask in a gym," Rindelaub says. "Because people are going to be doing high endurance workouts, they're going to be breathing more heavily, which means you're going to be creating more aerosol particles, which means it's going to be a higher risk."

Number of readings: 11. Median: 1216ppm. Percentage of readings above 800ppm: 91 percent.


"Cars are gross," says Rindelaub. "You're in a tiny area, there's no ventilation if the windows are up. So these CO2 ratings and these particles are going to get super high super quick, especially if you have more than one or two people in the car."

The highest reading of 5040ppm was taken in an Uber with a driver and one passenger. This equated to one in every eight breaths being backwash.

Opening a window, even just a small amount, can help, says Rindelaub.

Even in your own car it's worth thinking about risk. Parents ferrying multiple children to Saturday sports for example, should take infection into account.

"I would definitely have people wear masks if you were bringing in multiple people from different families into one tiny, tiny, enclosed place because that would be high risk."

Number of readings: 6. Median: 2458ppm. Percentage of readings above 800ppm: 100 percent.

Public transport

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."

Photography and visuals by Cole Eastham-Farrelly

Data notes:

The CO2 monitor used for all readings was a Qingping Air Monitor Lite and was recalibrated regularly. It uses a Senseair S8 LP sensor, which has an accuracy of ± 40 ppm ± 3% of reading. This is the same brand of sensor which is used in the Aranet4 monitor, but a different model. The sensor model used in the Aranet4 has an accuracy of ±30 ppm ±3% of reading. This monitor was purchased by the author independently.