RNZ's 'Whose Breath Are You Breathing?' project revealed carbon dioxide levels in gyms and yoga studios regularly exceeded limits considered safe for limiting the spread of Covid-19 aerosols.
The highest indoor CO2 level considered safe for Covid-19 transmission is 800 parts per million, but readings taken recently with a portable CO2 meter in Auckland gyms far exceeded that, in one case reaching 2086ppm.
Readings were taken in group fitness classes, cardio machine and weight training areas, in a cycling class, a sauna, and in changing rooms.
The worst space, with the highest CO2 reading, was taken in a closed-door spin class, and the lowest reading taken was 630ppm in the weight training area of a large gym during a quiet weekend morning.
The situation was worse in the closed confines of yoga studios, where in two classes RNZ recorded levels of 2170ppm and a whopping 4228ppm.
All facilities RNZ visited allowed people to exercise mask-free.
In enclosed spaces, CO2 levels can be used as an indicator of air's freshness, and can thus work as a proxy for risk of Covid-19 transmission High readings can indicate stale air lingering in a space.
Air exhaled is full of saliva particles of different sizes; some are droplets and fall to the ground a short distance away.
However, University of Auckland aerosol chemist Dr Joel Rindelaub said some breath particles were much smaller and more persistent.
"There's also going to be these tiny, tiny small ones that maybe are less than five microns, so less than 10 times the width of a hair, for instance; so small, you can't even see them," he said.
These particles are so small they float in the air.
"And these are going to linger around without proper ventilation for even hours."
If someone has Covid-19, the particles can be infectious.
Read more from this series:
- Whose breath are you breathing?
- Cost concerns scupper fresh air plans for Auckland buses
- Lack of classroom CO2 monitors may increase Covid-19 risk
Research conducted by fluid engineer and Universität der Bundeswehr München PhD student Benedikt Mutsch shows people breathe out a dramatically higher number of particles during exercise than when at rest.
Mutsch said although Covid-19 outbreaks had been linked to Zumba and cycling classes, there was not a lot of research looking into how exercising could impact the number of particles a person expels.
At rest, people breathe out about 500 to 600 particles per minute. Mutsch's research showed that during exercise, that number was 130 times higher on average, and his work also showed participants emitted 76,000 particles per minute.
Fitness buffs also have more particles in their puff, Mutsch said. Results taken from people who work out regularly showed they emitted more than those who don't.
He cautioned that the study only looked at 16 men and women, all healthy 18-24 year olds. For ethical reasons, people infected with Covid-19 couldn't be studied.
"There is evidence from rhesus monkeys that the number, concentration [of particles] can be higher in an infected individual," he said.
For those eager to avoid catching Covid-19 at the gym, he suggested steering clear of high-puff spaces.
"If there are people working out really intensively then maybe avoid this area; [we found] the really strong increases only for high intensity workouts."
Wearing a good-quality mask can also reduce risk.
Mutsch suggested asking your gym about ventilation. Systems with HEPA (high efficiency particulate air) filters can remove virus particles from the air.
For gyms without HEPA filters, CO2 monitors can show how fresh the air is, and be used as an indicator of when windows should be opened.
The industry body for gyms and yoga studios, Exercise New Zealand, said almost 800,000 New Zealanders have memberships to exercise facilities. Chief executive Richard Beddie said the industry took ventilation and Covid-19 seriously.
He was not convinced CO2 monitors displayed in gyms would be helpful.
"CO2 by itself is an indicative factor, it's not a causal factor. So higher CO2 doesn't cause more transmission, it would just indicate that the airflow isn't high."
The organisation had been encouraging its members to increase natural ventilation, using windows and doors as much as possible, he said.
"The challenge of course is when you go into some rooms and literally there are no windows and doors, or if there are they're relatively limited, and it will never be enough because the space is quite small. That's where forced air circulation comes in."
He said he knew of yoga studios and gyms with cycle classes who already used, or were investing in, CO2 monitors and HEPA filtration.
"If it's important for you, then you can ask. And similarly, it's like saying, 'What's your policy of cleanliness and how often you clean it?' If that's something that's important for you, you have every right to ask."
Potential benefits of exercise shouldn't be overlooked, he said, pointing to a 2020 study which found people suffering from Covid-19 who didn't exercise regularly were twice as likely to be admitted to hospital as those who exercised for 150 minutes a week.
He does not believe gyms are hotspots for Covid-19 transmission, saying research done by a non-profit exercise industry body showed low transmission at United Kingdom gyms in 2021. However, this was not a peer-reviewed study.
There are no regulations in New Zealand mandating indoor fresh air levels inside gyms, bars or restaurants.