A person at rest exhales roughly 0.5 litres of CO₂ per minute. The same person on a treadmill at 12 km/h is exhaling close to 1.8 L/min. A spin class hits 2.2 L/min during the working intervals.
A 20-person spin studio for 45 minutes is producing the respiratory equivalent of an 80-person quiet meeting.
The ventilation rarely matches.
Most gym fit-outs use the same air-changes-per-hour figure as a retail unit (~6 ACH). Specialist cardio rooms need closer to 10–12 ACH to keep CO₂ under 1,500 ppm during a class. We have measured spin studios at 2,400 ppm peak — a level that triggers a measurable VO₂max reduction in the riders themselves and an audible "this room feels heavy" comment in the post-class chat.
The signal nobody tracks.
- Operators measure footfall, member retention, and class popularity.
- They rarely measure the air the class is delivered in.
- Members do not articulate "the CO₂ peaked at 2,200 ppm and my last-set output dropped 8%." They say "I felt sluggish today."
The reviews channel the felt sense; the cancellations follow the felt sense. The number behind both is sitting unread in a wall plate.
What a measured studio does differently.
- One sensor per cardio room. The fit-out cost is comfortably under one month of one cancelled membership.
- Boost ventilation tied to the booking schedule, not the building's daytime profile. Spin at 7 AM and 6 PM needs the boost — not the boardroom upstairs.
- Class capacity tied to the air. If the studio cannot keep CO₂ under 1,500, the cap is too high. Members will not thank you for the policy. They will return more often, which is what the policy was for.
A gym sells the felt experience of the workout. The air is half of that experience. Treat it like programming, not like plumbing.