Exercise Multiplies CO2 Output
A person at rest exhales roughly 35-40 grams of CO2 per hour. During vigorous exercise, metabolic rate — and therefore CO2 output — rises 5 to 10 times. A spin class of twenty riders produces as much CO2 as a hundred-plus person seated audience, packed into a room a fraction of the size.
The result: fitness studios are among the worst-performing indoor spaces ever measured. Studies of gyms and group-exercise rooms routinely log 2,000-5,000+ ppm mid-class, deep into the range where people report headaches, dizziness, and premature fatigue.
Why Gym-Goers and Owners Should Care
For members, elevated CO2 doesn't just feel stuffy. Perceived exertion rises in poorly ventilated rooms — meaning the workout feels harder without being more effective — and the same conditions that let CO2 accumulate also concentrate aerosols, which drove so much pandemic-era attention to ventilation. Heavy exercise means heavy breathing: litres more shared air per minute, per person.
For owners, air quality is now a visible differentiator. A displayed CO2 reading under 1,000 ppm during a full class is marketing that no poster can match — the same trend we cover for restaurants and cafes. It also gives HVAC contractors hard data instead of guesswork, often revealing that ventilation is set for the room's size rather than its metabolic load.
Targets for Fitness Spaces
Because occupants are breathing hard, hold fitness spaces to a stricter standard than offices:
- Under 800 ppm — excellent; ventilation is keeping pace with the class.
- 800-1,200 ppm — acceptable between classes; investigate if mid-class.
- 1,200-2,000 ppm — under-ventilated for exercise; increase fresh-air supply or cap class size.
- Over 2,000 ppm — meaningfully degraded conditions; act now (doors open, fans on, break between sets).
What to Buy
A gym needs continuous, wall-mounted, always-powered monitoring with a display everyone can see:
- Studio floor: a WiFi display unit like the Airthings View Plus gives members a visible number and owners remote dashboards and history. More options in our WiFi monitor roundup.
- Multiple rooms: professional multi-zone hardware — see our professional & commercial guide.
- Personal use: carrying a portable monitor like the Aranet4 to classes is an entirely reasonable way to vote with your feet.
Mount monitors at breathing height on a wall away from doors, fans, and any single person's exhalation path — the placement guide applies doubly in rooms with twenty heavy breathers.
Fixes That Work in Practice
- Measure a full week first — peaks track the class schedule precisely. Reading the data tells you which classes overwhelm the ventilation.
- Increase outdoor air fraction during classes — many commercial HVAC systems idle at minimum fresh air by default; schedule boost periods around the timetable, or use CO2-triggered automation.
- Stagger and cap class sizes in rooms that can't keep up — occupancy is the lever when airflow is maxed.
- Use the ventilation calculator — our tool estimates required airflow; for gyms, size for 2-3x the sedentary per-person rate.
Bottom Line
Gyms concentrate more exhaled breath per cubic metre than almost any other space, and CO2 is the cheapest honest measure of whether the ventilation keeps up. One wall-mounted NDIR monitor per studio — visible to everyone — is a small cost for a healthier, demonstrably better-run facility.