Field measurements taken in Montreal between winter and spring 2026 with the Aeris View CO₂ monitor. Last updated: June 2026.
Most people have never seen a CO₂ number for the spaces they spend their lives in. We have — because for the past few months, we've been carrying our own monitor into gyms, cinemas, restaurants and shops around Montreal and recording what it shows. Some results were reassuring. One was genuinely alarming.
How to read these numbers
Outdoor air sits around 420 ppm of CO₂. Indoors, a well-ventilated space stays under 800 ppm. Around 1,000 ppm, a room starts to feel stuffy and research begins to detect effects on concentration. Above 1,400 ppm, a 2015 Harvard study found pronounced declines in cognitive performance. CO₂ itself isn't toxic at these levels — the point is that it's the best single indicator of how much of the air in a room has already been through someone else's lungs, and how well the building is ventilating.
How we measured
These are real-world spot checks and timelapses taken during normal visits — a workout, a movie night, a Sunday brunch — with an Aeris View running its NDIR sensor at 5-minute intervals. This is not a controlled laboratory study: occupancy, weather and HVAC settings all vary. Treat each reading as an honest snapshot of one place at one moment, which is exactly how indoor air works in real life.
The results
| Location | Situation | CO₂ reading | Verdict |
|---|---|---|---|
| Downtown sidewalk | Sunny afternoon, baseline check | ~500 ppm | Fresh air |
| Outdoor concert | Large crowd, open air | Near outdoor baseline, one brief spike | Crowds outdoors are fine |
| Ice cream shop | Lineup inside a small shop | Climbed to ~1,000 ppm | Acceptable — you can feel the crowd |
| Cinema lobby | Crowded pre-show evening | ~1,200 ppm | Stuffy — above the comfort line |
| Brunch restaurant | Full house, ~90-minute visit | Stayed in the healthy range throughout | Excellent ventilation |
| Budget gym | Mid-workout, occupied floor | 3,500–3,700 ppm, sustained | Seriously poor — roughly 8× outdoor air |
The gym: 3,500–3,700 ppm while people exercise
This was the measurement that motivated this article. At a low-cost gym, the monitor held steady between 3,500 and 3,700 ppm for our entire visit — not climbing, not falling, just parked at roughly eight times outdoor levels while members did cardio and lifted weights. A stable reading that high suggests ventilation deliberately kept to a minimum, likely to save on heating costs. If you've ever felt unusually wrecked after a workout at a budget gym, the air — not just the workout — may be part of the reason. Exercise multiplies your breathing rate, which means you're processing far more of that stale air per minute than someone sitting still.
The cinema: the lobby is the problem, not the theatre
On a busy evening, the crowded lobby — ticket lines, popcorn counter, people waiting for showtimes — reached about 1,200 ppm. Interestingly, our screening room itself was nearly empty that night. By the time we walked out around midnight with the building emptying, the lobby had dropped noticeably. The lesson: in cinemas, the pre-show crowd in a packed lobby can be the worst air you breathe all night.
The ice cream shop: a crowd you can watch in real time
Outside on the sidewalk: about 500 ppm. Inside a small shop with a lineup: the number climbed to about 1,000 ppm within minutes — still acceptable, but a perfect live demonstration of how quickly people fill a small space with exhaled air. Back outside, it returned to ~500 ppm.
The brunch restaurant: proof that good ventilation exists
We expected a packed Sunday brunch spot to be one of the worst readings. It was the opposite: through a roughly 90-minute visit at full capacity, kitchen running, the reading never left the healthy range. Somebody invested in that HVAC system, and it shows. Good indoor air is an operational choice — some businesses make it.
The outdoor concert: crowds are not the problem, enclosure is
At an open-air concert surrounded by people, readings hovered near the outdoor baseline the whole time, with one brief spike when someone exhaled right next to the sensor before settling back down. Thousands of people outdoors barely move the number; a dozen people in a sealed room can triple it. The variable that matters is the walls, not the crowd.
And one more: your car
The fastest CO₂ buildup we've ever recorded wasn't in any public building — it was inside a car in winter with the windows up. That test got its own write-up: CO₂ levels inside cars in winter.
What you can actually do with this
You can't fix your gym's HVAC, but you can choose a gym — and a quick glance at a portable monitor on your first visit tells you more about the place than any tour. At home, the same numbers are fully under your control: a bedroom door left open, a window cracked for ten minutes, a kitchen fan running while you cook. The first step is simply being able to see the number. That's why we built the Aeris View — a monitor you can read at a glance, at home or anywhere in this article.
Related reading: What is a safe CO₂ level indoors? · Is 1,000 ppm CO₂ dangerous? · Aeris View vs Aranet4
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