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"1000 ppm CO2 and I Can't Breathe!?"

Imamura here. When explaining CO2 levels in a vertical farm, I once let the number “1000 ppm” take on a life of its own.

The 1000 ppm Explanation

Whenever a new staff member joined the vertical farm, I’d give them the same facility tour.

“This is the propagation room.” “This is where we do final planting.” “This is the harvest area.”

And then, without fail, I’d explain CO2 levels.

“At this facility, we maintain CO2 concentration at 1000 ppm to promote plant growth.”

For us, this is the most routine of explanations. Supplement CO2, support photosynthesis, concentration around 1000 ppm. For anyone familiar with the work, the natural response is: “Oh, sounds like a great environment for the plants.”

But for someone hearing it for the first time, it doesn’t land that way.

The number 1000 seems large. ppm is a unit you almost never encounter in everyday conversation. And when you’re then told that “CO2 concentration is high,” feeling alarmed — thinking “wait, is that dangerous?” — is entirely understandable.

At the time, I hadn’t thought any of that through. I was giving a fully inside-the-facility explanation, from the perspective of someone who’d been there long enough to forget what it was like not to know.

The Day A Collapsed

It was an unremarkable Tuesday afternoon.

A (name changed), a new employee, was working inside the facility when she suddenly went pale and collapsed.

I immediately brought A outside and had her rest with a cup of tea in the break room. I kept checking her color and talking to her — my own heart rate wasn’t exactly settling down either. The second hand on the break room clock seemed to tick unusually loud.

After about ten minutes, A’s color started coming back.

“I’m sorry… I suddenly felt like I couldn’t breathe…”

She looked embarrassed as she said it. I asked whether she’d like me to call an ambulance. She said, “No, I’m fine now” — but just to be safe, I had her leave early for the day.

The next day, A came in looking perfectly well. A medical checkup showed nothing wrong. So why had she collapsed?

As we talked things through, the reason became clear.

“To be honest… I heard that the CO2 level was high, and I’d been thinking the whole time that it might make it hard to breathe…”

The moment I heard those words, the explanation I’d given rewound in my head. CO2. 1000 ppm. High. Hard to breathe. Oh. There it was. I’d meant to describe a safe growing condition. For A, it had become a source of anxiety.

“But — what concentration is 1000 ppm, exactly? It’s harmful, right?”

That question was what finally made me understand. For most people, “1000 ppm” isn’t a meaningful number. It just looks large. The unit I’d been throwing around as a matter of course had given her no reference point at all.

About the Same as Inside a House

From that day on, I added one line to my explanation for every new employee.

“The CO2 level inside the facility is 1000 ppm. That’s about the same as inside a house.”

That one change makes a visible difference. When I said “1000 ppm” without context, a small question mark seemed to hover over the listener’s head. Add “about the same as inside a house,” and they settle down almost instantly.

In fact, in an unventilated room, CO2 can easily reach 1000 ppm just from people breathing. By the morning after a night’s sleep, your own bedroom may well have exceeded 1000 ppm.

Outside air is around 400 ppm. Inside the facility, 1000 ppm. The numbers differ, but almost no one can sense that difference physically.

These days, I sometimes add a bit more detail:

“The CO2 level inside the facility is 1000 ppm — about the same as inside a house. For reference, the air inside submarines typically runs at 4000 ppm, and the International Space Station at around 5000 ppm, yet everyone aboard manages just fine. Effects on the human body don’t begin to appear until around 5000 ppm or above, so there’s nothing to worry about.”

When I get that far, new employees sometimes laugh and say, “I feel like an astronaut.” That’s probably not what they expected to hear on their first day. I never plan on getting there either — it just happens.

Beliefs Are Part of the Environment Too

The human brain is a fascinating thing: simply believing “this might be dangerous” can be enough to produce real physical symptoms. This is the reverse of the placebo effect, a phenomenon known as the nocebo effect.

I believe A genuinely felt short of breath and temporarily lost consciousness because she was convinced that “high CO2 = trouble breathing = dangerous.”

A vertical farm confronts newcomers with all kinds of unfamiliar stimuli. High humidity. The unusual quality of LED light. The constant sound of circulating water. For someone not used to it, the environment already puts you on edge. Add a number like “CO2 level: 1000 ppm” — one they have no frame for — and that tension can take shape as anxiety.

What A’s experience taught me was this: explaining technical terms requires everyday analogies.

“About the same as inside a house” does something that “1000 ppm” alone cannot. Even when the scientific content is identical, it only works if it actually reaches the other person.

The anxiety around CO2 levels almost disappears the moment you add those words — “about the same as inside a house.” It sounds too simple, but that’s genuinely how it works on the floor.

Even now, writing this, I can picture A in the break room, holding her tea with an apologetic look on her face. What she needed at that moment wasn’t more technical knowledge. It was simpler words.

The power of what people believe can override scientific reality. A lesson in human psychology I learned at a vertical farm — and one that turned out to matter more than I expected.