This is junk science on so many levels it's laughable.

First of all, the population of large modern cities is already exposed to CO2 levels equal to or greater than the test level (900 ppm) nearly 24/7. I just took my CO2 meter and walked around the house. The levels range from 980 to 1165 ppm, the garage having the lowest levels.

Atypical? I don't think so. Not at all. I'll go measure my neighbor's homes and the office when I get a chance. It only stands to reason that a sealed environment would have higher CO2 levels. Modern buildings, in order to be energy efficient, are sealed-up fairly tightly. This makes both air conditioning and heating more efficient. Therefore, without constant air exchange program (which nobody does) CO2 levels indoors are going to be higher.

In fact, outdoor CO2 levels right now are 425 in my backyard and 613 in the front of the house. Measurements taken about 30 cm from the ground and away from any vegetation.

Are people dropping dead? NO

Are people ending-up in the hospital with horrible respiratory system problems? NO

If these numbers were statistically significant one would be able to point out equally significant pandemic-like respiratory and other issues across a massive portion of the population.

What's even more interesting is that 2020 caused everyone to lock down indoors FOR A WHOLE YEAR! This means billions of people were exposed to potentially higher CO2 concentrations than normal. The assumption here being that a more active population moves between indoor and outdoor environments, at least on a transitory basis while going to work, market, etc.

People are not dropping deal like flies at CO2 levels in the 1,000+ ppm range.

This also brought-up another question: We spend hours in our cars. What's the CO2 level in there?

I measured three vehicles, small, medium and large. I took measurements with the air conditioning system set to bring in outside air as well as recirculate (what you might do on the highway to avoid sucking-in fumes). The levels ranged from 680 ppm to 1264 ppm. Now I am interested in getting numbers while driving on the highway with a full load of people in the car. I'll get those numbers tomorrow.

Back to the junk science experiment.

First, as someone pointed out, the size of the test chamber was, objectively speaking, ridiculously small. 60 x 50 x 140 cm. Since it is somewhat difficult to visualize volumes sometimes, I went and measured a couple of household items to get a sense of proportion.

All measurements in cm
Refrigerator: 91 x 83 x 174 volume: 1314 liters
Dishwasher: 60 x 50 x 75 volume: 225 liters

The test chamber's internal volume was 420 liters.

Here's what's odd. They list the internal dimensions as 60 x 50 x 140, which is 420 liters. In the article they claim the internal volume was 393 liters. I'll assume they used-up 27 liters with equipment.

That being the case, the volume of air the mice had available to them was in the order of 1/3 of the volume of a refrigerator and 75% greater than that of a dishwasher.

AND THEY KEPT 53+ MICE IN THERE FOR ELEVEN WEEKS!

Go stand in front of your refrigerator right now and imagine 53 mice in that volume. Go do it. I'll wait...

What's the ratio of air available? 393/53 = 7.4 liters per mice

What's the ratio of air available per human, for a family of four, in, say, a typical 2000 square foot (186 square meter) home? 113,267 litters per person

What's the weight ratio between a mouse and a typical person? About 2,700 to 1

Scaling the air available per person, this would translate into a requirement of nearly 42 liters of air per mouse. The test chamber had SEVEN.

While my calculations are not meant to be precise, the idea is to get a sense of proportion here. If you wanted to conduct an experiment that truly simulated free space exposure to any given level of CO2 concentration you would have to have a chamber of a volume sufficient to eliminate the possibility of creating deficiencies as well as contaminating the very air you are relying on to test for exposure. I mean, what do 53 mice farting for 12 weeks do to a chamber 1/3 the volume of a refrigerator? Their control loop had a single variable, CO2 concentration. Nothing else triggered an any kind of an adjustment to the environment in that chamber.

I could go on, but I'll stop here. There's a whole segment of society --government, academics and zealots-- who are going way out of their way to attempt to create FUD powerful enough to scare people into doing things we should not be doing. Yes, yes, yes, climate change is real and we contributed greatly to atmospheric CO2 concentration. None of that is in doubt at all. This isn't about denying any of it. This is about suggesting we need to start having the right conversations and stop lying to each other.

It is probably fair to say hundreds of millions of people have been living a great deal of their lives in >1000 ppm CO2 levels inside of cars and buildings for decades, maybe 50 or 100 years. Maybe more. Last I checked, the sky hasn't fallen yet.

Of course, one might say: Well, when outside levels get to 900 ppm what will inside levels be?

This is interesting. The test was conducted at the predicted atmospheric CO2 levels in EIGHTY YEARS. In other words, not any time soon. For a sense of proportion, anyone born today will likely be dead by then. So, yeah, a while from now.

This exposes a level of dishonesty (or hubris, if I want to be kind) that is far too common for those wanting to push junk science: Reality is reduced to a single variable. In this case, for a whole 80 years nothing, absolutely nothing changes in the world and the one-and-only variable --atmospheric CO2 concentration-- climbs and climbs and climbs. Like I said, dishonest. I'll add ignorant and manipulative to that.

We know, without a shadow of a doubt, that the planet --because this IS a planetary scale issue-- reacts to CO2 concentration through changes in weather. In fact, we know, also without a doubt, that the two things that happen are: Lots of "water events" (rain, hurricanes, cyclones, etc.) and lots of vegetation growth (trees, forests, etc.). Do you know what indoor farmers add to their environment in order to promote rapid plant growth? CO2. Exactly.

And so, reality isn't a single variable problem. It's a complex multivariate machine. Will outdoor atmospheric CO2 concentration ever get to 900 ppm? Who knows? Nobody knows. And nobody can say one way or the other. What is far more likely to happen is that the planet will continue to react to increased availability of CO2 --as it has been for millions of years-- and possibly prevent this.

Of course, I think we can expect to have shifted almost entirely away from fossil fuel for ground transportation. I can see that being the case in, say, 25 years or so. In other words, anyone who bought a new car today and keeps it for, say, ten years, will likely have lots of good options by that time to buy an electric vehicle. I am doubling that and adding five years to my electric conversion estimate.

The scientific and technical communities need to start pushing for honesty in this field or we are going to waste valuable time and resources on things that will be utterly pointless for the future of humanity.

For now, plant lot of trees and stop cutting down forests. That seems like the most proactive and harmless things we can do without requiring massive government spending programs.