I don't think that's safe to assume at all, for two reasons:

1. CO2 has effects on the human body of its own that aren't simply a lack of oxygen, and vice-versa. [0]

2. The baseline proportions involved aren't close, so even doubling CO2 isn't going to show up easily as a large swing in in oxygen%.

For example, the article references a study where the CO2 proportion going from 0.04% -to 0.25% correlates to mental problems.

Even if the watch could sample atmosphere directly, is it sensitive enough to detect a shift from 21.00% -> 20.79% oxygen?

As it's estimating oxygen in the owner's blood, it might not detect anything different at all... not if the owner's body has already compensated by breathing harder or by "underclocking" their brain to make dumber decisions.

[0] https://en.wikipedia.org/wiki/Asphyxiant_gas

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ErroneousBosh 6 hours ago | parent [-]

> The article references a study where the CO2 proportion going from 0.04% -to 0.25% causes mental problems. In other words, a difference in 0.21% of the air.

I'm finding that pretty difficult to believe, to be quite honest with you.

And before you say "aha, carbon dioxide brain fog!" consider that I'm about a mile from the sea with a 40mph onshore breeze. This air is about as oxygenated as it gets.

	▲	anon7000 6 hours ago \| parent \| next [-]
		It makes a lot of sense actually. You get severe symptoms when CO2 makes up only a couple % of the air. And can become fatal at like 5%. There’s not like a hard line where you suddenly die, it’s a gradual thing. It very much makes sense that we’d notice minor symptoms at a few thousand PPM when it only takes like ten thousand to feel it severely.
	▲	Terr_ 6 hours ago \| parent \| prev \| next [-]
		1% (10,000 ppm) is enough for the person to become aware something is odd through drowsiness or an elevated heart rate. I don't think it's too far-fetched for a quarter of that to cause subconscious cognitive effects, that could be measured in tests.
	▲	Hikikomori an hour ago \| parent \| prev [-]
		I got a monitor as we had an old apartment with bad ventilation. When I started feeling it I would check and it was always around 1200ppm and would open a window for a bit. Outside air is around 420ppm, but that's not the problem, enclosed and badly ventilated rooms are if you spend a few hours in there.