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But if I open a window at 700ppm, so what?

Maybe you live in a place where the room temperature is the same as outside. Here in winter, it means sitting in the cold.

No I live in Sweden.

You can open the window for a bit to air out, should be done even in the winter :)

Suddenly there's not enough CO2 in the room and you get overly awake! Bummer! /s