However, reliably detecting 1000~2000 ppm isn't hard, but will require some training. And that's the level where you want to leave the place in a state that's

However, reliably detecting 1000~2000 ppm isn't hard, but will require some training. And that's the level where you want to leave the place in a state that's not dangerous, followed by getting out of there and waiting for someone with an oxygen mask to go leak searching. But yeah, CO2 is the thing that makes you want to open a window long before acute toxicity.