This link was posted by kloch 1 hour ago on HN. It received 192 points and 173 comments.

I got a ppm-meter in my living room because back in the day I would study there for uni with mates. Opening the window and making a coffee-pause always at roughly 800-1000 ppm.

It's insane to think about how bad the air feels and smells at 800ppm and knowing that I was born in the 340s ppm and nowadays the air outside won't dip below the 400s ppm even with a fresh breeze from the forest in mid summer.