Why Finnish babies sleep in cardboard boxes (2013)

Disclaimer: this is just my general impression after googling around for how much CO2 buildup/concentration might be a factor/cause of SIDS after becoming a parent. It's by no means any kind of reputable research, just the googling of a dad.

The relative differences in weight/density between CO2, nitrogen, and oxygen just aren't big enough to matter much compared to the other forces that mix room air: convection, currents from forced air HVAC, air leakage, etc.

Air is already 80% nitrogen, and when we exhale we only drop the % of oxygen from 20% to 16%. And consider the fairly small lung volume of the kind of infant who would be sleeping in the box.

I'm sure you could get CO2 buildup in the box in very controlled lab settings, but in general practice as a crib it doesn't seem to be an issue.

And in case you were wondering about the general case: is CO2 buildup an issue for infants? The answer seems to just be: no, and it doesn't even depend much on how much you run your HVAC or if you have a fan or how hole-y your crib is, etc. Room air just mixes enough normally that it's not that big of a deal.

Getting pregnant too early again is not recommended. The body of the new mother should get some time too fully recover.

A cushioned cardboard box is enough for a newborn. Its weakness could be moisture and high flammability. But aside from that it is perfect.

Women can be very fertile right after birth, although most don’t get their periods for months while breastfeeding.

We got like 100 condoms after birth, which would mean years of sex given the stress and discomfort after birth.

I mean, they've been doing this for decades. Don't you think their dataset is probably more informed than your speculation?