"Environmental damage is taken on your current HP every 24 Hours."
Do you only take Environmental damage once per day (e.g. once per day you'll suffer the 30% HP loss from being on a Hot No Atmos planet) or, once per day will it apply all damage (e.g. if you spent 2 hours on a Hot No Atmos planet, you'll be hit with a 60% reduction at a single time)?
Not sure when the timers go, but either you spend 24h outside then it hits you with the damage, or it hits you at 17:00 with full damage. You won't get partial damage if you leave before the counter hits 0.
It was per hour, but changed afterwards. I suspect it might have something to do with newbies walking out of the starting city and not being able to return before the atmo damage killed them off - takes days to cross terrain on foot.