I think that's mentioned on the novel, idk why they didnt add that tho...
It is mentioned...more like shown in the webtoon too...remember the crack in the ceiling of the carriage where a bit of dirt slip through and fell... that's where the oxygen is coming from...same was mentioned in the novel (●'◡'●)ﾉ
It was… looks like whoever uploaded the chapters didn’t include all the parts. a lot of the recent chapters has so many parts missing that I ended up just reading the raws
For reference....ദദ(˵ • ᴗ - ˵ ) ✧
https://www.mangago.zone/read-manga/i_ll_be_the_matriarch_in_this_life/uu/mk_v-3-chapter-158/pg-8/
One issue that isn’t really being mentioned is that have limited oxygen. That’s what is more likely to kill them than anything else.