no need watch survival tv show, when on the comments suddenly there's many "expert" explain it to you ~
you all should know that MC is basically same as us, most of us never experienced real wildlife and just from watching tv or articles from internet, he never say he is the best or master, he just say his hobbies for camping and knowledge about it finally useful... he didn't even knew if the knowledge is actually true or not, just following it. and daily reminder that isekai world and Earth is different, Earth is already Polluted enough even to drink water from seeming clean rivers would still dangerous for us, while long long ago our ancestors still healthy even when their entire life drinking from the rivers without boiling it first.
while Isekai, as you can say, still clean from pollution, and there's magics as well ~
lucky for MC that he has skill to tell if those ingredients is edible or not.
as for why there's "smog" is probably mistranslation or the author could not explain better, probably what MC mean is "pollution"...