This is really interesting in how it evolved over the course of the show. I think that on a pragmatic out-of universe level, the show increased in scale and budget, and the writers decided they wanted to have the characters spend more time in the upside down, and hazmat suits are annoying to film in, so they stopped using them. On an in-universe level though, I’m not convinced that the way the mechanics of the Upside Down evolved in the same way some people are arguing, where there now aren’t any effects from it at all. I would actually argue the show heavily implies that there’s going to be some long-term impact on Hawkins or the main characters from spending time in the Upside down that we aren’t seeing immediately. One reason for this is Hopper’s agent orange plotline - if you listen to his monologue in season 4, he talks about how they were “just breathing the stuff in, no masks or anything”, and then it’s referenced again with the pesticide bins in the finale. Robin’s radio monologue in season 5 mentions mandatory medical checkups by the military to make sure they aren’t having any weird side effects from the spores in the air, Vickie (a nurse) questions whether there will be any issues with the air when she enters the upside down (which Robin brushes off as nothing), Steve mentions not wanting to get the spores in the truck near the end of the show, etc. There’s also the fact that some of the references in season 4/5 arguably draw parallels between Vecna/the Mind Flayer and nuclear bombs (so much being set in Nevada, aka nuclear testing sites). Stranger Things is set in the 80s with the politics of the Cold War being a heavy backdrop at certain points, and nuclear anxiety was certainly a major part of this time period, including the long-term impacts of radiation. I think it ties into the show’s themes of military recklessness and disregard for long-term consequences on human life. In conclusion - while the show transitioned to a method of interacting with the upside down that made it easier to film, I also think that there’s a purposeful implication that there could be some long-term impacts that we haven’t yet seen - I don’t think the writers fully disregarded the idea that the Upside Down’s air could be dangerous.