In season 1 of Stranger Things the environment in the Upside Down was shown to be toxic. Prolonged exposure was dangeorus and the scientists exploring it all wore full hazmat suits. Will was also substantially effected by his stay there.
In season 3 everybody is running around the Upside Down like it was nothing.
Has something changed, is this a plot hole, or were the scientists just being overly cautious? Is this ever addressed in the show or, the video games, or the comics, in canon?
Throughout the series, people who have access to hazmat suits (and the time to put them on) do wear them. US government scientists/soldiers (and Hopper and Joyce) in Season 1, and Russians in season 3 all wear hazmat suits in and around the Upside Down. During each season, some characters enter the Upside Down without hazmat suits, either because they went unexpectedly or because they did not have time or access (or both) to put on a hazmat suit when driven to the Upside Down by necessity.
When Will was saved, the main characters learned that long term exposure to the Upside Down wasn't great for your health (though also not lethal or unrecoverable). The government already knew this when it started sending people into the Upside Down and thus used hazmat suits because it had the resources to do so. This would enable the same people to go into the Upside Down over and over without the long term effects. Nancy and Jonathan went to the Upside Down in Season 1 without protection because they were exploring and did not know any better, and because they did not have access to hazmat suits.
The US government operation to explore the Upside Down ends after Season 1. At that point, in season 2, the only characters going into the Upside Down are either going unintentionally or intentionally for a very specific purpose that they believe (or hope) will take a very short time. Nobody goes there thinking they'll be gone for weeks like Will. They are also generally do not have access to hazmats suits or the time to go get them, but it should be noted that most/all of the characters in the upside down wear cloth over their faces to mitigate exposure. Driven by necessity, they accept the risks of exposure (if they know about them) and count on keeping that exposure as brief as possible and mitigating it when they can.
In Season 3, the Russians did wear hazmat suits (see image below), because they had the resources at their disposal to have them on hand. And in Season 4, the characters going to the Upside Down are again driven by necessity and the intention of making their stay as brief as possible.
