"As long as they wear masks to protect themselves from inhaling the Miasma, they should be good."
Some eyes were sensitive to Miasma, so it was better for the mask to cover their faces completely. That way, they could protect most of the openings that allowed stuff into the body.
"Wearing basic Adventurer wear should also protect the body, though Miasma is only dangerous to a person's health if it is taken into the system, where the Mana network really is."
It had such a simple prevention method, yet Miasma Poisoning was one of the most difficult things to treat. Of course, depending on the severity of the poisoning, the difficulty could go down a few notches.
The reason for it being so complex was due to the Mana Network being corrupted.