I haven't played the game, I've only watched, but as far as I understand there are different stages of the infection, in the first one they become fast biting zombies that don't need a gas mask, but in places where the infection has grown and there are closed rooms, the air changes there argue, there are level 2 and 3 monsters and gas masks are worn. There are no spores in the open, only indoors, where they already turn into a more advanced nasty version of the mutation, those that are infected.