If you stay up high you can avoid the poison in Broken Valley. The top of Lovis' tower and the quarry walls and building are above the gas, and you can drop down to them in human form. Most of the platforms are also free from poison, but unless you see a camp (tent, fire, crates, chests, etc) it wouldn't hurt to try flying low to the ground, and pull up if you hit poison, or save before transforming to drop down.