Once you have Infusion unlocked, go to the Northern part of the map and find Harriet in a warehouse. Kill her to acquire Harriet’s Fourpounder Pistol; a weapon that shoots toxic gas clouds for AoE damage and if you shoot the gas traps, they will detonate for explosive damage. If you equip the Steel Lungs perk while using this weapon, you can enter clouds of toxic fumes which regens your health!