After the first pool scene, Lt. Rick Janssen reads to his son "nitrogen which can be used to produce breathable oxygen".
Nitrogen being a "pure" element (ie not a compound) and so being oxygen, it is not possible to "produce" oxygen with nitrogen. See more
As nuclear fallout over the Pacific continues to fuel sandstorms over southern California. The last remaining pockets of residents were evacuated by the National Guard. The federal government, already struggling to keep up with flash floods in Long Beach and Venice, has declared Los Angeles uninhabitable.