A lungful is how much air — or smoke, or gas — you pull in with one breath, enough to fill the lungs. It is not a science measure with a number on a machine. It is a homely amount-word, like a handful or a mouthful: as much as that space will hold this time. People say a lungful of fresh air when they step outside, or a lungful of smoke if they breathe in from a cigarette or a fire. You almost always say a lungful of something. It can sound a little heavy or old-fashioned, but writers still like it when they want you to feel the breath itself, not just the idea of 'air'. It is ordinary enough in stories and spoken English; doctors doing real measurements would use other words.