The lungs are divided, anatomically, into progressively smaller portions; lung to lobes to segments and then sub-segments. Atelectasis means collapse. Thus, x-ray findings that suggest collapse of a sub-segment would be a very small piece of lung. In almost all instances, this finding has no implication, serious or otherwise. When larger portions of lung collapse, one wonders about their being a bronchial obstruction, by mucus or tumor. This is not so with sub-segmental atelectasis, which is frequently reversible over time. The most one would ever do, and this with a history of heavy smoking, would be to repeat the x-ray in a year.
Thanks so very much for your clear and concise explanation.