5mm deviation of the nasal septum to the left. is this serious?

Probably not: That degree of septal deivation is serious if it causes nasal obstruction. Depending on the status of other adjacent structures in the nose, 5 mm of septal deviation may cause little obstruction or it may cause significant obstruction. If so, straightening it with a surgery called a septoplasty might be helpful.

Deviated septum: A deviated septum is not an uncommon problem. It describes a situation when the cartilagenous/bony wall that divides the two nostrils is not midline. It can lead to nasal obstruction, snoring & even sinus issues. You only need surgery if you are having symptoms.Be sure to have your nose evaluated by a board certified otolaryngologist.