The commonest cause in children is digital trauma (nose picking) but may occur spontaneously due to colds, allergies, or dry air. This bleeding usually comes from the big vessels on the septum anteriorly (Littles Area or Retrocolumna Veins).
Adults differ and systemic factors (e.g. hypertension, diabetes, blood thinners) need to be excluded. They often bleed from big arteries towards the back of the nose (e.g. sphenopalatine arteries) or abnormal rosettes of vessels (haemangiomas) anywhere in the nose.
What is Epistaxis (Nosebleed)?