Viral Upper Respiratory Infection (URTI): This is the most common cause of green nasal discharge, often resulting from a viral infection that leads to inflammation and increased mucus production in the nasal passages.

Bacterial Sinusitis: A bacterial infection of the sinuses can cause green or yellow nasal discharge due to the presence of pus. This condition often follows a viral upper respiratory infection.
Allergic Rhinitis: Although typically associated with clear nasal discharge, allergic rhinitis can sometimes produce greenish discharge, especially if there is a secondary bacterial infection.
Vasomotor Rhinitis: This non-allergic condition can cause chronic nasal discharge, which may appear green if there is an associated infection.

Foreign Body in the Nose: Especially in children, a foreign body in the nasal cavity can cause persistent, foul-smelling green discharge from one nostril. This condition requires prompt medical attention to avoid complications.
Tumor or Cancer: Although rare, nasal or sinus tumors can cause abnormal nasal discharge, including green discharge, due to obstruction or infection.
Granulomatosis with Polyangiitis (GPA): A systemic vasculitis that can affect the respiratory tract, GPA may present with green nasal discharge among other symptoms like nasal crusting and destruction.

Primary Ciliary Dyskinesia (Kartagener Syndrome): A genetic disorder affecting the cilia in the respiratory tract, leading to chronic infections and potentially green nasal discharge due to impaired mucus clearance.
Cystic Fibrosis: While more commonly associated with respiratory and gastrointestinal symptoms, cystic fibrosis can lead to chronic sinusitis and green nasal discharge due to thick, sticky mucus.
Fungal Sinusitis: In immunocompromised individuals, fungal infections of the sinuses can cause green or black nasal discharge, depending on the type of fungus involved.