Nosebleeding in adults is primarily caused by local trauma and dryness of the nasal mucosa. Common triggers include:

• Direct injury such as blows to the face or nose picking.
• Dry air or climates with low humidity, which dry out the mucous membranes inside the nose, causing cracking and bleeding.
• Frequent nose blowing, irritation from allergies or infections like sinusitis.
• Use of medications such as blood thinners, nasal steroids, or nonsteroidal anti-inflammatory drugs.
• Exposure to irritants like smoke, chemical fumes, or illegal drugs inhaled nasally (e.g., cocaine).
• Underlying medical conditions such as liver disease (which affects clotting), bleeding disorders, or tumors in the nasal cavity.
• Anatomical issues such as a deviated septum.

Nosebleeds can be classified as anterior (from the front nasal septum, usually less severe) or posterior (from deeper arterial branches, often more serious). In many cases, nosebleeds are not serious but sometimes signify underlying health concerns requiring medical evaluation.

Therefore, nosebleeds in adults are mostly caused by nasal mucosal dryness, trauma, irritation, medications, and sometimes more serious health conditions.