What causes a child's clear nasal discharge to turn yellow?

A child's clear nasal discharge turning yellow usually indicates disease progression. Clear mucus is often seen in the early stages of viral infection, when nasal secretions are relatively thin. As inflammation worsens, the interaction between white blood cells and bacteria produces purulent material, causing the mucus to turn yellow. This may be a sign of upper respiratory tract infections such as the common cold or sinusitis. Another possibility is allergic rhinitis—when a child is exposed to allergens, an allergic reaction can trigger nasal inflammation, leading to yellow nasal discharge. Additionally, factors such as cold air irritation, nasal polyps, or nasal injury may also cause changes in the color of a child's nasal mucus.