Catarrh is caused by the body’s natural defenses or the immune system reacting to an infection or irritation. When this occurs, it sends white blood cells to the source of the infection or irritation, which then releases molecules that cause the mucous membranes to swell and produce mucus. Due to this swelling, the cavity is also narrowed. This results in further congestion and this catarrh can either be thick and colored or clear and runny. For the unknown reasons, some people have very sensitive blood vessels that significantly react to environmental triggers, such as pollution and cigarette smoke. As a result, the affected blood vessels swell in a similar way to an infection or allergic reaction. This swelling also leads to congestion and catarrh. It is known as non-allergic rhinitis or vasomotor rhinitis.
Catarrh, itself, is not a condition. It is often a symptom of other conditions such as hay fever or cold and other types of problems triggered by allergies. Cigarette smoke and pollution are common triggers for this, with others including perfume, alcohol, spicy food, changes in the weather and stress.