Catarrh is caused by the immune system reacting to an infection or irritation in an airway or a cavity of the body, such as inside the nose.
Your immune system sends infection-fighting white blood cells to the source of the infection.
The white blood cells cause the mucous membrane that lines the affected area to swell and produce mucus. The swelling will also narrow the cavity, resulting in further congestion.
The most common triggers of catarrh are:
Other triggers include non-allergic rhinitis and nasal polyps.
Some people have abnormally sensitive blood vessels that react to environmental triggers, such as cigarette smoke and pollution. The reasons for this over-sensitivity are unknown.
The affected blood vessels become enlarged (swollen) in a similar way to their response to an infection or allergic reaction. The swelling leads to congestion and catarrh.
As well as cigarette smoke and pollution, other triggers of non-allergic rhinitis include:
- chemical irritants, such as perfume or paint fumes
- changes in the weather
- spicy food
Read more about non-allergic rhinitis.
Nasal polyps are non-cancerous, fleshy swellings that grow from the lining of your nose or your sinuses (the small cavities inside your nose).
The polyps can prevent mucus from properly draining out of your nose or sinuses, leading to congestion and catarrh.
Read more about nasal polyps.