Mucus build-up can be caused by various factors, including infections, allergies, and smoking. When you have an infection, it can make mucus thicker and stickier, leading to inflammation in the mucous membranes that line the nose and the rest of your airway. This can cause certain airway glands to make more mucus, which can get thick with bacteria and cells that arrive to fight the infection, stimulating even more mucus production. Allergies can also cause your body to make extra mucus. When you have an allergy, your immune system overreacts to a harmless substance, like pollen, dust, or animal dander, and cells in your airway release substances like histamine, which can cause mucus production. Smoking can also cause excess mucus production. Other factors that can cause mucus build-up include a dry indoor environment, low consumption of water and other fluids, high consumption of fluids that can lead to fluid loss, certain medications, and irritants, fragrances, chemicals, and pollution. Chronic catarrh, which is a build-up of mucus in an airway or cavity of the body, can be caused by the immune system reacting to an infection or irritation, which causes the lining of your nose and throat to become swollen and produce mucus.