The term 'causes of catarrh' can be summarized by the answer to the crossword clue 'allergens' (9 letters). Allergens are substances that cause an allergic reaction in some people, and they are one of the most common causes of catarrh. Catarrh is a condition characterized by inflammation of the mucous membranes in the nose and throat, which can lead to a runny nose, sneezing, coughing, and other symptoms. Allergens can include pollen, dust mites, pet dander, mold, and certain foods.