 House Dust Mites HDM are arachnids that infest bedding, carpet, upholstered furniture and fabric. Like many other allergens, exposure to HDMA in sensitised patients is associated with poorer lung function, greater medication requirements and more asthma symptoms as well as chronic rhinosinusitis symptoms. In contrast to other allergens, there is evidence that HDMA leads to the development of asthma, in addition to exacerbating pre-existing asthma in HDM-sensitised patients. 