Inclusion Criteria:  age 18 years or older capable to understand the purpose of the study female/male ratio approximately 50%/50% in both of the following age groups: 50% aged 18- 50 yrs, and 50 % aged 51 - 90 years  Exclusion Criteria:  Current daily smoking Ex-smoking with a greater than 10 pack years' daily smoking history or with stopping of daily smoking less than one year ago Pregnancy and breastfeeding A doctor's diagnosis of any respiratory diseases, including both upper and lower airway diseases, lung parenchymal diseases and the sleep apnoea syndrome A doctor's diagnosis of gastro-oesophageal reflux disease or symptoms suggestive of it Current use of angiotensin-converting enzyme inhibitors Upper respiratory tract infection ('flu') within 4 weeks Any current (within 4 weeks) cough 8 Chronic (over 2 months' duration) cough during the last 12 months Wheezing during the last 12 months Attacks of shortness of breath or cough at night during the last 12 months Symptoms of rhinitis in a prolonged fashion during the last 12 months Heartburn or regurgitation once a week or more often during the last 3 months Abnormal anatomy of the respiratory organs, which may affect the deposition of the mannitol powder Any previous operations to the respiratory organs that can affect the deposition of the mannitol powder Unable to provide written informed consent to participate in the study