<qa>  What is the bronchial provocation testing performed?On the first visit subjects are asked to perform a lung function test spirometry, body plethysmography with helium. Further levels of eNO and eCO are determined. A blood sample is drawn to describe the inflammatory status. Bronchial inflammation will be measured in induced sputum. At the second visit, a non-specific bronchial provocation testing PD20 FEV1 methacholine is performed. 