Lung transplantation (LTx) is the only effective treatment for patients with end stage lung disease. Of the major organs transplanted, survival following LTx is the lowest with a mean of 5 years. Despite improvements, primary graft dysfunction (PGD) remains the leading cause of early mortality and contributes to the development of chronic lung allograft dysfunction (CLAD) that remains the leading cause of late mortality. Earlier detection of rejection after LTx is of substantial importance as it would improve the possibilities of treatment and could increase survival. The investigators have shown in previous work that exhaled breath particles (EBP) reflect the composition of respiratory tract lining fluid (RTLF). EBP and particle flow rate (PFR) can be used as non-invasive methods for early detection and monitoring of airway diseases such as acute respiratory distress syndrome (ARDS). It has also been shown that the particle flow prolife after lung transplantation differs between patients who develop PGD and those who do not and that the composition of EBP differs between patients with and without bronchiolitis obliterans syndrome (BOS), an obstructive form of CLAD. Samples of EBP and measurements of PFR will be collected from lung transplanted patients. Membranes with EBP will be saved for molecular analysis. The investigators aim to identify potential particle flow patterns and biomarkers for earlier detection of rejection after lung transplantation.