The aim of the study is to find out how blood gas parameters change under the use of surgical mask, FFP2 masks and FFP3 masks (with and without exhalation valve) in patients with known lung disease and long-term oxygen therapy.

In particular, the question arises whether patients become hypoxemic or hypercapnic by using a mask. Or whether wearing a mask increases oxygen saturation by its reservoir function.

Inclusion Criteria:

• Known lung disease and
• LTOT
• informed consent

Exclusion Criteria:

• Severe disease with contraindication for stress testing (e.g. myocardial infarction, acute pulmonary embolism, acute heart failure)
• Acute exacerbation of lung disease

Patients with known lung disease hospitalized at normal wards.