When patients undergo major surgery, they are connected to a ventilator during anesthesia. A heat and moisture exchange filter (HME filter) is routinely placed in the ventilator tubing to ensure that the inhaled air is warm and humidified. Today, HME filters are used routinely in all ventilator treatments. They are typically replaced once a day and discarded as regular waste after use.

The purpose of this study is to investigate whether HME filters contain proteins and small biological particles from the patient's exhaled air.

Inclusion Criteria:

• ≥18 years, able to understand study information in Danish.
• Admitted for elective surgery for non-malignant conditions with either intravenous or inhalation anaesthesia and invasive mechanical ventilation according to standard practice.
• Expected invasive mechanical ventilation > 1 hour.
• Possess legal capacity.
• Informed, signed consent is obtained.

Exclusion Criteria:

• Patients who are transferred to the ICU for continuous invasive mechanical ventilation after end of elective surgery.
• Known chronic lung disease.

The investigators will include adult patients who are scheduled for elective surgery for non-malignant conditions under general anesthesia with invasive mechanical ventilation. The written and oral study information will be provided at the standard preoperative consultation with the anesthesiologist the day before elective surgery.

All data analyses will be performed in our institution as only conventional biochemical analyses will be applied. No external collaborators are needed and hence the investigators have no need to share IPD outside the instutution.