The study will be measuring aerosol generated from the point of tracheal extubation after cardiac surgery in patients who then receive either high-flow nasal oxygen therapy (HFNT) or standard low-flow oxygen therapy, who have been consented to participate within another randomised controlled trial (The NOTACS Study, IRAS ID 278290). Aerosol generation will be compared at various sizes of particles < 10 microns in size and sampled in different locations in the standardised patient care rooms in the ICU. The rate of change of aerosol generation, maximum aerosol generation and duration of aerosol production greater than background will be investigated. The aim is to compare high-flow and standard low-flow oxygen therapy to determine if there is a difference in aerosol generation between the therapies.

Inclusion Criteria:

1. Aged 18 years or over.
2. Undergoing elective or urgent first-time or redo cardiac surgery (CABG, valve surgery, surgery on the aorta or any combination).
3. Have one or more clinical patient-related risk factor for postoperative pulmonary complications (COPD, asthma, lower respiratory tract infection in last 4 weeks as defined by use of antibiotics, body mass index ≥35 kg/m2, current (within the last 6 weeks) heavy smokers (> 10 pack years))(26, 27).

For the purposes of the study, the following definitions apply:

Asthma is a disease characterized by recurrent attacks of breathlessness and wheezing, and patients will have been prescribed medication by inhalers or nebulisers (either bronchodilators or steroids).

Chronic Obstructive Pulmonary Disease (COPD) is an umbrella term used to describe chronic lung diseases that cause limitations in lung airflow. The more familiar terms 'chronic bronchitis' and 'emphysema' are no longer used but are now included within the COPD diagnosis. The most common symptoms of COPD are breathlessness, or a 'need for air', excessive sputum production, and a chronic cough. Patients suitable for the NOTACS study will have been prescribed medication by inhalers or nebulisers (either bronchodilators or steroids).

Exclusion Criteria:

1. Requiring home oxygen therapy.
2. Deep hypothermic circulatory arrest planned.
3. Contraindication to HFNT, e.g. nasal septal defect.
4. Requirement for home respiratory support (including: CPAP, BiPAP).
5. Requiring emergency cardiac surgery defined as surgery required within 24 hours of the decision to operate.
6. Patient's not fluent in English.