Current participant exclusion criteria as of 04/05/2021: 1. Planned intubation and mechanical ventilation imminent within 1 hour 2. Known or clinically apparent pregnancy 3. Any absolute contraindication to CPAP or HFNO 4. Decision not to intubate due to ceiling of treatment or withdrawal of treatment anticipated 5. Equipment for both CPAP and HFNO not available Previous participant exclusion criteria: 1. Planned intubation and mechanical ventilation imminent within 1 hour 2. Known or clinically apparent pregnancy 3. Any absolute contraindication to CPAP or HFNO 4. Decision not to intubate due to ceiling of treatment or withdrawal of care anticipated 5. Equipment for both CPAP and HFNO not available

Current participant exclusion criteria as of 04/05/2021: 1. Planned intubation and mechanical ventilation imminent within 1 hour 2. Known or clinically apparent pregnancy 3. Any absolute contraindication to CPAP or HFNO 4. Decision not to intubate due to ceiling of treatment or withdrawal of treatment anticipated 5. Equipment for both CPAP and HFNO not available Previous participant exclusion criteria: 1. Planned intubation and mechanical ventilation imminent within 1 hour 2. Known or clinically apparent pregnancy 3. Any absolute contraindication to CPAP or HFNO 4. Decision not to intubate due to ceiling of treatment or withdrawal of care anticipated 5. Equipment for both CPAP and HFNO not available

1. Planned intubation and mechanical ventilation imminent within 1 hour 2. Known or clinically apparent pregnancy 3. Any absolute contraindication to CPAP or HFNO 4. Decision not to intubate due to ceiling of treatment or withdrawal of care anticipated 5. Equipment for both CPAP and HFNO not available

1. Planned intubation and mechanical ventilation imminent within 1 hour 2. Known or clinically apparent pregnancy 3. Any absolute contraindication to CPAP or HFNO 4. Decision not to intubate due to ceiling of treatment or withdrawal of care anticipated 5. Equipment for both CPAP and HFNO not available