High flow nasal cannula（HFNC）oxygen therapy is among oxygen delivery methods. HFNC can decrease anatomical dead space and create slight positive nasopharyngeal pressure in addition to providing high F_IO₂ and a completely humidified condition. Therefore, HFNC is now widely used in addition to noninvasive ventilation（NIV）. Although the positive pressure by HFNC is small and high transpulmonary pressure associated with spontaneous vigorous inspiratory effort could occur during HFNC therapy for patients with coronavirus Disease 2019（COVID-19）, it is preferred by patients because HFNC needs no face or nasal mask. Recently, reports were published that showed the usefulness of HFNC oxygen therapy. Without mechanical ventilation or an NIV instrument, HFNC oxygen therapy can be performed with a special delivery system and humidifier. Since HFNC can produce high F_IO₂ with slight positive airway pressure, it is sometimes too late for physicians to switch from HFNC oxygen therapy to mechanical ventilation with intubation, which might make patients’ prognosis worse, as would NIV.

  Although HFNC oxygen therapy could be useful for COVID-19 patients with hypoxemia, we should recognize concerns for the increased risk of viral transmission by bio-aerosol dispersion. In addition, although HFNC could be used without mechanical ventilation, there is also a concern about piped oxygen supplies in hospitals.

  In summary, there are many unresolved issues about HFNC oxygen therapy for patients with COVID-19. Therefore, some members of the Japanese Society of Respiratory Care Medicine and the Japan Association for Clinical Engineers have reported their consensus, including unresolved issues, about the usage of HFNC, other types of oxygen therapy and NIV based on current worldwide information.