Can high-flow nasal cannulae be used without humidification and how? 

Posted on April 30, 2020 by Raymond Cho


To the best of our knowledge, there is no evidence to support usage of HFNC without humidification and may actually cause harm to patients. However, the theoretical increased risk of COVID-19 transmission has no evidence to support it and sparse evidence suggests that HFNC may not affect infectivity.

I.   Benefits of High-flow nasal cannula (HFNC) in patients with hypoxemic respiratory failure

  • Clinical and physiological benefits (1)
    • Maintains high FiO2 by delivering high flows and reducing room air oxygen dilution (2).
    • Decrease anatomic dead space by washing out CO2 from upper airways.
    • Reduce work of breathing, decreases risk of atelectasis and reduces V/Q mismatch by delivering warmed and humidified gas optimized to physiological conditions.
    • Delivers positive pressure to increase lung volume and improve gas exchange.
  • The Surviving Sepsis guidelines give a weak recommendation to use HFNC on patients with acute hypoxemic respiratory failure (3). A meta-analysis used in the guidelines using 2093 patients suggest that HFNC reduces the need for intubation compared with conventional oxygen (RR 0.83, 94% CI) but does not change mortality or ICU length of stay.
  • ANZICS (Australian) guidelines state that HFNC is recommended for hypoxia related to COVID-19 as long as staff are wearing optimal airborne PPE. Risk of airborne transmission to staff is low with fitted, newer HFNC and negative pressure rooms are preferable (4).

II.   Evidence against increased viral transmission due to HFNC

  • There is widespread concern that HFNC can increase risk of viral transmission but a lack of evidence to support this concern (5).
  • WHO guidelines suggest that “newer HFNC and non-invasive ventilation with good interface fitting do not create widespread dispersion of exhaled air and therefore should be associated with low risk of airborne transmission (6).
  • Reasons HFNC may not increase viral transmission (5)
    • Because HFNC has a rate of 40-60 L/min and a normal cough has a flow of ~400 L/min, it is unlikely that a patient on HFNC is more contagious than a patient on standard nasal cannula.
    • Although ventilation does not have the same theoretical risk of transmission as HFNC, intubation certainly carries significant risk  of transmission to HCPs.
    • Existing evidence, although sparse, does not support the concept that HFNC increases pathogen transmission, including patients with bacterial pneumonia and an abstract looking at dispersal of particulates with volunteers in an experimental setting (7,8)

III.   Consequences of unwarmed, unhumidified HFNC

  • Bronchoconstriction: cold air causes bronchoconstriction (i.e. patients with asthma) and has been shown that 5 minutes of unconditioned gas can induce a significant decrease in pulmonary compliance and conductance in infants (9).
  • Mucociliary function: conditioned gas improves mucociliary function to facilitate clearance of secretions and reduce atelectasis, resulting in improved oxygenation. Prolonged ventilation with dry gas could also result in thickened secretions that may cause airway obstruction (10).
  • Patient discomfort: conventional non-humidified devices are associated with mask discomfort, nasal dryness, oral dryness, eye irritation, nasal/eye trauma and gastric distension. Therefore, many patients are unable to tolerate high-flow unhumidified gas for long periods of time (10).

IV.   Overall recommendations

  • Unfortunately, we were unable to find evidence supporting use of unhumidified HFNC in patients with hypoxemic respiratory failure (nor a protocol dictating its use). There is a potential for harm in patients with respiratory failure.
  • Most protocols recommend that HCPs use adequate airborne protection, negative pressure rooms and a max flow rate of 30 L/min to reduce the theoretical risk of airborne transmission.

However, there is a lack of evidence to support that HFNCs are truly associated with increased viral transmission (we have insufficient evidence for or against using HFNC in settings without a negative pressure room).

Questions? Comments? Does this need to be updated? Do you have valuable points to add ? Please email


  1. Rali AS, Nunna KR, Howard C, Herlihy JP, Guntupalli KK. High-flow Nasal Cannula Oxygenation Revisited in COVID-19. Cardiac Failure Review. 2020Apr11;6.
  2. Lodeserto F. High Flow Nasal Cannula (HFNC) – Part 1: How It Works [Internet]. REBEL EM – Emergency Medicine Blog. 2020 [cited 2020May1]. Available from:
  3. Alhazzani W, Møller MH, Arabi YM, Loeb M, Gong MN, Fan E, et al. Surviving Sepsis Campaign. Critical Care Medicine. 2020Mar20;
  4. Holley A. The Australian and New Zealand Intensive Care Society (ANZICS) COVID-19 Guidelines [Internet]. ANZICS. 2020 [cited 2020May1]. Available from:
  5. Farkas J. COVID-19 [Internet]. EMCrit Project. 2020 [cited 2020May1]. Available from:
  6. Clinical management of severe acute respiratory infection when COVID-19 is suspected [Internet]. World Health Organization. World Health Organization; 2020 [cited 2020May1]. Available from:
  7. Leung C, Joynt G, Gomersall C, Wong W, Lee A, Ling L, et al. Comparison of high-flow nasal cannula versus oxygen face mask for environmental bacterial contamination in critically ill pneumonia patients: a randomized controlled crossover trial. Journal of Hospital Infection. 2019Jan;101(1):84–7.
  8. Roberts S, Kabaliuk N, Spence C, O’Donnell J, Abidin ZZ, Dougherty R, et al. Nasal high-flow therapy and dispersion of nasal aerosols in an experimental setting [Internet]. Abstracts. [cited 2020May1]. Available from:
  9. Nishimura M. High-flow nasal cannula oxygen therapy in adults. Journal of Intensive Care. 2015Mar31;3(1).
  10. Nishimura M. High-Flow Nasal Cannula Oxygen Therapy in Adults: Physiological Benefits, Indication, Clinical Benefits, and Adverse Effects. Respiratory Care. 2016Apr25;61(4):529–41.


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