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More respiratory infections in adults followed the use of humidified oxygen compared with non-humidified low-flow oxygen therapy. Bacterial contamination was common in the humidified oxygen bottles across various hospital departments including respiratory wards.

UK guidelines currently recommend using non-humidified oxygen for adults requiring low-flow oxygen. However, in countries such as China, oxygen is routinely humidified regardless of flow rate to prevent respiratory dryness.

This review identified 27 trials, most conducted in China. Infection rates were low, less than 2%. There was also no difference in reports of dry nose, throat or chest discomfort.

Adults on long-term, low-flow therapy should continue to receive non-humidified oxygen in-line with UK guidance.

Why was this study needed?

Long-term oxygen therapy involves the delivery of oxygen for a minimum of 15 hours per day. It helps to maintain blood oxygen levels and reduce feelings of tiredness and breathlessness in people with chronic obstructive pulmonary disease as well as other long-term respiratory conditions, heart failure and cancer. Long-term oxygen therapy is often delivered at a low-flow rate of fewer than five litres of oxygen per minute via soft plastic nasal cannulae.

Oxygen can be humidified with the aim of reducing sensations of dryness in the upper airways. This may be important in high-flow oxygen therapy but the benefit of humidifying low-flow oxygen delivered through nasal cannulae has been uncertain. UK national guidance does not recommend it but some local guidelines support its use. In some countries, such as China and Japan, oxygen is routinely humidified regardless of flow rate.

This review aimed to clarify the effectiveness of humidifying low-flow oxygen.

What did this study do?

This systematic review identified randomised controlled trials and quasi-randomised trials comparing humidified or non-humidified low-flow oxygen therapy given to adults in hospital.

A total 27 studies including 8,876 adults were pooled in the meta-analysis. Three studies were conducted outside China. Sterile distilled water was mostly used as the humidifier fluid (19 of 27 studies) and oxygen flow rate varied from one to four L/min.

Objective outcome measures included bacterial contamination of the humidifier bottle and the time taken for medical staff to administer oxygen. Subjective measures included nose, throat and chest discomfort.

Common study limitations included the potential for biased group allocation and patients or staff being aware of treatment given. Individual study results varied considerably for most outcomes. The size of any effect demonstrated should be treated with caution; however, the direction of effect, given the contamination of humidifier bottles and the link to respiratory infections, seems plausible.

What did it find?

  • Non-humidified oxygen reduced the likelihood of respiratory infection by almost two-thirds compared with humidified oxygen, with an incidence rate of 1.6% vs. 3.8% (odds ratio [OR] 0.39, 95% confidence interval [CI] 0.21 to 0.73, four studies).
  • There was a high rate of contamination by bacteria in the humidifier bottles. 34.5% of humidified bottles were contaminated compared with 12.9% of non-humidified oxygen container bottles (OR 0.16, 95% CI 0.06 to 0.43, eight studies).
  • There was no significant difference between groups in rates of dry nose or throat, nosebleed, chest discomfort, the smell of oxygen and blood oxygen levels.

What does current guidance say on this issue?

British Thoracic Society guidelines advise that humidification is not required when delivering low-flow oxygen (via mask or cannulae) or high-flow oxygen for short periods. They advise that there is ‘little scientific evidence’ of any benefit from humidified oxygen.

The exception is patients with a tracheostomy tube, where the air is bypassing the nose and throat where it is normally warmed and humidified. Humidification of oxygen is recommended for these people.

People requiring high-flow oxygen for more than 24 hours, who report dryness or discomfort of the upper airways, or who have problems with sputum retention may benefit from humidification.

What are the implications?

The findings support current guideline recommendations that routine humidification of low-flow oxygen offers no benefit and should not be used.

Almost all evidence was from studies conducted in China, so population and demographic differences might limit applicability to the UK. For example, in some parts of China environmental air humidity may be a relevant factor to consider. One trial found that non-humidification in a dry climate led to more complications.

The low quality of the evidence means that it’s not possible to rule out a benefit of humidification in certain settings or patient groups.


Citation and Funding

Wen Z, Wang W, Zhang H, et al. Is humidified better than non-humidified low-flow oxygen therapy? A systematic review and meta-analysis. J Adv Nurs. 2017. [Epub ahead of print].

This study received no specific grant from any funding agency in the commercial, public or not-for-profit sectors.



British Lung Foundation. What is home oxygen therapy? London: British Lung Foundation; 2017.

Hardinge M, Annandale J, Bourne S, et al; British Thoracic Society Home Oxygen Guideline Development Group; British Thoracic Society Standards of Care Committee. British Thoracic Society guidelines for home oxygen use in adults. Thorax. 2015;70(1):i1-43.

NHS Choices. Home oxygen therapy. London: Department of Health; 2015.

O'Driscoll BR, Howard LS, Earis J, Mak V; British Thoracic Society Emergency Oxygen Guideline Group; BTS Emergency Oxygen Guideline Development Group. BTS guideline for oxygen use in adults in healthcare and emergency settings. Thorax. 2017;72(1):ii1-90.

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  • Long-term oxygen therapy is defined as oxygen used for at least 15 h per day in patients with chronic oxygen deficiency (chronic hypoxaemia).
  • Chronic hypoxaemia is defined as a partial pressure of oxygen in the arterial blood (PaO2) of ≤7.3 kPa or, in certain clinical situations, ≤8.0 kPa.
  • Low-flow oxygen systems deliver oxygen at flow rates below the patient's inspiratory flow rate.
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