In developing countries, oxygen therapy in hospitals is frequently humidified with unheated bubble-through humidifiers. We assessed the efficacy of humidification under such circumstances. The water temperature in the humidifier and the ambient air temperature were measured and compared to the oxygen flow rate. It was calculated that oxygen was cooled in the humidifier by 6.12 degrees C (95% CI: 5.88; 6.35) per liter of flow for flow rates up to 2 l/min. Using the average temperatures on the hospital ward in January and August, and the WHO-recommended flow rates of 0.5 l/min and 1 l/min, the relative humidity of the oxygen delivered to a child was estimated to be between 34% and 56%. We conclude that unheated bubble-through humidifiers achieve low humidity in oxygen in tropical climates. Some of the complications associated with the use of nasopharyngeal catheters for the delivery of oxygen might be explained by this, as oxygen of low humidity and temperature is delivered directly into the posterior nasopharynx.