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| Study | HH | HME | No of participants |
| Boots 2006 | 9 | 7 | 381 |
| Diaz 2002 | 4 | 4 | 43 |
| Hurni 1997 | 13.80 | 11.10 | 104 |
| Kollef 1998 | 5.30 | 5.70 | 310 |
| Lacherade 2005 | 25.3 | 21.4 | 369 |
| Roustan 1992 | 9.30 | 13.90 | 116 |
HH: heated humidification; HME: heat and moisture exchangers.
