Emergency services responded in force after a group of diners fell ill while dining in a large tent at a restaurant in the town of Giswil in canton Obwalden on Saturday evening.
Initial investigations show there was a build-up of carbon monoxide.
35 were in the tent - several started to feel unwell, some lost consciousness - others left the tent to get fresh air.
17 had to be hospitalized.
Reinforced rescue workers were called in from other cantons and two helicopters were used.
An investigation has been launched.
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