A further 13 people were treated, with five of those affected judged by doctors to be in a critical condition.
More than 200 people had gathered in the cave to dance to the sound system, Oslo police wrote on Twitter, with neither the participants nor organisers realising that using a diesel generator in a confined space was so dangerous.
Oppdatering: Totalt 7 bevisstløse personer ble kjørt til sykehus. Det skal være fra alvorlig til kritisk for flere av disse. 2 polititjenestemenn ble også utsatt for kullos og får behandling.
— OPS Politiet Oslo (@oslopolitiops) August 30, 2020
“It was a rave party in honour of someone who had a birthday,” Arve Røtterud from the Oslo police told the VG newspaper. “Carbon monoxide poisoning should be taken seriously, and several there were under the influence of drugs when they left.”
According to Ronny Andersen, from the fire services, the party-goers, who were between 20 and 30 years old, had brought in several portable generation units, with the oxygen level in the cave as low as 16 percent when the emergency services arrived.
“They started these up to generate power for the music, this created carbon monoxide and removed the oxygen, and that is why there was such a poor breathing atmosphere in there.”
Anders Bayer, press officer at Oslo University Hospital, said that it was lucky fewer had not been affected.
“What is serious about carbon monoxide poisoning is that you do not notice it, and then can become unconscious and die in a relatively short time.”
Two of the 25 patients were police officers who entered the cave to help evacuate the partygoers.
According to Røtterud, police discovered the accident when a car was flagged down by one of those affected.
The cave is approximately 70 meters deep and shaped like an 'E', Andersen said, and was built as an air raid bunker.