Twenty-five people, mainly young people, were taken to hospital in the Norwegian capital, Oslo, on Sunday with suspected carbon monoxide poisoning after attending a party in an underground bunker.
Five were later reported to be in critical but not life-threatening condition, Oslo dailies Aftenposten and VG said.
"Fortunately, they are all improving. We can say that they are out of danger," chief physician Fridtjof Heyerdahl at Oslo University Hospital told Aftenposten.
A Oslo University Hospital spokesman said the 25 patients admitted were mainly between 20 and 30 years of age, news agency NTB reported.
A few have been able to leave hospital, while others were to remain for observation.
Two of the patients were police officers who helped evacuate party goers at the rave.
Oslo University Hospital and police urged every person who attended the party to seek medical attention if they experienced symptoms such as headaches, dizziness, nausea or if they felt faint.
Oslo police and the fire service said they suspected the poisoning was caused by portable generators used to power audio systems.
Ronny Andersen of the fire service told VG that the air was poor in the bunker.
Carbon monoxide (CO) is a colourless, odourless gas which replaces oxygen in the blood when inhaled and continues to inflict damage even after the victim returns to clean air.
200 at the party
Up to 200 people reportedly attended the party, police wrote on Twitter.
Cathrine Sylju of the Oslo police said a probe would be launched into the incident, but none of the partygoers has been interviewed by police.
It was not known who organized the party.
According to the police, the partygoers illegally entered the bunker, which was likened to a cave.
Public broadcaster NRK quoted a man who attended the rave but left a few hours before police arrived at the scene.
The man, who was not named, said he arrived at around 11 pm and there were about 50 people in the bunker. During the night more people, some uninvited, arrived. He said he soon noticed that the air was beginning to get poor.
"When you rave in a room without windows with 50 to 60 others, the air becomes heavy," he said. "I had to go out several times and breathe fresh air."
He added that he did not suspect that the air was dangerous.
According to the man, several parties have been arranged at the bunker this summer.
A police patrol happened on a first group of partygoers near the bunker early Sunday. Several were dizzy and incoherent, triggering a large response from the authorities.