Six people suffered carbon monoxide poisoning from fumes from a charcoal barbecue they are believed to have been using in their conservatory.
Ambulance crews were called to a house in Gosmore Road, Hitchin, on Monday night after a man in his 50s collapsed and five other people had fallen ill.
The six, who were all said to be breathing normally, were taken to the Lister Hospital, Hertfordshire.
The ambulance service warned people about the use of charcoal barbecues.
An East of England Ambulance Service spokeswoman said: “Unfortunately at this time of year we do start to see incidents relating to the use of charcoal barbecues and want to take this opportunity to emphatically warn people to use them safely and properly.
“Last July we were called to a woman who sadly died after inhaling carbon monoxide fumes from a barbecue in her tent in Norfolk and we do not want to see a repeat of that tragedy.
“Please make absolutely sure your barbecue is used and stored, even after the heat has died down, in a clear open space to ensure enough ventilation.”