A family of four were taken to hospital in North Yorkshire after a gas cylinder they were using in their tent during a camping trip leaked.
Two adults and two children were found at the Castle Howard camp site near Malton in the early hours of Sunday, North Yorkshire Fire Service said.
They were drifting in and out of consciousness, vomiting and unable to stand or leave the tent, crews said.
Fire officers gave the family oxygen until the ambulance arrived.
The family were in a six-berth dorm tent and had three gas cylinders inside. At least one of the cylinders, a small one being used to power a lamp, was leaking, the fire service said.
Campers have been warned by firefighters not to use gas cylinders inside tents.