Okay , take it from a spieliologist (one who studies caves & their formations) {also a nutter who like nothing more than to crawl into utter darkness & get muddy & wet}...Fires in caves , even small caves are extreemly bad for the occupants .
Most caves in northern europe are formed by water , more often than not running through soluble rock such as lime stone & most with healthy calcium deposits, diluted calcium & the soot, ashed ect from the fire will make Clacium carbide which is highly toxic & gives off a gas called acetylene which aside from being very flamable is also heavier than oxygen .
Light a fire in a badly ventilated lime stone cave & you have a BIG bomb as the acetylene pools at the base of the cave until there is enough to be ignited by the fire.
The heat from a fire will rise sharply in most caverns quickly drying out the rock structure above , which will over time fracture due to the stress from the surrounding material & either send shards of material down from the cavern roof or more likely colapse the entire cavern. (water acts like a glue in most "live" cave systems the absence of air in the rock causing slight vacum)
Most cave dwelling peoples in ages past always lit thier fires at the cave mouth .....the romans favorite method of mining was to light fires in caves & wait for partial colapse ...be a cave man , not a roman