I was spearing all day by myself once and was getting real tired, figured i'd have a smoke before i packed up, New lighter wouldn't light, When i opened the door to see if something was stuck in it it worked fine and i woke up enough to go stand outside with a bad headache for a while. That was with a mr heater for heat. Ventilation is important, Especially on a spearing shanty where all the light leaks are sealed up.