Welling Ware Stainless When Do Firefighters Ware (i Dont Know If I Spelled That Right) The Gasmask/helmet?

When do firefighters ware (i dont know if i spelled that right) the gasmask/helmet? - welling ware stainless

appear in movies, photographs and even books descriptic Most firefighters do to protect the consumer ... I think I would call a gas mask? it is when goods? when required? Why wouldnt ever use a blazing fire?