Recently, a researcher left a lit Bunsen burner inside a BSC, closed the sash and walked away. The type of biological safety cabinet she was using recirculates about 70% of the air with 30% of the air goes out the exhaust. When the sash is closed there is no bypass to allow fresh air into the cabinet. Thus, no exhaust was leaving the cabinet. Heat within the BSC built up quickly. The situation was discovered only after the flame had burned for a few minutes. The BSC was hot to the touch on the outside.