Backdrafts are of great danger. They often ＿＿（51）＿＿ even highly experienced firefighters. A backdraft can occur when a compartment fire has little or no ＿＿（52）＿＿ . Due to this, little or no oxygen can flow into the compartment. Then, because fires ＿＿（53）＿＿ oxygen, the oxygen concentration decreases. When the concentration becomes too ＿＿（54）＿＿ to support combustion, this may cause a phenomenon in which when material is heated enough, it begins to break down into smaller compounds, including hydrogen. However, the hydrogen and smoke remain at a hot ＿＿（55）＿＿ enough to auto-ignite. If oxygen is then re-introduced to the compartment, e.g. by opening a door or window to a closed room, while the gasses are still hot enough to auto-ignite, combustion will often restart abruptly.