All combustible materials produce some amount of toxic smoke when they burn. How much toxic smoke will be emitted depends on the material, the amount of oxygen available and how long it burns. In sufficiently high concentrations over a long enough time, toxic smoke creates hazardous health conditions for people exposed to it.
In the early stages of a fire before flashover, (see “What happens in a fire”), the smoke produced will come from the first items ignited, often furniture and other contents of the room. As the fire grows and hits the flashover point, the volume and toxicity of the smoke it produces increases greatly.
If not extinguished or exhausted on its own, the fire continues consuming room contents as well as combustible building materials (including on the building exterior if the fire breaks through windows), which feeds the fire further and expands the amount of toxic smoke. Fire and smoke spreading through the building and up the façade will threaten occupants in parts of the building remote from the fire’s start point, making escape more difficult.
With fires developing 5-10 times faster today than in the 1950s, the safety of building occupants and first-responders depends overwhelmingly on how the building performs during fire, including the release of thick toxic smoke, which kills more people than the fire does. In fact, statistics show that smoke causes more than half of building fire casualties in the UK.
 Department for local communities and government, Fire Statistics Great Britain 2013 – 2015, January 2015.