Smoke is a mixture of heated gases, aerosolized liquids, and solid particulate matter. This mixture is a byproduct of incomplete combustion or thermal decomposition of materials. The temperature of smoke is highly variable, depending entirely on the heat source, the type of fuel, and the surrounding environment.
The Thermal Threshold for Smoke Production
Smoke is a physical manifestation of pyrolysis, which is the thermal decomposition of organic material in the absence of oxygen. This process must occur before the material can ignite into flame. Initial decomposition, which produces the first wisps of smoke, begins at a relatively low thermal threshold.
For materials like wood, pyrolysis begins when the material reaches temperatures between 200°C and 300°C (392°F and 572°F). The least stable components, such as hemicellulose, break down, releasing gases and combustible vapors. This initial breakdown is distinct from smoldering, which is flameless combustion sustained at higher temperatures, typically 350°C to 700°C (662°F to 1,292°F).
A fast, high-heat application results in rapid pyrolysis and flaming combustion that consumes most of the fuel, creating less dense smoke. Conversely, slower, lower-heat applications, such as smoldering, create dense, cooler smoke rich in uncombusted particles and toxic gases. Examples of smoldering include a cigarette or a decaying log.
Factors Determining Smoke Plume Temperature
Once smoke leaves its source, its temperature changes rapidly based on environmental factors. The smoke closest to the fire is the hottest, often consisting of gases that have not yet fully cooled. In the plume directly above a flame, gas temperatures can still be extremely high, frequently exceeding 400°C (752°F).
In an enclosed space, such as a structure fire, the smoke forms a distinct layer beneath the ceiling, which traps the heat and gases. Temperatures within this ceiling-level smoke layer can range from about 150°C (300°F) in the early stages to over 540°C (1,000°F) just before flashover occurs. This hot gas layer requires occupants and firefighters to crouch low to avoid extreme thermal exposure.
Ventilation plays a role in smoke temperature, as the introduction of cooler air rapidly mixes with and cools the hot gases. Smoke encountered outdoors, such as from a distant wildfire, has traveled far enough to equilibrate with the ambient air temperature. While this distant smoke may not pose a thermal threat, its temperature is no longer an indicator of the toxic components it carries.
The Dual Threat of Hot Smoke Inhalation
The temperature of inhaled smoke poses a threat through thermal injury to the respiratory system. The upper airway (nose, mouth, and pharynx) is effective at cooling inhaled gases to protect the lungs. However, exposure to superheated smoke can overwhelm this mechanism, causing thermal burns to the upper respiratory tract and leading to severe swelling and airway obstruction.
Thermal damage is limited to the area above the vocal cords because the efficient heat exchange in the upper airway cools the gas before it reaches the lungs. An exception is the inhalation of steam or superheated wet air, which carries significantly more thermal energy and can bypass the protective cooling mechanism to cause deep tissue burns in the lower airways.
Regardless of the temperature, the second threat is chemical injury from the toxic gases and particles carried within the smoke. Toxic products of incomplete combustion, such as carbon monoxide (CO) and hydrogen cyanide (HCN), are the primary cause of death in fire victims, and their lethality is independent of the smoke’s heat. CO disrupts the blood’s ability to carry oxygen, while HCN interferes with cellular respiration, creating systemic toxicity present even in cooled smoke.