This rule states that the amount of heat released during the consumption of a given quantity of oxygen is relatively constant for most combustibles. In other words, each kilogram of oxygen used in the combustion of common organic materials results in release of 13.1 MJ of energy. This means that the heat released per unit of oxygen consumed is about the same for wood or plastic. In a ventilation-controlled fire, where the amount of air entering through openings in a room governs the fire, the heat release rate in the room cannot exceed what the available air supply will support.

Air supply may limit the heat release rate in the compartment but that unburned gases (those that could not burn in the room) can burn outside of the compartment. But in the late 1970s, fire researcher C. Huggett at the National Institute of Standards and Technology (NIST) verified Thornton’s Rule using the oxygen consumption calorimetry technique, developed at NIST in the early 1970s. In “Estimation of Rate of Heat Release by Means of Oxygen Consumption Measurements,” Huggett shows how much energy was released per gram of oxygen for common combustibles. Where Thornton was only able to estimate the energy release based on the oxidation of carbon-carbon and carbon-hydrogen bonds, Huggett, with modern technology, was able to make actual measurements. Huggett simply verified Thornton’s earlier observation, which is the reason it is known today as Thornton’s Rule.