Terms and Definitions

Burn: undergo combustion.

Boundary conditions: the value of a mathematical function at the boundary of a calculation domain either known or assumed to be known.

In context of fire calculations these might be surface temperatures, heat or ventilation fluxes or gas pressure. For chemically active surfaces it may include species production or annihilation.

Note: the concept may also be applied to fire tests in relation to physical features of the boundaries of a space containing a fire and influencing heat loss and ventilation. For example, the fire development and combustion conditions in a room enclosure fire depend on features of the boundary conditions, including the surface area and thermal properties of the walls and ceiling, the dimensions and locations of vents such as open doors or windows, wind velocity and direction, ambient temperature, humidity and pressure.

Combustion: exothermic reaction of a substance with an oxidizing agent.

Combustion product: solid, liquid or gaseous material resulting from combustion.

Equivalence ratio: fuel : air ratio divided by fuel : air ratio required for a stoichiometric mixture.

Exposure dose: measure of the maximum amount of a toxic gas or fire effluent that is available for inhalation, calculated by integration of the area under a concentration–time curve.

Note 1: for fire effluent, typical units are grams time minute per cubic metre (g min m⁻³).

Note 2: for a toxic gas, typical units are microliters per litre (µL L⁻¹) at T=298 K and P=1 atm.

Fire: process of combustion characterized by emission of heat and fire effluent and usually accompanied by smoke, flame, glowing or a combination thereof.

Fire scenario: qualitative description of the course of a fire with respect to time, identifying key events that characterize the studied fire and differentiate it from other possible fires.

Note: it typically defines the ignition and fire growth processes, the fully developed fire stage, the fire decay stage and the environments and systems that impact on the course of the fire.

Fire effluent (also known as “smoke”): totality of gases and aerosols, including suspended particles created by combustion, or pyrolysis in a fire.

Fractional effective concentration (FEC): ratio of the concentration of an irritant to that concentration expected to produce a specified effect on an exposed subject of average susceptibility.

Note 1: as a concept, FEC may refer to any effect, including impairment of escape capability, incapacitation, lethality or other endpoints.

Note 2: when not used with reference to a specific irritant, the term FEC represents the summation of the FEC value for all measured irritants in a combustion atmosphere.

Fractional effective dose (FED): ratio of the exposure dose for a specific combustion product or mixture of products to that exposure dose predicted to produce a specified effect on an exposed subject of average susceptibility.

Note 1: as a concept, FED may refer to any effect, including incapacitation, lethality or other endpoints.

Note 2: when not used with reference of any specific substance, the term “FED” represents the summation of the FED values for all measured components of a combustion atmosphere.

Flame retardant: substance added, or treatment applied, to a material in order to suppress or delay the appearance of flame and/or reduce the flame-spread rate.

Heat release rate: rate of thermal energy production generated by combustion (essentially the burning rate).

Glowing combustion: combustion of a material in the solid phase without flame but with emission of light from the combustion zone.

Flaming combustion: combustion in the gaseous phase, usually with emission of light.

Flashover (stage of fire): transition to a state of total surface involvement in a fire of combustible materials within an enclosure.

Fuel-rich combustion: combustion in which the equivalence ratio is greater than unity.

Note: in ventilation-controlled fires the fuel/air mixture is fuel-rich and relatively high concentrations of pyrolysis products and incomplete combustion gases result.

Fuel-lean combustion: combustion in which the equivalence ratio is less than unity.

Note: in well-ventilated fires, the fuel/air mixture is fuel-lean and complete combustion tends to occur.

LC₅₀ (lethal concentration 50%): the concentration statistically calculated to cause the deaths of one half of the animals exposed to a toxicant for a specified time. This may be expressed in terms of volume fraction/volume fraction (ppm, %) or mass/volume (g m⁻³).

Note 1: care must be taken when comparing LC₅₀ values of both the exposure duration and the post exposure period over which deaths were scored. In combustion toxicology, LC₅₀ values have most commonly been measured using a 30 minute exposure period followed by a 14 day post exposure observation period.

Note 2: in combustion toxicology, an LC₅₀ concentration may be related to a specific combustion product such as carbon monoxide, of may be related to the test material rather than its actual products, expressed in terms of mass charge or mass loss concentration. For example, the LC₅₀ (mass loss) for wood decomposed under a specific combustion condition might be expressed as 100 g wood m⁻³. This means that when 100 g of wood were decomposed in the furnace system and the products were dispersed into 1 m³, exposure to the resultant combustion product atmosphere resulted in a 50% death rate.

Nominal atmosphere concentration mass charge (mass charge concentration): the mass of a test specimen exposed to heating in a furnace system divided by the volume of air into which the combustion products are dispersed (g m⁻³).

Nominal atmosphere concentration mass loss (mass loss concentration): the mass loss (mass decomposed) of a test specimen exposed to heating in a furnace system divided by the volume of air into which the combustion products are dispersed (g m⁻³).

Potency: the toxic potency is a measure of the amount of a toxic substance required to elicit a specific toxic effect—the smaller the amount required, the greater the toxic potency. Where the toxic effect is lethality, the potency can be expressed as the reciprocal of the LC₅₀ concentration.

Pyrolysis: chemical decomposition of a substance by the action of heat (cf. thermal decomposition).

Note 1: pyrolysis is often used to refer to a stage or fire before flaming combustion has begun.

Note 2: in fire science, no assumption is made about the presence or absence of oxygen (but chemically it is possible to distinguished between endothermic pyrolysis under nitrogen or in air, the latter involving some degree of exothermic oxidation).

RD₅₀ (respiratory depression 50%): statistically calculated concentration of a sensory irritant required to reduce the breathing rate of laboratory rodents (usually mice) by 50%.

Note: in combustion toxicology, an RD₅₀ concentration may be related to a specific combustion product such as hydrogen chloride, or may be related to the test material rather than its actual products, expressed in terms of mass charge or mass loss concentration.

Smouldering combustion: combustion of a material without flame and without visible light.

Stoichiometric combustion: mixture of chemical reactants having proportions in accordance with the equation for a specified chemical reaction.

Note: in a fire context this means that the fuel : air equivalence ratio is at unity, so that there is exactly the right amount of oxygen available for complete combustion of the fuel.

Thermal decomposition: process whereby the action of heat or elevated temperature on an item cause changes to the chemical composition.

Yield: mass of a combustion product generated during combustion divided by the mass of the test specimen. Note: yield can be expressed as mass charge yield or mass loss yield (g product per g specimen).

Recovery fraction (also known as conversion efficiency): The yield of a specified fuel element in a combustion product as a fraction of the maximum yield obtainable from complete conversion of the fuel element to that in the product. This can be calculated from the measured yield of the product of interest relative to its notional yield from complete conversion. For example, if all the carbon in a fuel specimen is burned to carbon dioxide, the recovery fraction for carbon dioxide is 1. If only half the carbon is recovered as carbon dioxide, then the recovery fraction for carbon dioxide is 0.5.