Opacity and visibility through fire smoke

Fires quickly produce large quantities of smoke. Smoke is made up of combustion gases, suspended droplets of water or fuel, and soot. The conditions under which smoke is generated depend not only on the material in question, but also on its combustion conditions. The phenomena involved in generating smoke, and soot in particular, are therefore highly complex.

One of the characteristics of smoke is opacity. Opacity represents the optical property of smoke to attenuate a beam of light between a source and an observer. The effect of this opacity on the observer takes the form of visibility. Visibility therefore depends on the individual, as well as on the object to be discerned through the smoke. Visibility through smoke is one of the key parameters for evacuating people. However, the link between opacity and visibility is difficult to establish.

Opacity measurement takes overall light attenuation due to all smoke components into account, by assessing the attenuation of light by the medium. Measuring devices are opacimeters, and several tests have been developed and standardized to quantify the propensity of a material to produce smoke when subjected to a heating scenario.

To consider the risk of visibility loss, it is necessary to determine the relationship between observed light attenuation and visibility. Various models have been established to represent this relationship, within a limited range of validity. These models are subject to considerable uncertainty.

The measurement of smoke opacity is included as an essential characteristic in various building and transport regulations. Visibility, on the other hand, is considered in Fire Safety engineering approaches, for example, because of its interaction with the evacuation of people. This involves calculating opacity, then using one of the opacity-visibility models to deduce the effect on people.