Hydrogen chloride decay, defined as the deposition of hydrogen chloride from fire (and other) atmospheres onto surfaces, has been studied as a function of atmospheric humidity, of size or configuration of the apparatus used, of type of surface in the apparatus and of rate of air movement. This decay means that, in a static system with a sorptive surface, hydrogen chloride has a half-life of 3 min or less, before disappearing from the atmosphere. A simplified mechanism has been devised to identify reactions which simulate the processes involved in hydrogen chloride generation, transport and decay. Mathematical fitting of the hydrogen chloride concentration-time profiles obtained from a large-scale and two small-scale apparatuses was carried out. The "parameters" Downloaded from 106 calculated in this way have been used as a method for screening system variables and identifying dependencies.It has been found that several factors affect hydrogen chloride decay, viz.: (i) The presence of humidity in the atmosphere greatly accelerates decay.(ii) The type of surface exposed to hydrogen chloride will have a large effect in determining peak atmospheric concentrations and rates of decay, with sorptive surfaces being responsible for much more rapid decay than smooth surfaces.(iii) The level of air agitation will also influence hydrogen chloride decay, because of the effect on the rate of transport of the molecules to the reactive surfaces.This work has two important implications in terms of fire hazard: (a) The peak airborne concentrations of hydrogen chloride which will be encountered in a real fire scenario will be much lower than would have been predicted from the chlorine content of the fuel; furthermore, these concentrations may soon decrease to very low levels because of decay.(b) The majority of small-scale smoke toxicity tests use non-sorptive surface environments; they are likely, therefore, to exaggerate the toxicity of the combustion products of chlorine-containing materials such as poly(vinyl chloride).

Large-scale combustion product evaluation experiments were carried out in a realistic room-plenum arrangement. A 30 ft. (9.14 m) length of electrical power wire with flexible PVC jacket and insulation was decomposed, in a plenum, by the action of an electrical overload. The combustion gases measured were HCl, CO, CO 2 and unburned hydrocarbons. The maximum concentration of HCl in the plenum was 3000 ppm (which represents roughly one third of the total chlorine in the wire). However, this amount decreased rapidly

Experiments in which PVC wire insulation was decomposed by an electrical overload in a plenum or in a 200~PMMA box showed that the highest concentration of HCl in the atmosphere was always less than 40% of the theoretical amount of chlorine in the original sample. Furthermore. this concentration quickly decays to a level that is dependent on humidity, but never more than 4% of the theoretical maximum. A mathematical model was used to determine a number of parameters that describe the system. The same model applies to both large and small scale tests. This treatment showed that less than 48% of the chlorine in the wire reached the atmosphere. It also showed that the rate of decay was primarily dependent on the rate of transport in the system, producing a half life of 5-6 min without external agitation. Furthermore, the final HCI concentration was found to be dependent on relative humidity, and on surface composition (painted gypsum or PMMA).