FOCUS Toxicity threats


necessary to use measured data and established models of toxicity. The sum of the contribution of individual toxicants is usually expressed as a fractional effective dose (FED). This is the ratio of the amount of each toxicant present to the level causing a particular effect (incapacitation or death). A FED equal to one indicates that the effects of the individual components will result in incapacitation or death for 50% of the exposed population. Preventing incapacitation, or ‘the inability


to effect one’s own escape’ is considered the critical point in fire safety in order to avoid fatalities. Incapacitation resulting from inhalation of the two asphyxiant gases found in fire effluents – CO and HCN – may be predicted from an equation in ISO 135711


. Over the timescales of fire exposure,


CO’s effect is dose dependent (CO concentration multiplied by each exposure duration). The stimulation of breathing caused by cyanide is predicted by raising the concentration to the power 2.36 (see Figure 3 below). The combined effect of all irritant gases can be estimated from the fractional effective concentration (FEC), which is the sum of all irritant concentrations divided by the concentration of each irritant to incapacitate 50% of the exposed population (IC50


Alternatively, death can be predicted from rat lethality data using equations in ISO 133442


, X)


The predicted FED is the sum of the contributions from each toxic gas. The ratio of each toxicant concentration to its lethal concentration (LC50 is multiplied by a factor VCO2


increased breathing rate caused by CO2


in a room or building, the FED can be related to the mass of material which would cause incapacitation or lethality for a given fire condition. The material-IC50


or material-LC50 is the specimen


mass – M – of a burning material, which would yield a FED of one in a volume, V, of 1m3 Figure 6).


(see


Comparing the smoke toxicities of different materials, the lower the material-IC50


or material- LC50 , the smaller the amount of material necessary


to cause incapacitation or lethality, and the more toxic the material’s smoke will be. The values are referenced to the fire condition under which they


were measured. As an example, the material-IC50 and material-LC50


were assessed for insulation


products under three fire conditions. The values are provided in Figure 7 (see next page). That table shows large differences between


the smoke toxicity of different insulation materials whether they are assessed on their capacity to incapacitate or kill. It also shows a progressive increase in smoke toxicity (or decrease in material- IC50


and material-LC50 ) for the combustible foams , X) (see Figure 4). .


with the transition from well ventilated to under ventilated flaming. As both the foams reported here contain gas phase flame retardants, their toxicity in well ventilated conditions is higher than may be expected. Clearly, neither incapacitation nor death are


, to account for the . The


lethality model assumes exposure for 30 min to fixed concentrations of toxicants (see Figure 5). It is important to note that only a FED or FEC


equal to one has a defined meaning. Smaller quantities of toxicants will have different effects. A FED equal to 2 would be sufficient to create a toxic atmosphere in twice the specified volume. In order to have an intrinsic value for the fire


effluent toxicity of a material, for example to determine the maximum permissible loading


desirable outcomes. When assessing smoke toxicity, fire safety engineers need an additional factor to ensure that their building occupants will never be exposed to life threatening fumes. In addition, allowance must be made for the parts of the population with greater susceptibility – for vulnerable occupants, the elderly and those with underlying health problems, all of whom are likely to be present in a given occupancy or workforce.


Assessing toxic hazard from smoke In Figure 7 (see page 33), the potential of a material to produce an incapacitating or lethal smoke is shown. It does not take into account its flammability. A flammable material will have a greater mass loss rate than a less combustible material. The toxic hazard depends on both the flammability of a material and the toxicity of its smoke.


Figure 3: [CO] and [HCN] are the concentrations of CO and HCN in ppm, and ∆t is the exposure time in minutes.


Figure 4


Figure 5 32 MARCH 2020 www.frmjournal.com


Figure 6


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60