Smoke, toxic potency

Knowing the impact of smoke toxic potency on escape from a fire is of sufficient importance that it has been the subject of research for over twenty years. As a result, we now have a realistic picture of proper contexts for the use of toxic potency data and a series of first-generation tools for measuring it. We also have a vision of the key technical issues to be resolved developing a proper small-scale fire simulator, relating rodent results to people, and validating the small-scale data. 

Wittbecker F-W, Bansemer B. A model for the scenario-related assessment of the smoke toxic potency. Interflam 2004, Proceedings of the Tenth International Conference, Edinburgh. Interscience Communication Limited London, 2004 pp. 1479-1490. 

The consequence of this is that any toxic potency (LC50) higher than 8mg/L will be subsumed within the toxicity of the atmosphere, and is of no consequence. Thus, values 8 or greater should be converted to 8mg/L for reporting purposes. Moreover, almost all common materials have virtually the same smoke toxicity their associated fire hazard will not be a function of smoke toxic potency. 

As these strategies are brought to fruition, there remains one related issue the determination of a smoke s potential harm per mass of material burned, i.e., the toxic potency of smoke. Accurate measurement of this key characteristic of fire smoke permits a more quantitative determination of the fire s toxic hazard which includes other factors as discussed below. Toxic potency assessment also tells us whether a small fire will produce smoke so toxic that only a small amount will kill. The presence of such "supertoxicants" has been a major topic of discussion within the fire community. 

All fire smoke is toxic. In the past two decades, a sizable research effort has resulted in the development of over twenty methods to measure the toxic potency of those fire smokes (6). Some methods have been based on determinations of specific chemical species alone. Values for the effect (e.g., lethality) of these chemicals on humans are obtained from (a) extrapolation from preexisting, lower concentration human exposure data or from (b) interpretation of autopsy data from accident and suicide victims. The uncertainty in these methods is large since ... 

A second approach to the problem of toxic potency measurement has been to expose laboratory animals, usually rodents, to the smoke from the combustion of small samples of a burning material. Measurement of their response to the smoke leads to one of several biological endpoints, such as the LC50 (the concentration of smoke lethal to 50% of the test animals). In this approach, the animals respond to all the toxicants that are present in the smoke. It presumes that rodent mortality can be related to human mortality or, more simplistically, that the relative toxicity of the smokes will be similar in humans and rodents. However, since the relative contributions of the individual toxic chemicals in the smoke are not determined, a quantitative relationship between man and rodent is impossible using this approach. 

LEVIN GANN Toxic Potency of Fire Smoke... 

The calculation of the fire s outcome in the third step includes the distribution of heat, smoke, and toxic gases throughout the building of concern. It allows the introduction of people into that building and monitors their movement in response to the fire. They may escape safely or fail to escape due to heat or the inhalation of toxic smoke. The benefits of changing some component of the defined fire problem is observed in the change in the number of deaths predicted, rather than by direct comparison of the toxic potencies of the different smokes. This mirrors the complexity of real-life fires. 

A large number of small-scale tests have been designed to measure the toxic potency of the smoke of materials. These tests differ in many respects the consequence of this is that the relative toxic potencies of smoke resulting from these various tests are different. The tests are not useful,... 

Fire safety can be improved by decreasing fire hazard, but is unlikely to be affected by small changes in toxic potency of smoke, since the toxic potency of most materials is very similar. 

During the 1970 s and early 1980 s a large number of test methods were developed to measure the toxic potency of the smoke produced from burning materials. The ones most widely used are in refs. 29-32. These tests differ in several respects the conditions under which the material is burnt, the characteristics of the air flow (i.e. static or dynamic), the type of method used to evaluate smoke toxicity (i.e. analytical or bioassay), the animal model used for bioassay tests, and the end point determined. As a consequence of all these differences the tests result in a tremendous variation of ranking for the smoke of various materials. A case in point was made in a study of the toxic potency of 14 materials by two methods [33]. It showed (Table I) that the material ranked most toxic by one of the protocols used was ranked least toxic by the other protocol Although neither of these protocols is in common use in the late 1980 s, it illustrates some of the shortcomings associated with small scale toxic potency of smoke tests. 

The toxic potency of the smoke of most common materials (natural or synthetic) is very similar (see Figure 1). In fact, the difference between the toxic potency of almost all combustible materials is less than one order of magnitude. Therefore, the relative rankings of materials are heavily dependent on the exact composition of the smoke being tested, i.e. on the combustion procedure being used. 

Categories of toxicity are classically distinguished by differences in orders of magnitude. The toxic potency of the smoke of most common materials is very similar, and thus, the toxicity of smoke is usually governed simply by the amount of material burnt per unit time. 

Toxic potency of smoke data can be used as one of the inputs in fire hazard assessment. In particular, they can be combined with average mass loss rates and times to ignition to obtain a quick estimate of toxic fire hazard. 

In recent years there has been much controversy surrounding the impact of smoke toxicity following a fire. This has included discussions regarding means to measure toxic potency, by one of a variety of small-scale methods, and how to use these results to evaluate fire hazard. There has been, in particular, much speculation regarding the hazards due to certain plastics, typically poly(vinyl chloride) (PVC). 

Other papers in this volume address the importance of a variety of fire properties on fire hazard, in particular the relative importance (or lack of it) of toxic potency of smoke (e.g. Ref. [1]). 

Undoubtedly, fire hazard is partially associated with the toxicity of the smoke itself. The smoke of a variety of common materials, as measured e.g. by the NBS cup furnace toxicity test [10], has recently been compared with the intrinsic toxic potency of other poisons and of toxic gases, as well as with toxicity categories [11]. It has been shown that toxicity is a relatively minor factor because there is very little difference between the intrinsic toxic potency of the smoke of the majority of common materials, with very few exceptions. 

The relevance of all this to the present paper is that the toxic potency of PVC smoke or of HC1 are fairly similar to those of other smoke or of carbon monoxide (CO) respectively. 

Researchers at the NIST developed a concept to minimize the usage of animals for the assessment of the toxic potency of a material in fires. This concept is based on the well-established hypothesis that a small number (N) of gases in the smoke accounts for a large percentage of the observed toxic potency. Research at NIST of toxicologically important gases and their interactions resulted in the development of the /V-Gas Model,65 which is expressed by the following equation ... 

Toxic potency values are most often assessed from the most suitable small-scale smoke toxicity test (NIST radiant test, using rats as the animal model, but only for confirmatory purposes, standardized in ASTM E 1678192 and NFPA 269193). The results from this test have been well validated with regard to toxicity in full-scale fires. However, such validation cannot be done to a better approximation than a factor of 3. This is illustrated by the fact that the range of the toxic potency of the smoke of almost all materials is so small that it pales in comparison with the ranges of toxic potencies of... 

Hirschler, M.M., Fire hazard and toxic potency of the smoke from burning materials, J. Fire Sci., 5, 289-307 (1987). 

Research in the field of combustion toxicology is primarily concerned with items 1, all of which are related to the toxic potency of the fire gas effluent. Toxic potency is defined by ASTM as a quantitative expression relating concentration (of smoke or combustion gases) and exposure time to a particular degree of adverse physiological response, for example, death on exposure of humans or animals. This definition is followed by a discussion, which states, The toxic potency of smoke from any material or product or assembly is related to the composition of that smoke which, in turn, is dependent upon the conditions under which the smoke is generated. One should add that the LCso is a common end point used in laboratories to assess toxic potency. In the comparison of the toxic potencies of different compounds or materials, the lower the LC50 (i.e., the smaller the amount of material necessary to reach the toxic end point), the more toxic the material is. 

The likelihood for the development of symptoms following inhalation exposure and the nature and severity of respiratoiy tiact injuiy depends on a number of factors, which include the chemical namre of the smoke, concentration and toxic potency of inhaled materials, particle size and vapor proportion, duration of exposure, water solubility, respiratory minute volume, and personal characteristics (e.g., differential susceptibility, exertion). During training and operational use, exercise will result in an increased respiratory minute volume (effect of tachypnea and increased tidal volume) and thus a greater inhalation exposure dose. Most of the more soluble inhaled material will tend to predominantly affect the upper airways, and the less soluble materials affect mainly the peripheral airways and alveoli. 

---