BSI Standards PublicationFire hazard testing Part 7-3: Toxicity of fire effluent — Use and interpretation of test results... EN 60695-7-3:2011 E ICS 13.220.40; 29.020 English version F
General
Fire hazard assessment involves predicting potential human harm or property loss due to fire incidents, while toxic hazard assessment focuses on the impact of inhaled fire effluents on individuals Guidance on the fire hazards associated with electrotechnical products is provided in IEC 60695-1-10, and IEC 60695-1-11 offers a detailed technical background for fire hazard assessment Additionally, ISO 13571 addresses the consequences of human exposure to life-threatening components of fire, particularly as occupants navigate enclosed spaces, including the effects of toxic fire effluents.
Toxic hazard assessment aims to evaluate the potential harm from exposure to toxic combustion products Traditionally, research focused on exposure durations leading to death, but there is a growing shift towards assessing exposure times that result in incapacitation, preventing victims from escaping fire-related dangers.
Certain toxic species function as asphyxiants, such as carbon monoxide and hydrogen cyanide, while others serve as irritants, including acrolein, hydrogen chloride, and sulfur dioxide These two categories of toxicants require distinct treatment approaches The impact of an asphyxiant is determined by the accumulated exposure dose, whereas the effects of an irritant are contingent upon reaching a specific threshold concentration.
Exposure dose
Haber’s rule suggests that the toxic effects of asphyxiant components in fire effluent are approximately proportional to their concentration and the duration of exposure This means that if the concentration of an asphyxiant is doubled while the exposure time is halved, the overall toxic effect remains relatively unchanged However, the toxic response can be more complex for certain components of fire effluent For further details, refer to ISO 13344 and ISO 13571.
The exposure dose, a key parameter, quantifies the amount of toxicant available for inhalation from fire effluent It is determined by integrating the concentration, C, over time, t.
In scenarios where concentration remains constant, the exposure dose can be calculated as the product of concentration and exposure time (Ct) However, during fires, the concentrations of toxicants typically fluctuate over time, making this calculation more complex.
NOTE Toxicologists sometimes use the symbol Ct for exposure dose even though it is normally calculated by integration
Figure 1 – Exposure dose as a function of time and concentration
Exposure dose units are calculated by multiplying concentration by time, typically represented as grams per cubic meter times minutes (g × min × m⁻³) In some cases, volume fraction is utilized instead of concentration, leading to exposure doses being expressed in different units.
The calculation of toxicant concentration relies on volume fractions, which assume a gas mixture temperature of 25 °C and a pressure of 0.1 MPa To determine the concentration, multiply the volume fraction by the density of the pure toxicant under these conditions.
Each contributor to fire effluent exhibits a unique concentration-time curve In numerous studies, significant toxic species are analyzed independently, and their effects are subsequently aggregated This methodology is referred to as the "toxic gas model."
An alternative method involves treating the fire effluent from a specific material or product as a single toxicant, provided its toxic potency is either known or can be assumed In this scenario, the exposure dose depends on the duration of exposure and a factor referred to as mass loss.
– 16 – 60695-7-3 © IEC:2011 concentration The different materials or products are considered independently and then their effects are summed This approach is known as the “mass loss model”.
Determination of concentration-time data
Concentration-time data can be determined through two methods: direct measurement during a full-scale fire scenario simulation or by calculating the mass loss rate of fuels in a modeled fire scenario.
The computational method for fire scenarios can be categorized into two types: hand calculations for simple cases with one or two burning items, and computer-based mathematical models for more complex situations While hand calculations may suffice in straightforward instances, as illustrated in Annex B, advanced fire models are necessary for intricate environments These models require detailed input, including the characteristics of the fire scenario and the time-based mass loss rate of all combustible materials, such as electrotechnical products.
Net mass loss for a product initiates once its specific ignition conditions, such as radiant flux or temperature, are achieved The rate of mass loss is directly proportional to the exposed surface area and the heat received from the fire This proportionality constant is established through laboratory measurements of mass loss per unit of surface area at various radiant flux levels Mass loss concludes when all the fuel is determined to be fully consumed.
Computer codes utilize mass loss rates and scenario-specific data to assess the impact of structure, ventilation, and victim location, calculating effluent temperature and concentrations over time at designated locations The time-dependent behavior of different fire hazard elements is illustrated in the output, as shown in Figure 2.
Initiation Detection Loss of Incapacity Death effective visibility
C om ponent s o f f ire ha (ar bi trar y s cal
Figure 2 – Time dependent components of fire hazard
Asphyxiants and the fractional effective dose, FED
General
The toxicity of an asphyxiant is determined by the exposure dose needed to elicit a toxic effect, known as the effective exposure dose 50 (ECt 50) A lower ECt 50 value indicates higher toxicity This principle is applicable to individual gases, gas mixtures, and fire effluents, regardless of their chemical composition.
Toxic hazard assessment calculates the exposure dose over time and divides it by the effective exposure dose 50, resulting in the fractional effective dose (FED).
The exposure dose, represented in the numerator, is influenced by the product's burning behavior and the specific fire scenario In contrast, the denominator, known as the effective exposure dose 50 (ECt 50), is where toxic potency is factored into the equation Further details on toxic potency can be found in Clause 6 When the exposure dose at the victim's location matches the effective exposure dose, critical implications arise.
50 (i.e when FED = 1) the defined effect, such as incapacity or death, is deemed to occur
In fire situations, exposure dose and the FED can be estimated using two closely-related approaches The first approach, known as the "toxic gas model," treats fire effluent as a mixture of toxic components The second approach, referred to as the "mass loss model," considers the effluent as a result of various burning products and materials.
Properties of the FED
The FED is a time-dependent quantity Its principle determinants are:
– the type and size of the fire,
– the time of exposure to the fire effluent and the relative location of those exposed,
– the volume of the compartment into which the effluent is dispersed, and
– the toxic potency of the fire effluent
In a specific scenario, the total Fire Effluent Dose (FED) is calculated by summing the toxic contributions from all components of the fire effluent Each component's contribution, denoted as \( f_i \), is determined by the effective dose of exposure to that particular effluent component.
[ ( 3 ) and the total FED = f 1 + f 2 + f 3 + (see Figure 3)
The contributors can be individual gases, as seen in the toxic gas model, or various burning items, as illustrated in the mass loss model.
Figure 3 – Total FED and contributors, as a function of time
Uses of the FED
The uses of the FED include the determination of the following:
– the time at which the atmosphere becomes untenable (this requires that the FED does not exceed a predetermined value chosen to provide tenability for continuity of operation, escape or rescue)
– comparisons of materials or products
– comparisons with a standard, e.g a reference standard material or a reference fire scenario.
Irritants and the fractional effective concentration, FEC
Sensory irritation in the upper respiratory system activates nerve receptors in the eyes, nose, throat, and upper respiratory tract The degree of incapacitation is primarily linked to the concentration of the irritant, with effects ranging from mild discomfort in the eyes and upper respiratory area to intense pain.
The assessment of irritant gas components in toxic hazard analysis primarily focuses on the concentration of each irritant Fractional effective concentrations (FECs) are calculated for each irritant at specific time intervals The cumulative FECs are monitored, and the point at which their total surpasses a designated threshold indicates the available escape time in relation to established safety standards.
FEC F ion i concentrat threshold ion concentrat gas irritant ( 4 ) where
[C] i is the concentration (or volume fraction) of irritant gas, i
F i is the threshold concentration (or threshold volume fraction) of irritant gas, i
Annex C provides a list of volume fractions of irritant gases that can significantly hinder an occupant's ability to escape effectively, highlighting critical F values for various important irritants.
NOTE Irritant toxicants can also be lethal, and in this case it appears that it is the exposure dose that is relevant, see 5.2.
Carbon dioxide
At low concentrations carbon dioxide is not toxic but it does cause hyperventilation and therefore increases the effective toxicity of other fire effluents Some formulae for calculating
FED values take this effect into account – see ISO 13344 and ISO 13571.
Oxygen vitiation
Low levels of oxygen are harmful and some formulae for calculating FED values take this effect into account – see ISO 13344 and [4].
Heat stress
Heat stress poses significant risks, potentially leading to incapacitation or even death It functions similarly to an additional toxicant, and its effects can be incorporated into the FED calculation as outlined in ISO 13571.
Effects of stratification and transport of fire atmospheres
Fire effluent concentrations are typically determined by the mass of fuel burned in relation to the volume of the dispersing environment Advanced models consider the impact of stratification and transport on the concentration of fire effluents in particular physical settings.
5 Methods of toxic hazard assessment
General approach
The goal of toxic hazard assessment is to determine the Fire Exposure Dose (FED) and/or Fire Exposure Concentration (FEC) related to a fire involving electrotechnical products This process begins with a thorough description of the product and its usage Next, the specific conditions under which the fire occurs are outlined, forming a "fire scenario." This scenario includes details about the enclosing structure, the ignition source, the involvement of the product in the fire, and the locations of individuals exposed to the hazard, as well as the potential impact on their safety.
The end effect that is considered is usually either death, or incapacitation such that the subject is rendered unable to escape from the fire
Electrotechnical products can present multiple scenarios, each linked to specific toxic hazards For every identified scenario, the corresponding FED and/or FEC values are computed.
Equations used to predict death
Simple toxic gas model
The toxic effects of the separate effluent components are generally additive, so the FED is the sum of the contributions of all the components
[ is the exposure dose of effluent component, i;
[LCt 50 ] i is the lethal exposure dose 50 of effluent component, i
As with a single toxicant, when the total FED reaches unity, death is predicted to occur.
The N-gas model
This use of the FED principle has been termed the "N-gas model” by the National Institute of Standards and Technology (NIST) [7]
This article examines the impact of carbon dioxide on the toxicity of carbon monoxide, based on empirical studies from NIST Additionally, it considers the potential significance of oxygen vitiation in this context.
The volume fraction of carbon monoxide (\$φ_{CO}\$) and carbon dioxide (\$φ_{CO2}\$) are critical in understanding the toxicity of carbon monoxide, which increases as the concentration of carbon dioxide rises The relationship between these gases is represented by an interactive curve characterized by the slope (\$m\$) and intercept (\$b\$) Additionally, the volume fraction of oxygen (\$φ_{O2}\$) plays a significant role in this context.
For volume fractions of carbon dioxide less than 5 %, m = –18 and b = 0,122
For volume fractions of carbon dioxide more than 5 %, m = 23 and b = –0,039.
Hyperventilatory effect of carbon dioxide
When the carbon dioxide (CO₂) volume fraction exceeds 0.02, it is essential to adjust the FED values by multiplying them with the frequency factor, ν CO₂, to account for the heightened rate of asphyxiant uptake caused by hyperventilation The frequency factor is calculated using the formula ν CO₂ = exp(X CO₂ / 0.05), where X CO₂ represents the volume fraction of carbon dioxide, as specified in ISO 13571.
Lethal toxic potency values
LCt 50 values used in Equations (5), (6) and (8) are given below in Table 1
Table 1 – Some toxic potency values
(30 min exposure, volume fraction value)
(30 min exposure, volume fraction value)
Mass loss model
In the mass loss model, fire hazard assessments are conducted by evaluating the mass contribution of specific burning products or materials This approach substitutes the effluent concentration term in the exposure dose with a mass loss concentration term, as detailed in section 4.2.
The total fire effluent includes contributions from each of the k burning materials or products The lethal exposure dose 50, denoted as [LCt 50 ] j, represents the toxicity level of the effluent from the j th product, as determined in laboratory combustion tests.
In fire hazard assessments of electrotechnical products, the mass loss model is commonly utilized This approach aims to compare different electrotechnical products or evaluate the contribution of a specific product when it represents a minor portion of the overall hazard.
Equations used to predict incapacity
Asphyxiant gas model
To assess the toxic hazard of asphyxiants, it is essential to evaluate the exposure dose, which is represented by the integrated area under the concentration-time curve Fractional effective doses (FEDs) are calculated for each asphyxiant at specific time intervals The cumulative sum of these doses indicates the time available for escape, measured against established safety criteria.
For carbon monoxide, the ECt 50 for incapacitation is 0,035 min [9]
For hydrogen cyanide, the incapacitating dose is not a constant, but varies depending on the volume fraction [5] The FED is calculated using an exponential expression t
{ ( 9 ) where φ HCN is the average volume fraction of hydrogen cyanide over the time increment ∆t
NOTE This equation is based on data obtained with values of φ HCN in the range 30 × 10 –6 to 400 × 10 –6
When the volume fraction of carbon dioxide (φ CO2) surpasses 0.02, the effective exposure doses of asphyxiants are significantly heightened due to hyperventilation This increase can be quantified by the factor of exp(φ CO2 / 0.05), as outlined in ISO 13571.
Irritant gas model
Fractional effective concentrations (FECs) are calculated for each irritant at specific time intervals The cumulative FECs indicate the duration available for escape when they surpass a defined threshold, in accordance with established safety criteria.
Mass loss model
Determining the concentrations of fire effluent toxicants over time can be challenging However, the fundamental concept of Fire Effluent Dispersion (FED) can still be applied by utilizing mass loss, the volume of dispersion for fire effluents, and established lethal toxic potency values.
LCt 50 value is recommended as an approximate exposure dose when relating incapacitation to lethality [10] Although based on experimental data obtained from exposure of rats, this relationship is also expected to be appropriate for human exposure,see ISO 13571
Generic values of toxic potency
First approximations for hazard assessment can often be conducted using average or generic toxic potency values, as the fire effluents from most materials tend to be similar within an order of magnitude.
An LCt 50 value of 900 g/m³ is recommended for well-ventilated, pre-flashover fires, while a value of 450 g/m³ is suggested for vitiated post-flashover fires For assessing occupant escape, ISO 13571 recommends values of 450 g/m³ and 220 g/m³, respectively The validity of these values can be verified by recalculating toxic hazard assessments with potency values differing by factors of 2 or 3 Significant differences in potential escape times may indicate the need for specific toxic potency data related to electrotechnical materials and the products involved.
Toxic potency values obtained from chemical analyses
Previous biological tests have established the lethal effective doses of major fire gases, with values available in published sources, including Table 1 (see 5.2.4) This data aids in hazard assessment through chemical analyses of fire effluents, a method gaining popularity due to the growing understanding of the toxic effects of individual fire gases and multicomponent fire effluents This approach minimizes the need for routine animal testing, as the toxic potencies of common fire gases have already been determined through animal exposure With adequate analytical data, toxic potency can be treated as a single value for a specific stage of fire.
Toxic potency values obtained from animal tests
Toxic potencies are determined by exposing animals, typically rats or mice, to specific concentrations of toxic gases or fire effluents while observing their behavior over time When materials burn, they generate a complex mixture of toxic substances that can chemically interact and have unpredictable biological effects upon inhalation By burning the material and exposing animals to the resulting effluents, researchers can capture the effects of these interactions, which often cannot be anticipated through chemical analysis alone.
7 Limitations on the interpretation of toxicity test results
Relying solely on toxic potency test results is insufficient for assessing fire hazards and ensuring fire safety These results should not be used to rank materials or electrotechnical products It is essential that toxic potency limits are excluded from material and product specifications Safety decisions and conclusions must be based on a comprehensive quantitative hazard assessment that includes all relevant fire test and scenario data.
Historically, toxicity testing was used to identify materials that produce highly toxic combustion effluents during thermal decomposition However, as of 2011, there have been no documented cases of fires where the danger stemmed from extreme toxic potency.
The mere presence or absence of chemical elements like nitrogen, halogen, or phosphorus in a product does not indicate the level of lethal toxic hazard It is essential not to draw conclusions solely based on specific toxic chemical species found in fire effluent A comprehensive hazard assessment is necessary to evaluate the overall threat posed by a fire and its effluent, integrating factors such as heat, smoke, toxicity, and oxygen depletion through a time-dependent quantitative analysis.
8 Effluent components to be measured
Minimum reporting
When organic materials are burned, they consume oxygen and produce carbon oxides, which are significant toxic components of fire effluents It is essential to report the levels of carbon dioxide, carbon monoxide, and oxygen.
Additional reporting
Gaseous fire effluent components
Other gaseous effluent components should be measured if their presence is known or is suspected
The presence of other elements in the fuel determines the necessary additional analyses Table 2 outlines the key gaseous effluent components expected from these elements, all of which, except for water vapor, contribute to the toxic hazard of the effluent.
Incomplete oxidation of fuel can lead to the production of various gaseous effluent components By understanding the fuel composition, one can estimate the organic fraction of the effluent through a carbon balance of the products Advanced techniques such as Fourier transform infrared spectroscopy and gas chromatography/mass spectrometry provide detailed insights into the composition of these gaseous emissions.
NOTE In the case of electrical insulating oils (see IEC 60695-1-40) the following toxic species can be produced: – acrolein and formaldehyde,
– dioxins and furans (for oils suspected of being contaminated with polychlorinated biphenyls,
– polyaromatic hydrocarbons (for mineral oils)
The production of these toxic species is not limited to electrical insulating oils
Element(s) in the fuel Principal effluent component(s)
Carbon, hydrogen, oxygen Water (H 2 O), Carbon dioxide (CO 2 ), Carbon monoxide (CO)
Acrolein (CH 2 =CHCHO), formaldehyde (HCHO) Nitrogen Hydrogen cyanide (HCN), nitrogen oxides (NO x )
Airborne particulates
Airborne particulates significantly enhance the toxicity of fire effluents, making it essential to measure the total particulate matter in milligrams per liter Additionally, understanding the particle size distribution of these particulates provides valuable insights into their impact.
Guidance for the use of LC 50 values
The toxic potency of effluent from burning or pyrolyzing products is primarily determined by its concentration, which can pose significant harm to individuals during exposure Fire can lead to a variety of adverse effects, with the most severe being death Additionally, symptoms like disorientation and eye irritation can impact survival and may result in lasting consequences.
Research on toxic hazards in fires primarily focuses on the effects that lead to fatalities The lethal potency of a toxicant is measured by the LC 50, which indicates the concentration of the toxicant that results in the death of 50% of the subjects after a specified exposure time, typically 30 minutes However, in fire scenarios, individuals encounter varying concentrations of fire effluent, necessitating the calculation of exposure based on the integral of concentration over time.
Fires pose various threats to life, primarily through inhalation of toxic fumes and burns, along with risks such as falling down stairs due to reduced visibility The initial danger encountered is known as the limiting hazard To conduct a toxic hazard analysis, it is crucial to determine if this limit is caused by the toxicity of the fire effluent.
A.3 Use of LC 50 values in specific types of fires
Current equipment for measuring the toxic potency of fire effluent does not assess self-sustaining, non-flaming combustion While it can be assumed that this mode resembles thermal or radiative pyrolysis, it remains unverified whether the combustion products or the LC 50 values are identical.
Fires with slow mass burning rates produce minimal effluent and heat, resulting in low concentrations of harmful substances in a room Unless the LC 50 value is extremely low, the risk to life safety remains minimal In the electrotechnical sector, these fires often start from overheated components, and individuals are typically not in close proximity to the smoldering source A person is only at risk of receiving a harmful dose if the effluent is confined to a small area.
LC 50 values are measurable for products involved in small flaming fires Most of these values fall in a narrow range, although there are a few combustibles with very high (low toxicity) or very low (high toxicity) values In both the measurement apparatus and the fire, there is an ample supply of oxygen
When the FED approach is employed, the toxic effluent components should be determined by chemical analysis
Common fuels release heat at a rate proportional to their oxygen consumption, which is often used to gauge heat release during a fire As materials burn, the heat causes hot gases to rise to the upper compartment layer, posing dual threats to nearby individuals: extreme temperatures and toxic fumes Identifying the primary hazard is crucial, as analysis indicates that burns or heat can become life-threatening before the toxicity of the effluent reaches critical levels Consequently, for hazard analysis in such fire scenarios, understanding that the toxic potency of the effluent is not excessively high is more vital than precise LC 50 measurements.
In various exposure scenarios, the heat from a fire spreads through the building via the movement of fire effluents before it reaches individuals In these instances, the toxic fire effluents are likely to pose the greatest threat to life.
In a compartment fire, when the flames grow sufficiently large, they deplete oxygen at a rate that exceeds the supply from doors and windows This lack of ventilation leads to incomplete combustion, resulting in more toxic fire effluents.
In a room engulfed in flames, the temperature and thermal radiation quickly reach lethal levels, posing a significant threat to individuals in adjacent and distant areas As the hot, toxic gases escape the room, they mix with outside air, losing heat through convection and conduction The extent of the danger is influenced by the rates of these processes, which vary depending on the building's characteristics.
LC 50 values can also be determined for products involved in large flaming fires, and most of these values again fall in a narrow range However, the measurement method requires inclusion of the effect of oxygen depletion in the flashed-over compartment This depletion results in enhanced yields of incomplete combustion products, notably carbon monoxide which is responsible for at least half of the FED in nearly all fires Thus its accurate inclusion in an
LC 50 determination is important Open (flow-through) systems can pre-determine the carbon monoxide yield by adjustment of the flow conditions Closed systems can post-determine the carbon monoxide yield by matching the results from real-scale fires
The determination of LC 50 can be simplified due to increased carbon monoxide production in post-flashover fires Laboratory studies indicate that carbon dioxide amplifies the toxicity of carbon monoxide, with an LC 50 of approximately 5 g × m –3 for carbon dioxide-potentiated carbon monoxide Analysis of various post-flashover room fire tests reveals a typical carbon monoxide yield of about 0.2 g/g of fuel burned, attributed to the fire compartment's underventilation Consequently, the LC 50 of post-flashover fire effluent is estimated to be around 25 g × m –3, based solely on the anticipated carbon monoxide and carbon dioxide levels Higher values are not feasible, as the presence of additional toxicants or increased carbon monoxide yields would likely reduce this estimate.
The accuracy of bench-scale measurement methods is crucial for replicating real-scale phenomena Pilot validation studies of a radiant apparatus for LC 50 measurement indicate that results can predict real-scale toxic potency within a factor of 3 Consequently, LC 50 values for post-flashover fire effluent range between 8 g × m –3 and 75 g × m –3 are indistinguishable Since all post-flashover fire effluent has an LC 50 value not exceeding 25 g × m –3, values above 8 g × m –3 determined by this method are also indistinguishable This calculation approach can be extended to other bench-scale devices once their accuracy is established.
Most common electrotechnical products have LC 50 values substantially higher than this Thus, for those combustibles one would conservatively use a common value of 8 g × m –3 in a post- flashover hazard analysis