Irritant gases

Irritant gases such as hydrogen chloride (HCl), hydrogen bromide (HBr), hydrogen fluoride (HF), sulphur dioxide (SO₂), nitrogen dioxide (NO₂), acrolein and formaldehyde act in two ways. Sensory irritation of the eyes and upper airways is felt immediately and depends on the concentration: it impairs escape at once. Damage to the deep lung depends on the inhaled dose and develops over hours. The literature treats the two with different quantities, and the standards and the handbook combine them differently.

Symbols follow the notation table. The equations below keep the sources’ own notation (FEC, $F_{\mathrm{FIC}}$, $F_{\mathrm{FLD}}$).

ISO 13571: a separate concentration endpoint

ISO 13571:2012 treats asphyxiants and irritants separately, because they are “physiologically unrelated and mechanistically independent” (§4.2.1). For irritants it considers only eye and upper-respiratory-tract sensory irritation. It computes a fractional effective concentration (FEC) [-] for each irritant at each time increment, from the concentration and not from the dose, and compromised tenability is predicted when the sum of the FECs exceeds a threshold (§4.2.3). The FEC is a separate endpoint: it is not added to the asphyxiant fractional effective dose (FED). The same threshold value must be used for FED and FEC in a given estimation (§5.4). Pulmonary irritation is excluded because its serious effects appear hours to days after exposure (§4.2.1).

Purser: a concentration term and a dose term

Purser and McAllister (2016, Society of Fire Protection Engineers (SFPE) Handbook Ch. 63) define the fractional irritant concentration (FIC) [-] as a sum over irritants, each term being the current concentration divided by the concentration predicted to cause a chosen endpoint:

$$ \mathrm{FIC} = \mathrm{FIC_{HCl}} + \mathrm{FIC_{HBr}} + \mathrm{FIC_{HF}} + \mathrm{FIC_{SO_2}} + \mathrm{FIC_{NO_2}} + \mathrm{FIC_{CH_2CHO}} + \mathrm{FIC_{HCHO}} + \sum \mathrm{FIC}_x \qquad \text{(Eq. 63.11)} $$

The denominators are tabulated for two endpoints, escape impairment and incapacitation (Table 63.6, SFPE columns). Purser states that a factor of 0.3 on the FEC for escape impairment should allow nearly all exposed people to escape (Ch. 63, p. 2414). Like the ISO FEC, the FIC is a concentration criterion and is not integrated over time. The same table lists the corresponding ISO 13571 values, which differ for several gases: for HCl, for example, the SFPE incapacitation concentration is 900 ppm and the ISO value 1000 ppm (Table 63.6).

The lung effects are a dose. The fractional lethal dose of irritants $FLD_{irr}$ [-] sums, for each irritant, the Ct exposure dose [ppm·min] divided by the dose predicted to be lethal to half the population (Eq. 63.15, Table 63.7, e.g. 114 000 ppm·min for HCl). Purser notes that lung irritation has relatively little effect on escape capability and “can be omitted from an escape calculation” (p. 2415), but in the simplified asphyxiant equation he includes $FLD_{irr}$ inside the sum that is multiplied by the CO₂ factor, because irritants impair lung function and add some hypoxia (Eq. 63.38; see Asphyxiant FED).

Purser: irritants and walking speed

Purser (2003, pp. 93 and 99–100) proposed that irritants do not slow people at low concentrations and stop effective movement at incapacitation, FIC = 1, with a sigmoid fall in between: “a curve has been fitted between these two extremes” (p. 93). Ch. 63 calls it an “estimated relationship” based on a concept (p. 2343, Fig. 63.17). In our reading, no walking data lie behind the curve. Ch. 63 gives it as

$$ F_{wv\,irr} = \frac{e^{-(1000\,x/b)^2} + (-0.2\,x + 0.2)}{1.2}, \qquad b = 160,\ x = \mathrm{FIC} \qquad \text{(Eq. 63.13)} $$

where $F_{wv\,irr}$ is the fractional walking speed (1 = normal walking speed of 1.2 m/s). It is 1 at FIC = 0, about 0.31 at FIC = 0.2 and 0 at FIC = 1 (Ch. 63, p. 2344). The smoke and irritant effects are combined by adding their losses (Purser 2003, p. 100; Ch. 63, Eq. 63.14, p. 2345):

$$ F_{wv} = 1 - (1 - F_{wv\,smoke}) - (1 - F_{wv\,irr}) $$

where $F_{wv\,smoke}$ is the fractional walking speed due to smoke obscuration (see Walking speed in smoke). The sum can fall below 0, and neither source clips it. It assumes a smoke term for non-irritant smoke, as in Purser’s 2003 fit to Jin’s data; pairing it with Eq. 63.10, which is fitted to pooled “moderately irritant” data, would in our reading count irritancy twice (see Walking speed in smoke).

Fractional walking speed against FIC from Eq. 63.13: 1 at FIC 0, 0.71 at 0.1, 0.31 at 0.2, 0.08 at 0.5 and 0 at 1

Fractional walking speed against fractional irritant concentration FIC from Ch. 63, Eq. 63.13, drawn thin because, in our reading, no walking data lie behind it. Script: scripts/figures/fundamentals_irritant_speed.py.

The equation as printed in Purser (2003)

Purser (2003, p. 100) prints the curve of his Fig. 1 as $F_{wv\,irr} = 1 - \big((1 - e^{-(x/b)^a}) + (-0.2x + 0.2)/1.2\big)$ with a = 2, b = 160, x = FIC. As printed it gives 0.83 at FIC = 0 and rises to 1 at FIC = 1, the opposite of his Fig. 1. We compared it with values read from Fig. 1 (about 1.0, 0.92, 0.71, 0.31, 0.14, 0.08 and 0 at FIC = 0, 0.05, 0.1, 0.2, 0.3, 0.5 and 1), scaling x by 1, 10, 100, 1000 and 10 000, in both terms or in the exponential term only. No scaling gives 1 at FIC = 0, because the linear term alone fixes 0.83 there. The best tested case, 1000 x in the exponent only, ends at 0 at FIC = 1 but dips to −0.10 in between, with a mean absolute error of 0.16 against the readings. Ch. 63’s Eq. 63.13, with 1000 x in the exponent and the whole numerator divided by 1.2, matches the readings with a mean absolute error of 0.002, and is the form given above. The figure on this page is computed from Eq. 63.13, not from the 2003 printing. A second, minor misprint: Purser (2003, p. 99) refers to “Figure 2” for the irritant curve, which is his Fig. 1.

The Purser / FDS+Evac guide form

The guide to FDS+Evac, the evacuation module of the Fire Dynamics Simulator (Korhonen 2021, Eqs. 12 and 17, Table 2), follows this structure: its total dose

$$ \mathrm{FED_{tot}} = \left(\mathrm{FED_{CO}} + \mathrm{FED_{CN}} + \mathrm{FED_{NO_x}} + \mathrm{FLD_{irr}}\right)\times \mathrm{HV_{CO_2}} + \mathrm{FED_{O_2}} $$

adds the irritant lethal dose into the incapacitation sum, and its Table 2 lists both the lethal doses $F_{FLD}$ and the incapacitating concentrations $F_{FIC}$. The values in that table agree with the SFPE columns of Tables 63.6 and 63.7. This sum is the Purser / FDS+Evac guide form. It is not the ISO 13571 form, which keeps irritants out of the FED altogether.

Known limits

Human measurements at incapacitating concentrations cannot be made for ethical reasons, and most of the experimental work behind the irritant values used animals, mostly rodents (Ch. 63, p. 2343). The endpoint concentrations differ between sources by up to an order of magnitude; for SO₂, Table 63.6 gives 24 ppm for SFPE escape impairment and 150 ppm for ISO incapacitation. Individual sensitivity is wide: for HCl, Purser proposes 200 ppm as the escape-impairment concentration for the average person, but notes that some people may be impaired at 60 ppm and others may escape through about 400–600 ppm (p. 2343). The concentrations of acrolein and formaldehyde are rarely known in a fire, and Ch. 63 suggests a smoke-density surrogate when they are not (pp. 2344–2345).

Sources

ISO (2012). ISO 13571:2012 Life-threatening components of fire — Guidelines for the estimation of time to compromised tenability in fires, §4.2 and §5.4. ISO, Geneva. iso.org/standard/56172. Read from the public preview.
Purser, D. A., & McAllister, J. L. (2016). Assessment of hazards to occupants from smoke, toxic gases, and heat. SFPE Handbook of Fire Protection Engineering, 5th ed., Ch. 63, 2308–2428. Eqs. 63.11, 63.13–63.15, 63.38, Tables 63.6–63.7. doi:10.1007/978-1-4939-2565-0_63
Purser, D. A. (2003). ASET and RSET: addressing some issues in relation to occupant behaviour and tenability. Fire Safety Science, 7, 91–102. doi:10.3801/IAFSS.FSS.7-91
Korhonen, T. (2021). Fire Dynamics Simulator with Evacuation: FDS+Evac. Technical Reference and User’s Guide (FDS 6.7.6, Evac 2.6.0 draft), §3.4. VTT Technical Research Centre of Finland. github.com/tkorhon1/FDS-Evac-Guide. Secondary source.

How pyFDS-Evac uses this: see Fractional effective dose.

How it is verified: the FED and FIC rows of the Verification index; the HCN, NOₓ and irritant terms are not yet verified end to end (#257).

Last updated on September 29, 2026

Asphyxiant fractional effective dose Heat