Smoke inhalation

Smoke inhalation is the intake of combustion products into the respiratory system. It can cause respiratory pain and injury, impaired vision, confusion, loss of consciousness and death. The outcome depends on gas and particle composition, concentration, exposure duration, breathing response and the exposed person's ability to escape.

Inhalation is not only a problem in the room of origin. Smoke can travel through openings and shafts, exposing occupants elsewhere while the fire itself remains out of sight. Conditions in the source compartment and transport through the building both determine the received dose.

The main purpose of assessment is often to predict incapacitation as well as death. Incapacitation can prevent escape and thereby lead to a later lethal exposure. Detection, pre-movement behaviour, visibility and physical response all interact with the inhaled atmosphere.

Smoke components and exposure

Smoke contains particles, droplets, gases and entrained air. Its composition varies with fuel, combustion mode and oxygen supply. Inhalation therefore exposes a person to several hazards at once rather than to one uniform substance.

Carbon monoxide is often the principal asphyxiant. It reduces oxygen carriage and can cause confusion and unconsciousness, with a dose depending on concentration, time and breathing rate. Well-ventilated and vitiated fires can produce very different yields.

Hydrogen cyanide is another asphyxiant and is particularly relevant to nitrogen-containing materials and some smouldering polyurethane foams. It can incapacitate rapidly at high concentration and has a different concentration-time relationship from carbon monoxide. A model limited to carbon monoxide can miss this pathway.

Carbon dioxide stimulates ventilation and is itself an asphyxiant at elevated concentration. Low oxygen adds hypoxia, while irritant products can cause eye and airway pain. These interactions can increase uptake and reduce the time available for escape.

Particles and droplets contribute to optical obscuration and can carry or consist of irritant material. Their size distribution affects where and how they travel and deposit. Some particles settle while toxic gases continue to spread, so clearer air is not necessarily safe to breathe.

Physiological effects

Purser groups incapacitating effects into impaired vision, respiratory tract pain and breathing difficulty, asphyxia, and pain or burns from heat. Impaired vision can result from optical opacity and from irritant effects on the eyes. These effects can directly obstruct orientation and movement.

Smoke inhalation can impair escape in several ways. Based on Purser (2002), pp. 2-85–2-86.

Respiratory tract effects include pain, inflammation, bronchoconstriction and laryngeal spasm, with collapse possible quickly in severe cases. Lung inflammation can also appear hours later and produce continuing respiratory distress. Removal from smoke therefore does not guarantee that all effects have ended.

Asphyxia results from toxic gases that reduce oxygen availability, transport or use. Confusion and loss of consciousness can follow, preventing purposeful escape. Carbon monoxide, hydrogen cyanide, carbon dioxide and low oxygen act through different but interacting mechanisms.

Heat adds pain, burns and hyperthermia, which can further prevent movement. Hot inhaled products can injure the respiratory tract as well as heat exposed skin. The complete exposure should therefore include thermal conditions alongside inhaled chemistry.

The exposed person's location changes both the dose and the available response. Someone below a stable layer can initially inhale little smoke and later encounter a descending layer, while someone travelling through a corridor can encounter several different atmospheres. A model should follow that movement rather than assigning one room-average exposure for the whole escape.

Measurement intervals should be short enough to capture severe changes. A brief high-concentration pulse can dominate a dose even when the longer-term average appears moderate. Time-averaging must therefore be justified against the endpoint and the person's exposure duration.

Reports should also distinguish pain and irritation from asphyxiation. Irritants can prevent effective movement before the accumulated asphyxiant dose reaches its reference endpoint, creating additional time in the hazardous atmosphere. Ignoring that feedback can underestimate the eventual dose and the chance of collapse.

For evacuation analysis, the exposure history should be compared with detection, pre-movement and travel times. A survivable total dose is not enough if confusion or loss of consciousness occurs before the exit is reached. The relevant endpoint is the person's condition at each stage of escape.

Fire-statistics categories should be interpreted with their definitions. Room of origin, remote location, injury and death can be classified differently between datasets, and building practices vary. These sources can guide which pathways deserve attention but should not be converted directly into universal probabilities.

A final evidence review should identify what remains unknown about the atmosphere and the person. Missing yield data, uncertain transport and assumed breathing rate can each change the predicted dose. Stating those limitations is preferable to assigning a precise incapacitation time that the evidence cannot support.

Fire toxicity includes asphyxiant and irritant effects caused by combustion products. A given endpoint occurs when the person has received an effective exposure, not simply when a particular gas appears. Concentration, duration, breathing and the selected endpoint must all be defined.

For short high-concentration exposures, uptake can be nearly linear with concentration multiplied by time. For longer exposures, breathing and body uptake produce a more complex relationship. Purser shows that the concentration needed for a lethal carboxyhemoglobin level at four hours differs from the proportion predicted by Haber's rule.

Carbon dioxide and hydrogen cyanide also depart from a simple universal Ct rule. Their concentrations strongly influence time to incapacitation, so each exposure segment needs an appropriate concentration-effect relationship. A single average concentration can hide a short severe period.

Activity and carbon dioxide-driven hyperventilation increase the volume inhaled and therefore the dose of other products. Dose assessment should state the person's activity and the atmosphere rather than treating elapsed time as the only variable.

Fractional effective dose

fractional effective dose (FED) divides the dose received during each time interval by the dose predicted to cause a specified effect, then sums the fractions. A value of one corresponds to the selected model endpoint, such as incapacitation or death. Different gases and populations require appropriate dose-effect data.

Purser evaluates asphyxiation, irritant effects and heat through endpoint-specific relationships. Only contributions that the selected model explicitly combines belong in the same sum. A reference fraction of one describes a model endpoint, while a design assessment may select a lower acceptance level for its intended population.

The dose calculation depends on the combustion-product yields supplied to it. A yield specifies the mass of a product emitted per mass of fuel burned, and oxygen-starved conditions can change that ratio substantially. The worked model comparison below illustrates the effect of this input on predicted exposure.

fractional effective concentration (FEC) addresses the concentration side of exposure assessment and should not be confused with accumulated dose. A high concentration for a short period and a lower concentration for a longer period can have different effects. The selected model determines how each is evaluated.

Dose accumulation and immediate irritation

For a specified cumulative endpoint, a fractional-dose model divides exposure into time intervals. Each interval contributes a fraction of the exposure associated with that endpoint under its stated conditions. Adding those fractions follows the exposure history rather than judging the atmosphere from its final concentration alone. $$F=\sum _j\mathrm{\Delta }{F}_j,\mathrm{\Delta }{F}_j=\frac{\mathrm{\Delta }{t}_j}{t_I(C_j)}$$

Symbol	Description	Unit
$F$	Accumulated fraction for the selected endpoint	dimensionless
$\mathrm{\Delta }{F}_j$	Contribution from interval j	dimensionless
$\mathrm{\Delta }{t}_j$	Interval duration	min
$C_j$	Concentration during the interval	units required by the selected gas model
$t_I(C_j)$	Time to the endpoint at that concentration	min

This notation does not supply a universal function for every gas. Carbon-monoxide uptake, hydrogen-cyanide effects and direct carbon-dioxide incapacitation have different relationships. Purser also treats carbon-dioxide-induced changes in breathing as an interaction, rather than simply another interchangeable dose term.

Immediate sensory irritation raises a different question: whether the current concentration impairs seeing, breathing or movement. A fractional effective concentration (FEC) assessment addresses concentration-based irritant effects, whereas cumulative dose follows exposure through time. Neither index can be substituted for the other merely because both are dimensionless.

Purser's illustrative CFAST calculation shows the importance of the combustion input before any dose model is applied. It represents a 2.44 m × 3 m × 2.44 m compartment, medium fire growth and a floor-level opening 1 m wide and 0.3 m high. The compared carbon-monoxide yields are 0.007 kg/kg for well-ventilated burning and 0.149 kg/kg for the vitiated condition, a factor of approximately 21.

In that theoretical case, accounting for changing yields produces a fractional effective dose near one at 200 s; fixed well-ventilated yields produce about 0.1. These are model-comparison results under the stated geometry and fire assumptions. They are not measured survival times, and the gas-yield ratio is not itself a ratio of physiological outcomes.

Transport and remote exposure

Gottuk and Lattimer discuss smoke-inhalation fatalities occurring away from the fire origin. The hazard therefore depends on transport through the building as well as production in the burning room. Smoke can expose people who never encounter the original flames.

Upper layers can spill through doorways and other openings, while shafts and ducts provide vertical paths. Pressure differences from buoyancy, wind and equipment determine the direction and rate of movement. A building-scale exposure model needs these paths rather than a source-room concentration alone.

Smoke inhalation can occur far from the fire. Based on Gottuk and Lattimer (2002), p. 2-54 and Australian Building Codes Board et al. (2005), p. 2.5-6.

Particle deposition and external oxidation change the mixture during transport. Visibility can improve while carbon monoxide remains high, and external burning can oxidise some toxic gases while releasing additional heat. Receiver conditions cannot be inferred from appearance alone.

Fire statistics provide evidence about where fatalities and injuries occur and which products dominate. Purser notes differences between UK and US patterns, including more deaths remote from the room of origin in the US data. Building style and cultural context can influence such comparisons, so the datasets are not universal frequencies.

Escape and incapacitation

Purser separates the developing fire hazard from the process by which occupants escape. Survival depends on both: detection and response must occur before exposure becomes incapacitating. A person who cannot see, breathe or remain conscious has reduced ability to use an otherwise available exit.

The first phase involves awareness and pre-movement behaviour, before the person is strongly affected by smoke and heat. The second involves exposure that impairs escape, and the third involves death. Toxicity is therefore most important during exposure and its consequences, not only at the final endpoint.

Visibility can decline within tens of seconds near a fire, while irritants make the eyes and lungs painful. This can lengthen the time spent in smoke and increase inhaled dose. Good escape guidance and early detection can change that sequence by reducing exposure time.

Incapacitation is not a binary condition with one universal concentration or time. Different people have different activity, breathing and susceptibility, and mixed smoke produces several effects at once. Assessments should use a suitable population and safety margin.

Yield data and test conditions

A toxic-product yield expresses how much of a species is produced per unit fuel consumed. It depends on material and combustion mode, so a value from a well-ventilated test cannot automatically represent a vitiated fire. Full-scale mass loss, ventilation and dispersal history are also needed.

Small-scale tests can examine specified equivalence ratios and material compositions. Full-scale tests add enclosure and flow effects, while incident data provide real-world outcomes with less control. Each evidence type addresses a different part of the problem and should be used accordingly.

Incorrect yield history can underestimate smoke-inhalation hazard. Based on Purser (2002), pp. 2-96–2-97.

The 2005 guidelines identify carbon monoxide as the principal toxic species for many fires but caution that atypical materials require additional species. Smouldering polyurethane foam is their example of a source that can yield significant hydrogen cyanide. Source terms should therefore be based on the actual combustible and combustion mode.

Source yield, receiver concentration and inhaled dose must remain distinct. Entrained air can dilute concentration while increasing total mass flow, and deposition can change particles without removing gases. A complete model follows each relevant component from source to receiver.

Prevention and engineering assessment

Limiting ignition and early fire growth reduces effluent production and can preserve more time for detection and escape. Suppression can also shorten exposure, but its effect on combustion products must be checked because incomplete combustion and yields can change. Total mass burned remains part of the assessment.

Smoke management can extract, dilute, pressurise or contain contaminated gas. The 2005 guidelines identify these as broad approaches, and their effectiveness depends on maintaining the intended flow and pressure paths. Nominal fan capacity alone does not establish safe inhaled conditions.

Alarm and escape analysis should compare the time needed for response and movement with the time to incapacitation. This includes visibility, irritants and mobility, not only a lethal FED threshold. A scenario can fail because a person becomes unable to escape before conditions become lethal.

Sensitivity analysis should vary yield, ventilation, source strength, transport and occupant activity. These uncertainties affect both concentration and dose. Presenting a range and its controlling assumptions is more defensible than an unsupported single exposure time.

Limits and common misunderstandings

Smoke inhalation is not only carbon monoxide poisoning. Carbon monoxide is often central, but hydrogen cyanide, carbon dioxide, low oxygen, irritants and heat can all contribute. The complete atmosphere determines the effect.

Good visibility does not prove safe air. Gases can remain after particles deposit, and diluted smoke can still contain hazardous concentrations. Optical and toxic assessments must be separate.

A lethal dose is not the only useful criterion. Incapacitation can occur earlier and prevent escape, leading to a later fatal exposure. Sublethal effects therefore deserve explicit attention in design.

there is no universal lethal smoke concentration for every person and fire. Fuel, combustion conditions, transport, breathing and individual susceptibility all matter. Numerical examples are evidence for their stated models and tests, not universal thresholds.

See also

• Tenability criteria
• Oxygen depletion
• Haber's rule

References

• Australian Building Codes Board, National Research Council of Canada, International Code Council, & Department of Building and Housing New Zealand. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
• Gottuk, D. T., & Lattimer, B. Y. (2002). Effect of combustion conditions on species production. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Jin, T. (2002). Visibility and human behavior in fire smoke. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Mulholland, G. W. (2002). Smoke production and properties. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Purser, D. A. (2002). Toxicity assessment of combustion products. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.