Effects of heat on people

Effects of heat on people are the physiological and behavioural consequences of thermal exposure during a fire. Fire engineering distinguishes impaired escape, incapacitation and injury because they are different outcomes. Heat can obstruct evacuation through pain before a person becomes physically incapable of movement or suffers the most severe consequences of exposure.

The principal thermal mechanisms are whole-body overheating, burns to the body surface and thermal injury to the respiratory tract. Their development depends on both the exposure and the person. Duration, incident heat, humidity, air movement, clothing and activity influence the response; air temperature alone does not describe every relevant condition.

Engineering assessments translate these effects into tenability criteria suited to a particular scenario and occupant population. A criterion based on pain, a cumulative thermal dose and a criterion for severe injury answer different questions. The endpoint and the conditions behind the chosen model must therefore accompany any numerical limit or predicted exposure time.

Heat transfer and the exposed person

A thermal exposure describes the transfer of energy to the person, rather than merely the temperature of the surroundings. The relevant heat flux expresses the rate of energy transfer per unit area. The resulting tissue heating depends on how that energy is absorbed, distributed and removed during the exposure.

Radiation, convection and conduction provide distinct transfer paths. Radiation can heat exposed skin from a flame, hot surface or hot gas region without direct contact. Convection transfers heat between moving gases and the body. Conduction becomes important when skin contacts a hot object.

Thermal exposure can act through several transfer paths and produce different effects. Based on Purser (2002), pp. 2-125–2-128.

These paths need different environmental information. A gas temperature does not specify the radiation received from a nearby opening. Equally, a radiant flux does not define the convective heating caused by surrounding hot gases. A calculation should identify which mechanisms are represented and which exposure quantities its response model requires.

Convection depends on air movement and humidity as well as temperature. The same nominal air temperature can therefore produce different heating under different flow conditions. For contact heating, the material properties of the object influence the interface temperature and heat transfer. A metal surface and a low-conductivity surface need not produce the same contact exposure.

The body is also not a uniform passive target. Different regions may be covered or exposed, and activity changes heat production and the demands of movement. Purser identifies the amount and type of clothing and the work being performed as significant influences on thermal tolerance.

A useful engineering exposure description consequently states the location, duration, principal transfer mechanisms and assumed protection. This creates a traceable connection between the fire calculation and the response model. A single reported temperature or flux is insufficient when the omitted conditions materially affect the chosen endpoint.

Hyperthermia, skin pain and burns

Hyperthermia results when the body accumulates heat faster than it can remove it. It can develop during prolonged exposure at conditions that do not immediately burn the skin. Activity and impaired evaporative cooling can increase the burden. It is therefore a different mechanism from the rapid surface heating associated with intense radiation.

Skin pain and burns depend on the thermal history of the exposed tissue. The balance between incoming heat and heat conducted away controls the temperature rise near the surface. Duration matters because the tissue continues to accumulate heat while exposure persists. A brief intense exposure and a longer moderate exposure cannot be compared by peak temperature alone.

Pain, partial-thickness injury and more extensive burns are separate endpoints. A relationship developed for the onset of pain should not be relabelled as a relationship for fatal injury. Conversely, an analysis that only predicts severe injury can miss an earlier condition that prevents occupants from continuing along a route.

Purser describes a continuum of impairment. Discomfort and pain can reduce the efficiency of escape before a sharply defined incapacitating event occurs. Some effects, such as loss of consciousness from other fire hazards, have a clearer endpoint than the point at which painful conditions stop effective movement.

Clothing has a conditional role. It can reduce immediate heat transfer to covered skin, while leaving the head or other regions exposed. During longer exposures, clothing can also restrict evaporative heat loss. Treating clothing as a single constant increase in tolerance would conceal these different effects.

These distinctions explain why a temperature limit requires context. The same number may be used as a screening criterion in one method and as an input to a time-dependent response model in another. Its meaning depends on the exposure duration, humidity, body region and endpoint for which it was selected.

Humidity and respiratory thermal exposure

Humidity affects both the removal of body heat and the delivery of heat to tissue. Moist air can restrict cooling by evaporation. Steam can also transfer substantial energy when it condenses. A dry-air exposure relationship therefore cannot automatically be transferred to an atmosphere containing a large amount of water vapour.

The distinction is especially important for the respiratory tract. Purser describes the limited thermal capacity of dry air and the cooling effect of the moist airway surfaces. Steam and other atmospheres with greater thermal capacity can produce a different pattern of thermal injury. Temperature alone does not capture that difference.

Fire atmospheres complicate interpretation because several mechanisms may act together. Inhaled irritants can cause chemical injury, hot gases can transfer heat, and body-surface burns can have subsequent systemic consequences. Observed respiratory damage cannot always be assigned to thermal inhalation alone.

This is a limitation on the engineering inference, rather than a reason to omit respiratory effects. A model using gas temperature as a thermal indicator should state whether humidity and other properties of the atmosphere are represented. Separate toxic effects remain relevant even when a thermal criterion is satisfied.

The source material also distinguishes immediate escape impairment from effects that develop after exposure. A person leaving the hazardous environment has not necessarily avoided every injury endpoint. Egress calculations address a defined period and outcome; they should not be interpreted as a complete clinical prognosis.

The practical consequence is to preserve the model's stated exposure conditions. A criterion derived for dry air, exposed skin or resting subjects represents those assumptions. Changing the humidity, protective clothing or physical activity changes the physical problem, even if the reported gas temperature is unchanged.

Thermal criteria and cumulative exposure models

The ABCB handbook distinguishes simple exposure limits from more detailed human-response models. It identifies fractional effective dose and probit functions among the available approaches, while noting that simple limits and dose models are commonly used for building fire assessments. These are different ways of representing an exposure-response relationship.

A simple limit compares a selected environmental quantity with a prescribed threshold. A dose model considers the accumulated effect of exposure over time. The distinction matters when conditions change during evacuation: an instantaneous value does not contain the same information as the complete exposure history.

A thermal fractional effective dose can accumulate contributions from radiant and convective exposure according to the selected model. The supplied New Zealand verification method explicitly requires both contributions for its thermal assessment. Their inclusion does not mean that every published thermal equation uses the same coefficients, assumptions or endpoint.

The choice of criterion also belongs to a stated design framework. The supplied 2020 ABCB handbook uses a thermal dose criterion of 0.3, evaluated at two metres above floor level, within its occupant assessment. That is a criterion of the documented method, not a universal boundary between injury and no injury for every person.

The occupant population matters independently of the numerical model. Purser discusses variation in susceptibility and uncertainty in predicting human responses to complex fire atmospheres. A nominal response endpoint cannot guarantee an identical response from every individual. The design interpretation must distinguish the model endpoint from the margin adopted for the assessment.

The relation to available safe egress time (ASET) is therefore conditional. ASET ends when the first applicable criterion is exceeded at the specified assessment location. Heat may govern that time, but visibility or toxic exposure can become limiting earlier. Passing a thermal check does not establish that all other conditions remain tenable.

Radiation while passing a window

Thermal radiation along an escape route provides a useful example of an explicitly defined endpoint. The supplied 2025 New Zealand method considers occupants passing a window opening or glazed panel. Its criterion requires the predicted time to onset of pain to exceed the time spent in the relevant exposure region.

For that method, the time to pain is calculated from the maximum incident radiant flux using the following empirical expression. The numerical coefficient requires the stated units.

t_p = (35/q″_r)^1.33

t_exp = D/V

Symbol	Meaning in this method	Unit
tp	Predicted time to onset of pain	s
q″r	Maximum incident radiant heat flux	kW/m2
texp	Exposure time while passing the window	s
D	Travel distance within the specified radiation exposure region	m
V	Assumed travel speed	m/s

The method assumes a speed of 1.0 m/s. Its exposed travel distance covers the part of the route where incident radiation is at least 2.5 kW/m². This is a specific way to define exposure for the calculation. It should not be interpreted as proof that lower fluxes have no possible effect over any duration.

The method compares a defined passage exposure with its predicted time to pain. Based on Ministry of Business, Innovation and Employment (2025), pp. 34–36.

The surrounding limitations are essential. The simplified analysis applies to a single window and limits the maximum radiation to 10 kW/m². Passage must occur within ten minutes after ignition unless fire-resisting glazing is used. The method is not appropriate where occupants are likely to have impaired mobility.

Multiple windows require a more detailed treatment of the cumulative, time-dependent exposure. The source also includes conditions concerning glazing, alternative routes and other protective provisions. Reproducing the pain equation alone does not reproduce the full verification method or establish compliance with all of its requirements.

Worked example and sensitivity

Consider an illustrative maximum incident flux of 5.0 kW/m² and an exposed path length of 8.0 m. Applying the method's assumed speed gives an exposure time of 8.0 s. The pain equation gives the following result, calculated without intermediate rounding.

t_p = (35/5.0)^1.33 = 13.3 s

The calculated exposure time is shorter than the predicted pain time for these selected inputs. This comparison illustrates the equation and its endpoint. It does not demonstrate that a particular route meets every requirement of the method, nor that smoke, toxicity and other exposures have been addressed.

Increasing the maximum flux to 8.0 kW/m², while retaining the same illustrative path and speed, reduces the calculated pain time to approximately 7.12 s. The eight-second passage would then exceed the predicted pain time. This sensitivity shows why the exposure intensity and travel duration must be assessed together.

The method predicts a shorter time to pain as incident radiant flux rises. Based on Ministry of Business, Innovation and Employment (2025), pp. 34–36.

The two flux values are selected calculation inputs, rather than measurements of a particular fire. A project application would first establish the incident flux using the geometry and radiation procedure appropriate to the source. The occupant-response calculation cannot supply that missing fire-exposure information.

It would also be inappropriate to extend the calculation to a slower, mobility-impaired population simply by substituting a different speed. The supplied method explicitly excludes that application. A different exposure and movement assessment would be required to address the changed population and its relevant response assumptions.

The example also illustrates the importance of units. The pain equation gives seconds for flux entered in kilowatts per square metre. Substituting watts per square metre without changing the empirical coefficient changes the numerical result. The unit convention is part of the equation's definition.

Interpretation within an evacuation assessment

A thermal assessment should identify the effect being prevented, the exposure model and the occupant assumptions before interpreting a calculated time. Pain avoidance, preservation of escape capability and prevention of severe injury are related objectives, but their response criteria are not interchangeable.

The exposure location also needs to be clear. A temperature measured within an upper gas layer does not automatically describe a person's whole exposure along an escape route. The relevant quantities must be evaluated where the selected method requires them, with any movement and shielding assumptions stated.

Thermal and non-thermal hazards can affect the same movement. Smoke may obscure the route, irritants may impair vision or breathing, and heat may make continued passage painful. Purser's discussion of escape impairment shows why these effects must be interpreted together, even when separate numerical criteria are used.

A useful engineering conclusion therefore reports which criterion controls, where it is exceeded and what exposure assumptions support the result. It retains the distinction between a method-specific threshold and an individual physiological outcome. This makes the calculation relevant to the evacuation decision without overstating what the thermal model predicts.

See also

• Fireball

References

• Australian Building Codes Board. (2020). Fire safety verification method: Handbook (Version 1.1).
• Ministry of Business, Innovation and Employment. (2025). C protection from fire: Verification Method C/VM2 (2nd ed.).
• Purser, D. A. (2002). Toxicity assessment of combustion products. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-83–2-171). National Fire Protection Association.