2.3 Human Behavior in Fire Emergencies

2.3 Human Behavior in Fire Emergencies

Building egress design and fire safety strategies must account for how people actually behave during fire emergencies. Designing a safe means of egress requires combining hydraulic calculations of human movement with an understanding of human psychology, evacuation delay, and the physiological effects of toxic gases.

Life Safety Egress Modeling and Calculations

Egress modeling describes how occupants move through a building's exit systems. The traditional engineering method is the SFPE Hydraulic Flow Model, which treats occupant movement similarly to fluid flow through a pipe network. The capacity of an egress route depends on its physical geometry, the occupant density, and the movement speed.

The flow rate ($F$) through a means of egress component is calculated as: $F = f_s W_e$ Where $F$ is the flow rate (persons per minute), $f_s$ is the specific flow boundary coefficient (persons/min/meter), and $W_e$ is the effective width of the component.

• The Effective Width Concept: Egress calculations do not use the physical width of doors or corridors. Instead, they subtract a boundary layer that occupants naturally avoid due to psychological comfort and physical clearance. For corridors and doors, a boundary layer of 3.5 inches (90 mm) is subtracted from each side. For stairs, 3.5 inches (90 mm) is subtracted from walls and 2.0 inches (50 mm) from handrails.
• Speed-Density Relationship: Walking speed ($S$) is highly dependent on occupant density ($D$, persons/m²). As density increases, walking speed decreases: $S = k - a D$ Where $k$ and $a$ are flow constants. Unimpeded walking speed is approximately 1.2 m/s.
• Critical Densities: Below 0.5 persons/m², occupants move unimpeded. At densities above 1.9 persons/m², walking speed drops significantly, and queueing occurs at pinch points. At densities above 3.8 persons/m², flow drops to near zero, posing extreme risks of crowd crush.

Panic vs. Rational Behavior

Popular media often portrays fire evacuations as chaotic scenes of irrational "panic," where crowds stampede and fight for survival. Empirical research shows that mass panic is extremely rare in building fires. Instead, occupants behave rationally based on their perception of the situation, the information available, and social structures.

Key sociological phenomena in evacuations include:

• Affiliation Behavior: Occupants are highly likely to move toward familiar people (family, friends) or familiar locations (the entrance they used to enter) rather than the nearest emergency exit.
• Role-Directed Behavior: Occupants maintain their social and occupational roles. Staff members assist customers, teachers guide students, and parents seek children before evacuating.
• Social Influence (Bystander Effect): Occupants look to others to confirm a threat. If a fire alarm sounds but others ignore it, an individual will likely ignore it to avoid embarrassment.
• Commitment to Activities: Occupants are reluctant to abandon tasks they have already started (e.g., closing a laptop, packing belongings, paying a bill) before evacuating.

Occupant Response Times and Pre-Movement Delay

The total time required to evacuate a building is modeled as: $t_{evac} = t_{det} + t_{not} + t_{pre} + t_{trav}$ Where $t_{det}$ is detection time, $t_{not}$ is notification time, $t_{pre}$ is pre-movement time, and $t_{trav}$ is travel time.

In many fires, the pre-movement time ($t_{pre}$) is the largest component of total evacuation time. Occupants rarely start moving immediately when an alarm sounds. This delay is influenced by:

• Alarm Ambiguity: Simple horns or bells are ambiguous; occupants often assume they are drills or malfunctions.
• Voice Notifications: Utilizing voice evacuation systems that provide specific, clear information about the fire's location and nature reduces ambiguity, reducing pre-movement times by up to 50% compared to traditional horns.
• Training and Familiarity: Trained staff and occupants who participate in drills respond much faster.

Toxicology of Fire Gases

Most fire-related deaths are caused by smoke inhalation, not thermal burns. The combustion of building contents produces a toxic cocktail of gases:

Carbon Monoxide (CO)

Carbon monoxide is an asphyxiant gas produced by incomplete combustion.

• Mechanism: CO binds to hemoglobin in red blood cells to form carboxyhemoglobin (COHb). Hemoglobin’s affinity for CO is 200 to 250 times greater than its affinity for oxygen. This blocks oxygen transport in the blood, causing systemic hypoxia.
• Fractional Effective Dose (FED): The FED model predicts incapacitation by summing the cumulative exposure to toxic gases over time. An FED of 1.0 represents the threshold where 50% of the population becomes incapacitated.
• Thresholds: COHb levels of 10-20% cause mild headaches; 30-40% cause confusion, dizziness, and motor impairment; and 50% or higher lead to unconsciousness and death.

Hydrogen Cyanide (HCN)

Hydrogen cyanide is a highly toxic cellular asphyxiant produced by burning nitrogen-containing materials (e.g., polyurethane, wool, nylon).

• Mechanism: HCN inhibits cytochrome c oxidase in the mitochondria, preventing cells from using oxygen to produce energy (ATP).
• Synergy with CO: HCN is 20 to 30 times more toxic than CO. When inhaled together, they act synergistically. HCN causes rapid hyperpnea (deep, rapid breathing), which increases the rate at which the occupant inhales CO and other toxic gases.

Hypoxia

Hypoxia is the depletion of oxygen in the fire room.

• Normal air contains 20.9% oxygen.
• At 15% oxygen, motor coordination is impaired, and breathing rate increases.
• At 10-12% oxygen, dizziness, confusion, and rapid fatigue occur.
• Below 10% oxygen, occupants lose consciousness quickly, followed by brain death.

Irritant Gases

• Sensory Irritants: Gases like hydrogen chloride ($HCl$, from PVC) and acrolein dissolve in the moisture of the eyes and throat, causing burning, tearing, and involuntary blinking (blepharospasm). This restricts visibility and slows movement speed.
• Pulmonary Irritants: Gases like nitrogen dioxide ($NO_2$) and phosgene penetrate deep into the lungs, causing delayed pulmonary edema (fluid buildup) hours after exposure.