How fires start and behave

Fire is one of the oldest workplace hazards and still one of the most destructive. A fire in a warehouse, a restaurant kitchen, a laboratory, a data center or an office can kill people in minutes, destroy a business that took decades to build and release smoke and contaminated water into the community. Most workplace fires are preventable, and most deaths in fires are caused not by flames but by smoke and toxic gases, often because people did not recognize the danger early or did not know how to get out. This course gives workers, supervisors and safety representatives a practical understanding of how fires start and spread, how to prevent them, how fire protection systems and portable extinguishers work, and how to respond to fires and other emergencies through planning, evacuation, shelter-in-place, drills and recovery. This first lesson explains the science of fire: what fire needs, how ignition happens, how fires grow and spread through a building, and why smoke is the main killer.

This is an awareness-level course. It supports the fire safety and emergency preparedness duties that employers have under OSHA standards, state plans and locally adopted fire codes, but it does not replace hands-on training in the use of portable extinguishers, your workplace's emergency action plan and fire prevention plan, or the specific training your employer must give you for your building, processes and role. Always follow your site's emergency procedures, the instructions of fire wardens and the fire department, and federal, state and local law. In an emergency, call 911.

The fire triangle and the fire tetrahedron

Fire is a rapid chemical reaction, called combustion, in which a fuel combines with oxygen and releases heat and light. For combustion to start and continue, three things must be present together: fuel, oxygen and heat. This is the fire triangle. Remove any one of them and the fire goes out. Modern fire science adds a fourth element: the self-sustaining chemical chain reaction in the flame, in which highly reactive molecular fragments called free radicals keep the reaction going. The four elements together form the fire tetrahedron. Some extinguishing agents, such as dry chemical powders and certain clean agents, work largely by interrupting this chain reaction rather than by cooling or smothering.

• Fuel: anything that can burn, including wood, paper, cardboard, plastics, textiles, flammable liquids, gases, metals in some forms, and dusts.
• Oxygen: normally supplied by the air, which contains about 21 % oxygen. Oxidizing chemicals, such as peroxides, nitrates and pure oxygen from cylinders, can supply oxygen even without air and make fires far more intense.
• Heat: the energy needed to raise the fuel to its ignition temperature, supplied by flames, sparks, hot surfaces, friction, electrical faults or chemical reactions.
• Chain reaction: the self-sustaining cycle of reactions in the flame zone.

Every method of fire prevention and fire fighting works by removing one or more of these elements. Keeping fuels away from ignition sources and storing them properly removes fuel or heat from the equation before a fire can start; water cools burning material below its ignition temperature; foam and carbon dioxide separate the fuel from oxygen; and dry chemicals interrupt the chain reaction.

How ignition happens

Solids and liquids do not burn directly. When they are heated, they give off flammable vapors or gases, and it is these vapors that mix with air and burn. Wood, paper and plastics break down under heat through a process called pyrolysis, releasing flammable gases. Liquids evaporate. Ignition occurs when the vapor concentration in air is within the flammable range and enough energy is supplied. Several properties describe how easily a material ignites:

• Flash point: the lowest temperature at which a liquid gives off enough vapor to form a flammable mixture with air near its surface. Gasoline has a flash point of about −40 °C, so it gives off flammable vapor in almost any weather; diesel fuel's flash point is above about 52 °C, so it is harder to ignite at room temperature but burns fiercely once heated.
• Flammable range: the range of vapor concentrations in air, between the lower flammable limit (LFL) and the upper flammable limit (UFL), within which a mixture can ignite. Below the LFL the mixture is too lean; above the UFL it is too rich. Propane, for example, burns between about 2.1 % and 9.5 % in air.
• Autoignition temperature: the temperature at which a material ignites without a spark or flame, simply from heat. Hot surfaces such as steam pipes, exhaust manifolds and overheated motors can ignite vapors and dusts with low autoignition temperatures.
• Minimum ignition energy: the smallest spark energy that can ignite a mixture. Some vapors and fine dusts can be ignited by static electricity discharges far too small to feel.

Safety data sheets (HSE-102) list flash points, flammable limits and autoignition temperatures in section 9, and the hazard classes in section 2 tell you whether a product is a flammable liquid, gas or solid. Knowing these properties explains many fire safety rules: why gasoline must never be used as a cleaning solvent, why containers of flammable liquids must be kept closed, and why sparks from grinding can start fires meters away.

Sources of ignition at work

Ignition sources in workplaces are everywhere, and many fires begin with an ordinary source meeting an unexpected fuel:

• Open flames: torches, burners, candles, pilot lights, matches and lighters, including smoking materials discarded carelessly.
• Hot work: welding, cutting, brazing, soldering and grinding, which throw sparks and molten metal that can travel more than 10 m and smolder unnoticed for hours (HSE-212 covers hot work permits).
• Electrical sources: overloaded circuits, damaged cords, loose connections, arcing switches, overheated motors and transformers, and lighting fixtures in contact with combustibles.
• Hot surfaces: ovens, dryers, heaters, steam lines, engine exhausts and process equipment.
• Friction and mechanical sparks: slipping belts, seized bearings, grinding wheels and metal striking metal or concrete.
• Static electricity: discharges when liquids are poured or pumped, when powders are moved, or when people walk across insulating floors.
• Chemical reactions: reactive chemicals mixed accidentally, oxidizers contacting organic materials, and self-heating materials such as oily rags, which can heat spontaneously as the oil oxidizes.
• Lithium-ion batteries: damaged, defective or improperly charged batteries can enter thermal runaway, venting flammable gases and burning intensely (Lesson 2).

Stages of fire growth

Fire safety engineers describe a typical fire in a room as passing through four stages. In the incipient stage, a small fire burns slowly, producing some smoke but little heat; this is the stage at which a portable extinguisher can be effective and the stage that smoke detectors are designed to catch. In the growth stage, the fire spreads across the fuel and to nearby materials, heat and smoke accumulate in a layer under the ceiling, and temperatures rise rapidly. If the hot smoke layer becomes hot enough, at roughly 500 to 600 °C, the radiant heat it sends down ignites everything combustible in the room almost at once, an event called flashover. After flashover, the fire is fully developed, the room is unsurvivable and the fire threatens neighboring spaces. Finally, in the decay stage, the fire burns down as fuel or oxygen runs out. A dangerous variation occurs when a fire in a closed room consumes most of the oxygen and then smolders; if a door is opened suddenly, the inrush of air can cause an explosive ignition of the accumulated gases, called a backdraft.

In modern buildings, the growth stage can be very short. Furniture and contents now contain large amounts of plastics and synthetic foams, which release heat and smoke far faster than traditional materials. Fire service research has shown that rooms furnished with modern contents can reach flashover in a few minutes, compared with much longer times for rooms furnished with older, natural materials. The practical lesson is that the time available to escape is short: people may have only a few minutes from the first signs of fire to untenable conditions.

How fire and smoke spread

Heat moves in three ways, and each contributes to fire spread. Conduction transfers heat through solid materials, such as a steel beam or pipe carrying heat into another room. Convection carries hot gases and smoke upward and outward; hot smoke rises to the ceiling, spreads horizontally, and moves through stairwells, shafts, ducts and gaps around pipes and cables. Radiation transfers heat as infrared energy in straight lines, igniting materials that are not touching the fire and making it impossible to approach a large fire closely. Fires also spread by direct flame contact and by burning debris falling or being carried by air currents.

Buildings are designed to slow this spread. Fire separations and fire-rated walls, floors and doors divide buildings into compartments; fire stopping seals openings around pipes and cables; and self-closing fire doors keep smoke and fire out of exit stairs and corridors. These features only work if they are maintained: a fire door propped open with a wedge, a ceiling tile left out, or a hole cut through a fire-rated wall for a new cable can allow smoke to reach escape routes in seconds. Lesson 3 returns to these everyday issues.

Smoke: the main killer

Most fire deaths are caused by smoke inhalation rather than burns. Smoke contains hot gases, fine particles and toxic products of combustion, including carbon monoxide, which prevents blood from carrying oxygen; hydrogen cyanide, released by burning plastics, foams and textiles and acting very quickly; and irritant gases that damage the airways and eyes. Smoke also blocks vision, so people cannot find exits they know well, and the heat of the smoke layer can burn the airways. Because smoke rises, the air near the floor is often cooler and clearer, which is why people escaping are taught to stay low.

Smoke spreads much faster and farther than flames. In a multi-story building, smoke can fill stairwells and upper floors well away from the fire, which is one reason why exit stair doors must be kept closed and why elevators must not be used during a fire. Fire alarm systems, sprinklers and smoke control systems (Lesson 5) all aim to detect smoke early, limit its production and keep it out of escape routes.

Fire classes

Fires are grouped into classes according to the fuel, because the right extinguishing method depends on what is burning. The classes used in Canada and the United States are:

Class C refers to the electrical hazard rather than a separate fuel: once the power is shut off, an electrical fire usually becomes a Class A or B fire. Using water on an energized electrical fire risks electric shock, and using water on a cooking oil fire causes violent spattering and fire spread. Lesson 6 explains extinguisher ratings and selection in detail.

Why fire science matters to every worker

Understanding how fires start and behave turns rules into common sense. A worker who knows that vapors, not liquids, burn will keep lids on solvent containers. A supervisor who understands flashover will insist on immediate evacuation rather than heroic attempts to fight a growing fire. A worker who understands smoke will close doors behind them as they leave, know the second exit route, and never re-enter a burning building. The rest of this course builds on these principles: Lessons 2 and 3 apply them to prevention, Lessons 4 to 7 to the laws, systems and equipment that protect buildings, and Lessons 8 to 12 to planning and responding to fires and other emergencies.