USFA Structure Fire Cause Methodology

Since the introduction of National Fire Incident Reporting System (NFIRS) Version 5.0, the implementation of the cause hierarchy has resulted in a steady increase in the percentages of unknown fire causes. This increase may be due, in part, to the fact that the original cause hierarchy (described in Fire in the United States 1995-2004, 14th Edition) does not apply as well to Version 5.0. Causal information now collected as part of NFIRS Version 5.0 was not incorporated in the old hierarchy. As a result, many incidents were assigned to the unknown cause category. As the hierarchy was originally designed for structures, incidents that did not fit well into the structure cause categories were also assigned to the unknown category.

Structure Fires

To capture the wealth of data available in NFIRS 5.0, USFA developed a modified version of the previous cause hierarchy for structure fires as shown in Table 1. The revised schema provides three levels of cause descriptions: a set of more detailed causes (priority cause description), a set of mid-level causes (cause description), and a set of high-level causes (general cause description). The priority cause description and the cause description existed previously as part of the original cause hierarchy, but have been expanded to capture the new 5.0 data.

Table 1. Three-Level Structure Fire Cause Hierarchy
Priority Cause Description
(in hierarchical order)
Cause Description General Cause Description
Exposure Exposure Exposure
Intentional Intentional Firesetting
Investigation with Arson Module Investigation with Arson Module Unknown
Children Playing Playing with Heat Source Firesetting
Other Playing
Natural Natural Natural
Fireworks Other Heat Flame, Heat
Smoking Smoking
Heating Heating Equipment
Cooking Cooking
Air Conditioning Appliances
Electrical Distribution Electrical Malfunction Electrical
Appliances Appliances Equipment
Special Equipment Other Equipment
Processing Equipment
Torches Open Flame Flame, Heat
Service Equipment Other Equipment Equipment
Vehicle, Engine
Unclassified Fuel Powered Equipment
Unclassified Equipment w/ Other or Unknown Fuel Source Unknown Unknown
Unclassified Electrical Malfunction Electrical Malfunction Electrical
Matches, Candles Open Flame Flame, Heat
Open Fire
Other Open Flame, Spark Other Heat
Friction, Hot Material
Ember, Rekindle Open Flame
Other Hot Object Other Heat
Natural Condition, Other Natural Natural
Heat Source or Product Misuse Other Unintentional, Careless Unknown
Equipment Operation Deficiency Equipment Misoperation, Failure Equipment
Equipment Failure, Malfunction
Trash, Rubbish Unknown Unknown
Other Unintentional Other Unintentional, Careless
Exposure (Fire Spread, Other) Exposure Exposure
Unknown Unknown Unknown

Note: Fires are assigned to a cause category in the hierarchical order shown. For example, if the fire is judged to be intentionally set and a match was used to ignite it, it is classified as intentional and not open flame because intentional is higher on the list.

The causes of fires are often a complex chain of events. To make it easier to grasp the "big picture," the 16 mid-level categories of fire causes such as heating, cooking, and playing with heat source are used by the USFA. The alternative is to present scores of detailed cause categories or scenarios, each of which would have a relatively small percentage of fires. For example, heating includes subcategories such as misuse of portable space heaters, wood stove chimney fires, and fires involving gas central heating systems. Experience has shown that the larger categories are useful for an initial presentation of the fire problem. A more detailed analysis can follow.

Fires are assigned to one of the 16 mid-level cause groupings using a hierarchy of definitions, approximately as shown in Table 2.1 A fire is included in the highest category into which it fits on the list. If it does not fit the top category, then the second one is considered, and if not that one, the third, and so on. (See Table 1 Note for examples.)

Vehicle, Outside, and Other Fires

While these new cause categories have usefulness for the other property types - vehicle, outside, and other fires - there are limitations. USFA plans to investigate and develop specific cause categories for vehicle, outside, and other fires. Until then, the causes of fires for these property types are based on the distributions of the NFIRS cause of ignition data element. This data element captures a very broad sense of the cause of the fire.

Other Considerations

An additional problem to keep in mind when considering the rank order of causes is that sufficient data to categorize the cause were not reported to NFIRS for all fatal fires in the database. The rank order of causes might be different than shown here if the cause profile for the fires whose causes were not reported to NFIRS were substantially different from the profile for the fires whose causes were reported. However, there is no information available to indicate that there is a major difference between the known causes and the unknown causes, and so our present best estimate of fire causes is based on the distribution of the fires with known causes.

Table 2. Mid-Level Cause Groupings
Cause Category Definition
Exposure Caused by heat spreading from another hostile fire.
Intentional Cause of ignition is intentional or fire is deliberately set.
Investigation with Arson Module Cause is under investigation and a valid NFIRS Arson Module is present.
Playing with Heat Source Includes all fires caused by individuals playing with any materials contained in the categories below as well as fires where the factors contributing to ignition include playing with heat source. Children playing with fire are included in this category.
Natural Caused by the sun’s heat, spontaneous ignition, chemicals, lightning, static discharge, high winds, storms, high water including floods, earthquakes, volcanic action, and animals.
Other Heat Includes fireworks, explosives, flame/torch used for lighting, heat or spark from friction, molten material, hot material, heat from hot or smoldering objects.
Smoking Cigarettes, cigars, pipes, and heat from undetermined smoking materials.
Heating Includes confined chimney or flue fire, fire confined to fuel burner/boiler malfunction, central heating, fixed and portable local heating units, fireplaces and chimneys, furnaces, boilers, water heaters as source of heat.
Cooking Includes confined cooking fires, stoves, ovens, fixed and portable warming units, deep fat fryers, open grills as source of heat.
Appliances Includes televisions, radios, video equipment, phonographs, dryers, washing machines, dishwashers, garbage disposals, vacuum cleaners, hand tools, electric blankets, irons, hairdryers, electric razors, can openers, dehumidifiers, heat pumps, water cooling devices, air conditioners, freezers and refrigeration equipment as source of heat.
Electrical Malfunction Includes electrical distribution, wiring, transformers, meter boxes, power switching gear, outlets, cords, plugs, surge protectors, electric fences, lighting fixtures, electrical arcing as source of heat.
Other Equipment Includes special equipment (radar, x-ray, computer, telephone, transmitters, vending machine, office machine, pumps, printing press, gardening tools, or agricultural equipment), processing equipment (furnace, kiln, other industrial machines), service, maintenance equipment (incinerator, elevator), separate motor or generator, vehicle in a structure, unspecified equipment.
Open Flame, Spark (heat from) Includes torches, candles, matches, lighters, open fire, ember, ash, rekindled fire, backfire from internal combustion engine as source of heat.
Other Unintentional, Careless Includes misuse of material or product, abandoned or discarded materials or products, heat source too close to combustibles, other unintentional (mechanical failure/malfunction, backfire).
Equipment Misoperation, Failure Includes equipment operation deficiency, equipment malfunction.
Unknown Cause of fire undetermined or not reported.

Source: USFA

NFIRS fire causal data can be analyzed in many ways, such as by the heat source, equipment involved in ignition, factors contributing to ignition, or many other groupings. The hierarchy of causes used has proven to be useful in understanding the fire problem and targeting prevention, but other approaches are useful too. Because the NFIRS database stores records fire-by-fire, and not just in summary statistics, a very wide variety of analyses is possible.

The cause categories displayed in the graphs of USFA’s NFIRS data-related reports are listed in the same order to make comparisons easier from one to another. The y-scale varies from figure to figure depending on the largest percentage that is shown; the y-scale on a figure with multiple charts, however, is always the same.

1 The hierarchy involves a large number of subcategories that are later grouped into the 16 mid-level cause categories, then the 8 high-level cause groupings.