Topical Fire research series Volume 7, Issue 2 / August 2007

Fires and Exposures

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Findings:

■ Source fires and the subsequent exposure fires account for about 30,700 fires, 275 deaths, and 875 injuries 

annually.

■ When structure fires are the source of exposure fires, they have the highest loss rates of any source fire. 

■ Structure source fires have over seven times the fire death rate, over five times the loss rate, and over four 

times the fire injury rate as the average structure fire.

■ March has the highest incidence (12%) of source fires. This is primarily due to the increased incidence (19%) 

of outdoor source fires.

I

n 2004, an estimated 12,100 fires spread beyond their 

boundaries and caused an estimated 18,600 additional 

fires. These source fires and the fires resulting from their 

fire spread (exposure fires) resulted in an estimated total 

of $803 million in direct losses, 875 injuries, and 275 

fatalities.1,2

Table 1. National Estimates for Source and 

Exposure Fires, 2004

Measure Source Fires Resulting 

Exposure Fires Total

Fires 12,100 18,600 30,700

Deaths 255 15 275

Injuries 825 50 875

Dollar Loss $605,000,000 $197,000,000 $803,000,000

Note: Totals may not add due to rounding

Source: 2004 NFIRS 5.0 data and NFPA Annual Survey

What is an Exposure? 

Exposures are defined as anything in the immediate range of 

a fire that is not burning but could start burning if the fire is 

not contained. When enough oxygen is available, combustible material burns once its temperature reaches its ignition 

temperature. Preventing exposure fires requires protecting 

the objects from heat spread and thus from reaching their 

ignition temperatures.3

 This is one of the most critical jobs 

in firefighting, as protecting exposures helps contain the fire 

and prevents fire spread. After rescuing individuals, protection of exposures is the highest priority for firefighters.4

Exposures can be internal or external. Internal exposures are 

within the property, such as a room or floor of a building. 

External exposures include other buildings, vehicles, and 

outside objects or vegetation such as trash, dumpsters, trees, 

or crops. External exposures are typically known as “exposure fires.” These exposure fires and the source fires that 

cause them are the topic of this report.

Why Fires Spread

A fire spreads for many reasons. The fire may go undetected 

for a substantial period of time because of the location 

of fire or, in the case of buildings, because of the lack of 

appropriate detection or extinguishing systems. There may 

have been a delay in notifying the fire department. The 

weather conditions or materials providing the fuel load may 

have contributed to the spread. Whatever the circumstance 

and whatever the reason, it is likely that fires with significant fire spread will extend to other areas, and can result in 

additional injuries, deaths, and dollar loss, as shown in Table 

1. The source fire, because of the extent of the fire spread, 

has substantially higher loss rates than the resulting exposure fires (Table 2). In general, exposure fires have much 

higher loss rates than all fires.

U.S. Department of Homeland Security • U.S. Fire Administration

National Fire Data Center • Emmitsburg, Maryland 21727

www.usfa.dhs.gov/statistics/reports/pubs/tfrs.shtm 
TFRS Volume 7, Issue 2 / Fires and Exposures Page 2

Table 2. Loss Measures for Source and 

Exposure Fires, 2004

Measure Source

Fire

Resulting

Exposure

Fires

Source

Fires and

Exposures

All Fires

Loss per Fire $54,248 $11,762 $28,728 $8,498

Injuries per 1,000 Fires 66.1 2.6 27.6 11.3

Deaths per 1,000 Fires 19.0 0.8 8.0 2.3

Note: Loss per fire is computed only for those fires where loss information was provided.

Source: 2004 NFIRS 5.0 data only

Wildland/Urban Interface Exposure Fires

The wildland/urban interface (WUI), the area where 

structures and other human development meet or intermingle with undeveloped wildland, creates an environment in which fire can move readily between structural and 

vegetation fuels. WUI fires usually include a large number 

of simultaneously exposed structures and the immediate involvement of residential and commercial areas. The 

continuing WUI expansion has increased the likelihood that 

wildfires will threaten structures and people. Large WUI 

losses resulting from wildland fires are a serious issue. Public awareness of the WUI problem has grown recently, and 

the importance of limiting fuel loads (vegetation) around 

structures in the WUI has been stressed. 

Source Fires

Approximately one-half of all source fires, those fires 

leading to exposure fires, occur in structures, as shown in 

Figure 1.

Source structure fires have the highest rates of deaths, injuries, and dollar loss compared to the average fire. Structures 

generally involve buildings, which have a higher percentage of individuals and property than other types of fires. 

Compared to the average fire, source fires have much higher 

rates of injuries, deaths, and dollar loss, as shown in Table 3. 

Generally, the dollar loss for structure fires decreases sharply 

in the first and second exposure fire following the source 

fire. Then in the third and fourth subsequent exposure fires 

the dollar loss increases but not to the level of dollar loss as 

the source fire, as shown in Table 4. This can happen when 

fire resources get fully committed before additional fire 

resources arrive.

Figure 1. Distribution of Source Fires by General Incident Type, 2004 (percent)

Source: 2004 NFIRS 5.0 data only

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TFRS Volume 7, Issue 2 / Fires and Exposures Page 3

Table 3. Loss Measures for Source Fires and Average Dollar Loss for All Fires, 2004

Structures Vehicles Outside Other

Source 

Fire

All Structure 

Fires

Source 

Fire

All Vehicle 

Fires

Source 

Fire

All Outside 

Fires

Source 

Fire

All Other 

Fires

Loss per fire $98,298 $17,327 $9,550 $4,562 $2,516 $517 $9,464 $2,734

Injuries per 1,000 fires 113.6 25.3 25.4 5.2 11.1 0.9 58.8 8.1

Deaths per 1,000 fires 33.9 4.7 7.8 2.2 0.9 0.1 0.0 0.6

Note: Loss per fire is computed only for those fires where loss information was provided.

Source: 2004 NFIRS 5.0 data only

Table 4. Dollar Loss for Exposure Fires by Number of Exposures and General Incident Type; 

Four or Fewer Exposures, 2004

Measure

Number of Exposures:

Source One Two Three Four

Structure Loss Per Fire $98,298 $15,348 $16,840 $22,148 $28,681

Vehicle Loss Per Fire $9,550 $6,155 $4,465 $3,520 $4,784

Outside Loss Per Fire $2,516 $2,009 $6,977 $4,929 $2,931

Other Loss Per Fire $9,464 $4,473 $2,215 $34,241* $990

Note: Loss per fire is computed only for those fires where loss information was provided.

Source: 2004 NFIRS 5.0 data only 

*This value includes a fire with a dollar loss 50 times the average dollar loss for “other” fires.

When Source Fires Occur

Overall, source fires occur at a relative constant rate, with a 

notable peak above average in March, as shown in Figure 2. 

Structure source fires typically occur at all times of the day, 

with a slight peak between the hours of 4 a.m. and 5 a.m., 

as shown in Figure 3. There is also a slight peak in the number of structure source fires during the month of January, 

and the incidence of these fires remain higher than average 

in the colder weather months through April, as shown in 

Figure 4. Vehicle source fires peak between the hours of 2 

a.m. and 4 a.m. Although these vehicle fires typically occur 

at a constant rate throughout the year, there is a slight peak 

in July. 

Outside source fires generally occur in the early spring when 

conditions are drier, with a sharp peak in March. These outside fires typically occur during the early afternoon, with a 

definite peak between the hours of 2 p.m. and 3 p.m.

Source fires spread to similar type properties. For example, 

source structure fires spread to other structures 70% of the 

time. Vehicle fires spread to other vehicles 68% of the time, 

and outside fires spread to outside fires 47% of the time, as 

shown in Table 5.

Table 5. Type of Exposure Fire by Source Fire, 

2004

Source 

Fire

Exposures

Structure Vehicle Outside Other Total

Structure 70% 22% 3% 5% 100%

Vehicle 25% 68% 5% 3% 100%

Outside 36% 13% 47% 4% 100%

Other 18% 14% 3% 65% 100%

Source: 2004 NFIRS 5.0 data only; some totals may not add to 100% due to rounding.

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TFRS Volume 7, Issue 2 / Fires and Exposures Page 4

Figure 2. Distribution of Source Fires by Month, 2004 (percent)

Source: 2004 NFIRS 5.0 data only

Figure 3. Distribution of Source Fires by General Incident Type and Hour, 2004 (percent)

Source: 2004 NFIRS 5.0 data only

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TFRS Volume 7, Issue 2 / Fires and Exposures Page 5

Figure 4. Distribution of Source Fires by Type of Fire and Month of Occurrence, 2004 (percent)

Source: 2004 NFIRS 5.0 data only

Causes of Source Fires

The top three causes of source fires include incendiary or 

suspicious (43%); open flame, ember, or torch (17%); and 

other heat, flame, or spark (14%), as shown in Figure 5. For 

example, numerous vehicle fires fall under the incendiary 

or suspicious category as a result of insurance fraud.5

 Open 

flame items such as candles, matches, lighters, and cigarettes 

can cause devastating fires. Fires from other heat, flame, or 

sparks can include fireworks or malfunctions of appliances 

and machinery.

Examples

Each year, newspapers describe various incidents where fires 

spread away from the original fire source. Below are four 

examples of such fires:

•	 February 2007—About 150 firefighters battled a 

Raleigh, North Carolina, townhouse complex fire 

for more than an hour. The flames were fanned by 

winds that reached 30 to 40 mph, which made the 

fire difficult to control because embers were blown 

over firewalls separating the units.6

•	 February 2007—Des Moines, Iowa, firefighters 

responded to a chimney fire at a single family home 

around 7 p.m. That fire quickly spread to two other 

connecting buildings.7

•	 February 2007—Pittsburg, Kansas, Fire Department officials say a trash barrel was the source of a 

fire that destroyed a two-car garage and sent heavy 

smoke up into the downtown sky.8

•	 February 2007—Two Looneyville, Texas, houses 

burnt to the ground and the fire spread to the 

adjoining woods. There were three propane tanks 

sitting in the middle of the blaze that kept the first 

responders from being able to initiate their attack.9

Conclusion

Although the incidence of fires with exposures varies by 

month, the risk remains serious throughout the year. Therefore, fire safety must be practiced constantly to ensure early 

detection and early notification. Preventing exposure fires 

involves controlling the spread of the source fire. It is necessary to make sure that smoke alarms work and sprinklers 

are installed whenever possible. Do not try to put fires out 

yourself, and call for fire assistance as soon as possible. 

Outside fires do have predictable fire seasons in early spring 

and late summer. The most successful means of protecting 

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TFRS Volume 7, Issue 2 / Fires and Exposures Page 6

Figure 5. Cause Distribution of Source Fires, 2004 (percent)

Source: 2004 NFIRS 5.0 data only

structures in the WUI is not necessarily advancements in 

fire technology but in educating communities in the WUI. 

Structures that are threatened by wildland fires need special 

attention during the fire season by clearing out nearby vegetation, brush, and other combustibles. It is recommended 

that homeowners take responsibility for securing fire-safe 

conditions and source-fire defense of their residences during wildland fire season by following these guidelines:10

■ Remove leaves, brush, and dry grass within 30 feet of 

residences and other structures.

■ Remove trees within 10 feet of residences, and space 

trees 10 feet apart. Cut branches below 6 feet from the 

ground to prevent fires from spreading into the treetops.

■ Clean roofs and rain gutters regularly to keep them free 

of twigs, leaves, and pine needles. Remove all tree limbs 

within 10 feet of your chimney or stove pipe.

■ Store firewood and other combustible materials, such as 

picnic tables, at least 30 feet away from any structures.

■ Apply a fire-retardant solution, such as phosphate salt, to 

wood shingles and shake roofs.

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TFRS Volume 7, Issue 2 / Fires and Exposures Page 7

More information about how you can help prevent exposure fires can be obtained from your local fire department 

or the U.S. Fire Administration.

To request additional information or comment on this 

report, visit 

http://www.usfa.dhs.gov/applications/feedback/

Notes: 

1NFIRS 5.0 contains both converted NFIRS 4.1 data and native 

NFIRS 5.0 data. This topical report includes only native 5.0 data. 

Incident type 110 (structure fire, other) is not included in this 

analysis as it is a “conversion only” code. That is, incident type 

110 is technically a version 4.1 incident and, as such, is not 

included in this analysis.

2National estimates are based on native version 5.0 data from 

the 2004 National Fire Incident Reporting System (NFIRS) and 

national fire loss estimates from the National Fire Protection Association’s (NFPA) annual survey of fire loss. Fires are rounded to 

the nearest 100, deaths to the nearest 5, injuries to the nearest 25, 

and loss to the nearest $million.

3Heat spreads in three ways. The first is through radiation, which 

transmits heat in the air. Any material close to a fire absorbs heat 

until it starts to burn. The second is through conduction, where 

heat is absorbed by an object or material such as metal that 

can transfer heat to another area. The third is through convection, where heat, smoke, and fire are drawn upward spreading 

throughout concealed spaces, floors, walls, roofs, or attic spaces.

4 Lloyd Layman, Fire Fighting Tactics, page 10.

5Consumeraffairs.com, “Some SUV Owners Burning More than 

Gas,” http://www.consumeraffairs.com/news04/2006/06/

suv_arson.html

6

WRAL.com, “Investigators Find Fire Cause; Burned-Out Residents 

Find Very Little,” http://www.localtechwire.com/news/local/

story/1212119/

7

WHOtv.com “Fire Spreads to Three Buildings,” http://www.

whotv.com/Global/story.asp?s=6070271

8Greg Grisolano, “Trash Fire Spreads to Garage,” Pittsburg State 

University, http://media.www.psucollegio.com/media/storage/

paper437/news/2007/02/22/FrontPage/Trash.Fire.Spreads.

To.Garage-2740864.shtml

9 Johnny Johnson, The Daily Sentinel, “House Fire Spreads into 

Woods,” http://www.news-journal.com/news/content/region/

ETtoday/stories/2007/02/26/looneyville.html

10 Forest Service Publications “On the Line of Fire The Men and 

Women Who Battle America’s Wildland Blazes,” http://www.dnr.

state.md.us/forests/otheragencies/fire.html