A simple rule of thumb was proposed in 1998 as a definition of minimum separation distance between firefighters and flames to prevent burn injury. The rule stated that the safety zone must be large enough to allow the firefighter to be at least four flame heights in distance away from the fire front. Since then, safety zone research efforts have focused on obtaining measurements of energy emitted by “real” fires. These measurements are needed to evaluate the accuracy of the theoretical safety zone model.

Unfortunately, such measurements are difficult to make in wildland fires. To date, measurements have been collected in fires burning through high-elevation sage brush in Montana; manzanita, juniper and pinyon pine in northern Arizona; tall grass prairies in Kansas; crown fires in the boreal forest of Northern Canada; and lodge pole pine forests in eastern Oregon.

The flame model

Technically speaking, wildland fires are composed of turbulent diffusion flames, which means that the temperature of the flame and the energy released by the flame is a function of the rate that oxygen in the air can mix with the combustible gases released by heating of the woody fuels. This also explains why wind is the dominant environmental factor affecting fire behavior.

Any firefighter who has worked on a fire has observed the strong influence that wind can play on fire size and intensity. The effect occurs in two ways:

1. Increased wind causes increased mixing of the air and combustible fuels, leading to faster burning and higher temperatures; and

2. Wind causes the flames to tilt forward, closer to the vegetation ahead of the fire front, leading to increased energy transfer to those fuels and thus faster heating and ignition.

If the temperature of the flame increases, then the radiant energy emitted by the flame also increases. In fact, the radiant energy is proportional to the flame temperature raised to the power of four. For example a change in flame temperature from 1,000°F, the typical temperature of the flame tip, to 1,500°F will increase the radiant energy emitted by the flame nearly four times.

The original safety zone research study assumed that the flame was essentially a flat plate of steel 66 feet wide with a constant temperature of 1,832°F. (See Figure 1, page 16.) This geometry was selected primarily because the mathematics for even this simple shape were relatively complex and presented a computer programming challenge.

However, in reality temperatures vary greatly in flames, with the highest temperatures — as high as 2,500°F — usually occurring in the lower third of the flame and the tip of the flame being roughly 1,000°F. We now use a commercial software package specifically designed to model radiant energy exchange. This new tool permits us to model the flames with varying temperatures throughout. (See Figure 2, at left.)

Wind affects firefighter safety zones in two ways: It can increase the maximum flame temperature, leading to longer and taller flames, and it tilts the flames forward, increasing the amount of radiant heat ahead of the flames. For example, if we calculate the minimum safe distance from a vertical flame front to a firefighter at its center as shown in Figure 2, the minimum safe separation distance is between 3 and 3H times the flame height. If that flame is now tilted toward the firefighter as would occur if the wind were driving the flame (see Figure 3, at left) then the minimum safe separation distance increases to between 3H and 4 times the flame height. The tilted semicircular configuration is chosen for the firefighter safety zone calculations because it represents the worst-case scenario in terms of heat impact on the firefighter.

Burn injury limits

The effect to the skin is the same regardless of whether the heating occurs by radiation from the fire, conduction from contact with a hot source, or convection from hot air or flames. The heating levels that cause burn injury aren't easily defined; burn injury severity depends on exposure time and heating magnitude. In other words, exposure to a low-level heating source like the sun for a long time would result in the same effect as exposure to a higher-energy source like a fire for a short time. The type, thickness, number of layers and fit of clothing, and even the rate at which the person wearing the clothing perspires, also are important.

The Society of Fire Protection Engineers Handbook indicates that exposure of bare skin to any type of heating greater than 0.23 Btu/ft²-s (2.5kw/m²) for a long period will result in burn injury. As a point of comparison, the maximum energy that a person could receive by exposure to the sun is less than 0.09 Btu/ft²-s (1kw/m²). Exposure of unprotected skin to heating levels greater than 4.5 Btu/ft²-s (50kw/m²) will result in severe burns in less than 15 seconds and, if the area of exposure is large enough, fatality in 40 seconds.

In the original safety zone study, 0.6 Btu/ft²-s (7kw/m²) for 90 seconds was selected as the level at which a firefighter wearing Nomex clothing would receive second-degree burn injuries. The 0.6 Btu/ft²-s limit is based on an experiment where Nomex cloth was located a half inch away from the burn sensor. If the cloth is touching the skin, then the time to burn injury drops to about 35 seconds.

The bottom line is that severe burn injury to skin covered with one layer of Nomex from radiant heating occurs when energy flux levels exceed 0.45 to 0.72 Btu/ft²-s (5 to 8 kw/m²) for a minute or two. At this time there's no clear reason to change the burn injury limit (0.6 Btu/ft²-s after exposure of 80 to 90 seconds) that is being used to define firefighter safety zone size.

While working on the Monument Fire in eastern Oregon this summer we stood about 40 feet away from flames that were 15 feet wide and 50 feet tall. We were receiving enough heat that it was very uncomfortable and even painful, forcing us and the firefighters around us to shield our faces. Calculations assuming a rectangular flame with temperatures similar to those used for the safety zone model suggest that we were receiving about 0.3 Btu/ft²-s (3 kw/m²); a rate about one half that selected as the burn injury limit for safety zones.

Table 1
Additional separation distance radius needed for crews and equipment*

Equipment (#)**		0	1	5	10
	1	18	20	27	34
20-person crews (#)***	5	41	42	46	50
	10	57	58	61	65
*Total separation distance (in feet) is 4 × flame height + added factor **250 ft2 per item ***50ft2 per firefighter

Given the increased heating for tilted flames and the uncertainty associated with estimating burn injury limits, flame heights and fire intensity we recommend that the 4 times the flame height rule be retained as the minimum separation distance model. We emphasize that this is a minimum. At this separation distance, under conditions where the flames are uniformly radiating from two or more sides of the safety zone, the firefighters probably will be subjected to heating levels that require shielding all exposed skin. They also will have to breath thick smoke and likely will experience ember showers.

The math

For purposes of calculating firefighter safety zone size, we propose the following geometric configuration where the safety zone is a circle and the radius of the circle or total separation distance is a combination of 4 times the flame height and the additional area needed for people and equipment; in other words, the person closest to the fire must be 4-times-the-flame-height away. The safety zone size (SZS) can be calculated using Equation 1.

SZS = 4F_H+[(A_FFN_FF+A_EN_E)/3]H

SZS is the total separation distance for a circular safety zone, or the radius of the circle. F_H is flame height or alternatively flame length; A_FF is area needed for each firefighter, we suggest 50ft² or the space needed to deploy a fire shelter); N_FF is the number of personnel who will be using the safety zone. A_E is the area needed for each item of heavy equipment (for example a crew cab pickup requires about 200ft², a D6 Caterpillar with blade and ripper attachments requires about 280ft², and a D8 with attachments requires about 360ft²), while N_E is the number of pieces of heavy equipment that are expected to use the safety zone. The dividing factor of three is an approximation to the numerical constant π (actual value 3.14159).

This equation is difficult to apply while working on a fire, so Table 1 on page 19 presents the solution for a range of numbers of firefighter crews and vehicles. The number obtained from the table should be added to 4 times the flame height to get total minimum separation distance or safety zone radius.

A third option is to use the following approximation to Equation 1:

The additional distance needed above the four times flame height for people and equipment (in feet) = 20 + 4 × the number of 20-person crews + the number of pieces of equipment.

This will give an approximation to the solution of Equation 1.

A fourth method is to use the 4-times-the-flame-height rule and simply estimate the area needed for people and equipment. The 4-times-flame-height rule represents a very rough approximation based only on radiant heating and should be taken as a minimum. It does not account for convective heating such as may occur under strong winds; in steep, narrow canyons; or on slopes.

Common questions

There is some evidence that the 4-times-flame-height rule does not hold true for flames less than 5 feet in height. The primary reason for this relates to the depth or thickness of the flames. Shorter flames are less efficient radiators than taller flames, thus they give off less energy. However, limited measurements in actual wildland fires indicate that as height or length of the flames increases, the flames radiate more energy per unit area.

Another factor is that the model is based on a uniform and continuous flame front oriented in a semicircle around the front of the firefighter. This is very seldom the case for short flames, which usually are less uniform and continuous and do not encircle the firefighter. For these basic reasons we have not modified the 4-times-flame-height rule for short flames.

But what about using water bodies as a safety zone? There are historical accounts of firefighters and others successfully using water as a safety zone. Two different cases can be distinguished. The first case is when firefighters are on the water, for example in a boat, during which the standard 4-times-the-flame-height minimum separation distance rule applies. Common sense dictates that all personnel on the water should have a personal flotation device.

The second case is when firefighters are in the water, swimming, floating, wading and the like. For this case, the separation distance model we have developed does not apply because the water, assuming typical stream and lake temperatures, cools the skin more effectively than air does. This suggests that the firefighter could be closer than 4 times the flame height away and not be burned from radiant heat. However, there are other factors that should be considered in this case, such as the risk of drowning and hypothermia.

Also, being closer to the flames could expose the firefighter to convective heat, which could lead to burning of the airways. In general, water should not be considered as a safety zone except as a last resort, when escape routes have been cut off and a deployment situation is imminent. Such action should include use of the fire shelter as a heat shield while in the water.

Further modeling and field measurements support the 4-times-flame-height rule of thumb for minimum safety zone size. It's important to realize that this should be considered a minimum — meaning that in all cases larger is better. It's also important to remember that the rule of thumb is based on radiant heating, and firefighters should always be cognizant of situations that may lead to convective heating.

Future work will focus on characterizing the parameters that influence convective heating. Up-to-date summaries of firefighter safety zone information can be found at www.firelab.org/fbp/reshome.htm.

Bret Butler is a research engineer with the U.S. Forest Service's Fire Behavior Research Work Unit at the Rocky Mountain Research Station's Fire Sciences Laboratory in Missoula, Mont. His research focuses on fundamental heat and combustion processes in wildland fire. Applications for his research include fire behavior models, links between fire behavior and effects, and firefighter safety. He came to the Forest Service in 1992 after receiving a Ph.D. in mechanical engineering from Brigham Young University.

Jason Forthofer is a mechanical engineer in the Fire Behavior Research Work Unit at the Rocky Mountain Research Station's Fire Sciences Laboratory in Missoula, Mont. He worked at the Missoula Technology and Development Center in 2000 and on fire crews on the Flathead National Forest for six years. His work has included heat transfer modeling using computer software and applications of computational fluid dynamics. He has contributed to the development of improved fire shelters and is currently working on safety zone research and wind modeling. Forthofer has a bachelor's degree in mechanical engineering from Montana State University.

Example calculation of safety zone size

Situation: You are a member of a crew of 20 firefighters that has just arrived at a fire burning south of Ely, Nev. You arrived the previous night. The morning briefing is scheduled for 20 minutes from now, and your crew boss asks you to provide him with some estimates of minimum safety zone sizes that will be needed for the morning and afternoon. He expects that you will be assigned to build and maintain fireline on the southeast flank of the fire. You may have one D4 dozer assigned to work with you.

Solution: Information needed is:

1. F_H, flame height or length for both the morning and afternoon.
2. N_FF, number of people that will be using the safety zone.
3. N_E, number of vehicles and/or heavy equipment that may need to use the safety zone.

Procedure: You are unfamiliar with the area and fire behavior, so you go to the fire behavior analyst, or FBAN, and ask for estimates of flame lengths given expected weather and fuels in the area you will be working. He says that flames have been 15 to 20 feet in the mornings and 20 to 25 feet in the afternoon. But today a dry cold front is expected to pass through about 14:30, resulting in higher westerly winds than previous days. The FBAN is predicting flame lengths of 28 to 35 feet during the cold front passage. The FBAN predictions correspond with observations from the previous day's burning. During initial attack an FBAN may not be available, but in most cases other firefighters who have observed fire in similar fuels and under similar conditions can provide estimates of flame height.

With this information you now can calculate the minimum safety zone size assuming a circular safety zone.

F_H, = 20 in the morning and 35 in the afternoon

N_FF, = 20 plus 2 (crew plus dozer operator and dozer boss)

N_E, = 4 (2 crew rigs, a dozer boss rig and a D4 dozer)

Using Equation 1:
Safety zone radius	= 4 × flame height + [(NFF, × 50 + NE × 200)/3]½
	= 4 × 20 + [(22 × 50 + 4 × 200)/3]½
	= 80 + [(1100 + 800)/3]½
	= 80 + [633]1/2
	= 80 + 25
	= 105 feet for the morning period and (140+25 due to the expected taller flames) or 165 feet in the afternoon
Using Table 1:
Safety zone radius	= 4 × flame height + (# from table for 1 crew and 4 pieces of equipment)
	= 4 × 20 + 24
	= 80 + 24
	= 104
	= 104 feet for the morning period and (140+24 due to the expected taller flames) or 164 feet in the afternoon
Using the simplified equation:
Safety zone radius	= 4 × flame height + 20 + 4 × (# crews) + (# of pieces of equipment)
	= 4 × 20 +20 + 4 × 1 + 4
	= 80 + 20 + 4 + 4
	= 108 feet for the morning and 168 feet (140 + 28) during the afternoon

You can now tell your crew boss that the safety zones need to be big enough to allow the firefighters to be more than 100 feet from the flames in the morning and more than 160 feet from the flames in the afternoon. For a circular safety zone these distances would be the circle's radius.