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After life safety is accounted for, incident stabilization becomes the next most important fireground objective. Incident stabilization means ensuring that the fire does not go past where it already is and in many cases means protecting the exposures.
An object, structure or an area can be considered an exposure if it is close enough for the energy generated by the fire to be transferred to it. How close is close enough depends on the size of the fire; that is how much energy it can produce, the type of heat transfer, and the type, size and configuration of the fuel exposed to heat.
When we discuss exposures, we should talk of two different types ó exterior and interior. Exterior exposures are the ones we think about the most. For single-family dwellings, the exposure buildings are usually the homes on either side of the burning structure.
Sometimes, especially in tightly packed neighborhoods, the homes to the immediate front and rear of the burning structure can also be considered exposures. When dealing with town homes, duplexes and other attached structures, the exposures are actually attached to the fire building.
Interior exposures can seem pretty straightforward; typically the floor above the fire is considered an exposure. However, the issue is not always so easy. For example, in homes with balloon frame construction, a fire in a second-floor bathroom might show signs of extension in the basement, making the basement an exposure. In garden apartments, the prevailing construction features make almost every apartment an exposure, but obviously the apartments immediately adjacent to the unit on fire, including the one directly above, are the ones of most concern. For high-rises, the fire floor, the floor below and the two floors above are usually considered interior exposures.
How are exposures threatened?
The fire from a fully involved house can create a significant amount of energy. If you remember back to fire school, heat, like pressure, moves from high to low. Heat is simply a form of energy. There are three basic ways that heat is transferred from one place to the next: conduction, convection, and radiation.
Conduction is surface-to-surface contact like a piece of paper on top of a steel plate. With conduction, the heat is actually transferred molecule to molecule. If you heat a plate to a high enough temperature, the paper sitting on top of it will burn. It is easy enough to reduce the rate of heat transfer when conduction is the primary mode of transfer; you move the threatened object away from the heat source. Conduction can be a concern with interior exposures, especially given materials such as metal pipes that traverse multiple compartments in the structure.
Convection is a flow of energy, that requires a medium to move through, like air or water. Convection, being a current, is a little more difficult to overcome. You can move objects out of the path of convection currents or you can stop the current by putting the fire out. Another option is to ventilate smartly and direct the heated gases contained in that current to the outside atmosphere. Convection currents are a huge concern in large buildings with stacked units and common void shafts. It is possible for these currents to carry heat along with toxic gases from the initial fire compartment to remote areas of the building.
Finally, there is radiation. Heat transferred via radiation is electromagnetic energy in the form of a wave, moving from one space to the next. This just like the heat from the sun heating the earth; no medium is necessary for this heat transfer. Radiation is the form of heat transfer of most concern when dealing with exterior exposures.
Protecting exposures with water
The idea of exposure protection is to find a way to absorb the heat the exposure is being subjected to. Our primary method for doing this is by putting water directly onto the threatened exposure, which protects the exposure in two ways. First, when the radiated energy from the fire encounters the water, it is able to absorb some of that energy, which prevents it from being absorbed by the structure. Second, when given a porous surface, the water will soak into it, increasing both its moisture content and the amount of energy it takes to make that surface burn.
Because most of the exterior exposures we protect have vertical surfaces, it means that while some of the water we apply adheres to it, a great deal does run off. Therefore it's critical that a continuous supply of water be applied to the surface as long as it is threatened.
For interior exposures, protection comes in the form of aggressive opening of concealed spaces by the operating crews. Crews should remember that while radiation is the primary mode of heat transfer to exterior exposures, conduction and convection are the primary modes of heat transfer to interior exposures.
While water is the main option for exposure protection, there are others. The first is Class A foam. The ability of Class A foam to lower the surface tension of water allows it to more quickly penetrate porous surfaces, allowing for a greater and quicker increase in moisture content, which provides protection. However, according to Daniel Madrzykowski and David Stroup, "There is a negative consequence of using surfactants in solution form if the material to which they are applied is not porous, it may run off at a faster rate than water would." 
The other main option for exposure protection is CAFS. On porous materials, CAFS is one-and-a-half to two times more effective than Class A foam . It creates a series of bubbles that tend to be uniform in shape and size and the radiant heat energy must pass through multiple layers, with each lowering the energy of the wave. According to Madrzykowski and Stroup, "In order to increase the time to ignition of wood which will be immediately subjected to a radiant heat source, the agent should be in CAFS rather than in solution form."
In the municipal setting with a steady supply of water, the primary mode of exposure protection should be and usually is a continuous stream of water directed onto the threatened surface.
If that surface is porous and Class A foam is available the foam will enhance the effectiveness of the protection. When the exposure problem presents in more rural areas, CAFS will provide extended unmonitored protection to the exposure without tying up sizeable amounts of available water.
A quick coating of the threatened exposure with CAFS may buy crews the time they need to put a knock on the main body of fire.
One of the most important things to remember is that exposure protection is simply a stop-gap measure. You are hoping to increase the amount of time for the original fire to be extinguished without the exposure becoming involved.
There may be cases where the original fire is so large that the only real exposure protection will be to at least darken down the original fire. Good judgment must prevail.
 D. Madrzykowski & Stroup, D. Ed. (1998). Demonstration of biodegradable, environmentally safe, non-toxic fire protection liquids. NSTIR 6191.