Wildland-urban interface (W-UI) fires are a significant concern for federal, state, and local land management and fire agencies. Research using modeling, experiments, and W-UI case studies indicates that home ignitability during wildland fires depends on the characteristics of the home and its immediate surroundings. These findings have implications for hazard assessment and risk mapping, effective mitigations, and iden-tification of appropriate responsibility for reducing the potential far home lass caused by W-UI fires, By Jack D. Cohen O nce largely considered a Cali-fornia problem, residential fire losses associated with wildland fires gained national attention in 1985 when 1,400 homes were destroyed nationwide (Laughlin and Page 1987). The wildland fire threat to homes is increasing and is commonly referred to as the wildland—urban interface (W-UI) fire problem. Since 1990, W-UI fires have threatened and destroyed homes in Alaska, Arizona, California, Colorado, Florida, Michigan, New Mexico, New York, and Washington. Extensive or severe fires in Yellowstone in 1988, Oakland in 1991, and Florida in 1998 attracted much media coverage and focused national attention on wildland fire threats to people and property Federal, state, and local land man-agement and fire agencies must directly and indirectly protect homes from wildfire within and adjacent to wildlands. Davis (1990) indicated that since the mid-1940s, a major population increase has occurred in or adjacent to forests and woodland areas. Increasing residential presence near fire-prone wildlands has prompted agencies to take actions to reduce W-UI fire losses. When an apparently all-encompass-ing, seemingly unstoppable W-UI fire occurs, the rapid involvement of many homes over a wide area produces a sur-real impression; some homes survive amid the complete destruction of sur-rounding residences. After the 1993 Laguna Hills fire, some termed this seemingly inexplicable juxtaposition a “miracle.’ Miracles aside, the charac-teristics of the surviving home and its immediate surroundings greatly influ-enced its survival. Wildland fire and home ignition re-search indicates that a home’s exterior and site characteristics significantly in-fluence its ignitability and thus its chances for survival. Considering home and site characteristics when designing, building, siting, and maintaining a home can reduce W-UI fire losses. W-UI Fire Loss Characteristics W-UI residential fire losses differ from typical residential fire losses. Whereas residential fires usually involve one structure with a partial loss, W-UI fires can result in hundreds of totally destroyed homes. Particularly during severe W-UI fires, numerous Journal of Forestry 15homes can ignite in a very short time. The usual result is that a home either survives or is totally destroyed; only a few structures incur partial damage (Foote 1994). The W-UI Fire commonly originates in wildland fuels. During dry, windy conditions in areas with continuous fine fuels, a wildland fire can spread rapidly, outpacing the initial attack of firefighters. If residences arc nearby, a wildland fire can expose numerous homes to flames and lofted burning embers, or firebrands. A rapidly spreading wildland fire coupled with highly ignitable homes can cause many homes to burn simul-taneously. This multistructure involvement can overwhelm fire protection Capabilities and, in effect, result in unprotected residences. Severe W-UI fires can destroy whole neighborhoods in a few hours—much faster than the response time and suppression capabilities of even the best—equipped and staffed firefighting agencies. For example, 479 homes were destroyed during the 1990 Painted Cave fire in Santa Barbara, most of them within two hours of the initial fire report. The 1993 Laguna Hills fire in southern California ignited and burned nearly all of the 366 homes destroyed in less than five hours. Figure 1. The structure survival process Whether a home survives depends initially on whether it ignites; if ignitions with continued burning occur, survival then depends on effective fire suppression. Figure 1 shows that home survival begins with attention to the factors that influence ignition. These factors determine home ignitability and include the structure’s exterior materials and design combined with its exposure to flames and firebrands. The lower the home ignitability the lower the chance of incurring an effective ignition. Ignition: A local ProcessIgnition and spread of fire, whether on structures or in wildland vegetation, is a combustion process. Fire spreads as a continuing ignition process whether from the propagation of flames or from the spot ignitions of firebrands. Unlike a flash flood or an avalanche, in which a mass engulfs objects in its path, fire spreads because the requirements for Figure 2.The incident radiant heat flux is shown as a function of a wall’s distance from a flame 20 meters high by 50 meters wide, uniform, constant, 1,200 K, black-body. The minimum time required for a piloted wood ignition is shown given the corresponding heat flux at that distance. Journal of Forestry 16combustion are satisfied at locations along the path. The basic requirements for combustion—the fire triangle—are fuel, heat, and oxygen. An insufficiency of any one of the three components, which can occur over a relatively short distance, will prevent a specific location from burning. “Green islands” that remain after the passage of a severe, stand-replacement fire demonstrate this phenomenon. Commonly one can find a green, living tree canopy very close to a completely consumed canopy. The requirements for combustion equally apply to the W-UI fire situa-tion. In the wildland fire context, fire managers commonly refer to vegeta-tion as fuel. However, for the specific context of W-UI residential fire losses, a house becomes the fuel. Heat is sup-plied by the flames of adjacent burning materials that could include firewood piles, dead and live vegetation, and neighboring structures. Firebrands from upwind fires also supply heat when they collect on a house and adjacent flammable materials. The atmosphere amply supplies the third necessary component, oxygen. A wildland fire cannot spread to homes unless the homes and their ad-jacent surroundings meet those com-bustion requirements. The home ig-nitability determines whether these re-quirements are met, regardless of