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The bulk of wildfire destruction in California happens in the WUI. The Kincade Fire has burned more than 75,000 acres—roughly five times the size of Manhattan—in rural areas and the WUI north of Santa Rosa. Last year’s Camp Fire killed 85 people and eliminated more than 10,000 homes in Paradise, a town situated in the WUI. The year before that, the Tubbs Fire killed 22 people and destroyed more than 5,000 structures, some in Santa Rosa proper and some in the WUI around it.

Although much of the WUI is naturally vulnerable to fire, human behavior is primarily to blame for the destruction. People start more than nine in 10 fires, according to reliable estimates. Dry trees and dry brush in the WUI might act as natural kindling, but built structures—houses, cars, hospitals, utility poles, barns—act as the most potent fuel, researchers have found. A house burns a lot hotter than a bush does; a propane tank is far more combustible than a patch of grass.

If building in the WUI is so dangerous, why do it? In part because building new housing is so very difficult in many urban regions in California, due to opposition from existing homeowners and strict building codes. The number of people living on the streets in San Francisco and Los Angeles is related to the extreme cost of rent in those cities is related to the statewide housing shortage is related to the pressure to sprawl into the periphery.

So housing sprawls into the periphery. And each time major fires happen—in the WUI, as well as in unpopulated regions and urban areas—the state’s housing crisis gets a little worse. Rental prices surge. Families struggle with displacement and homelessness. Vacancy rates fall to near zero. The cost of homebuilding goes up. And resources for families without stable housing get stretched even thinner.

Read: The worst is yet to come for California’s wildfires

One solution to the state’s twin problems is to build more dense housing in urban areas: An aggressive infill-building push would lower rental prices and shift the state’s population to less fire-prone areas, as well as help reduce carbon emissions. California Governor Gavin Newsom has signed a number of measures to boost construction and hold down housing costs, and legislators are pushing many more ambitious policies. But the state is far, far away from satisfying the demand for housing, and the climate is only getting hotter.

In the meantime, California isn’t doing enough to discourage building in fire-prone areas. “I cannot recall any development project that was denied, or where the density was substantially reduced, because of known wildfire hazards,” one now-retired planning director for Sonoma County noted. Even if the state reins in future development, millions of Californians already live in the WUI, at risk of having their homes destroyed and their lives endangered by fire—and at risk of being unable to insure their homes, or of seeing their housing values fall and their economic security imperiled.

California’s housing crisis has exacerbated its wildfire crisis, and its wildfire crisis has exacerbated its housing crisis. That vicious cycle is nowhere near ending.

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