What does 'containment' of a fire mean, exactly?

Fall in Southern California means decorative gourds, Halloween fun and unhealthy, smoky air from a wildfire burning somewhere nearby.

Fire season may be back but it also never left, really. And it's way scarier than any slasher flick or haunted house. East Coast kids get snow days off from school; SoCal kids get fire days.

As your phone buzzes with push alerts or tweets announcing another wind-fueled fire is burning in the dried-out hills, you've likely noticed one statistic often repeated in media coverage: "the fire is __% contained."

So what does containing a fire actually mean? We asked fire officials to help explain.

The basics

Basically, containment is the work fire crews do to remove potential fuel around a fire's perimeter, Los Angeles County Fire Department Capt. A.J. Lester explained for us.

"When we're talking about containment, we're talking about separation of fuels (that are) unburned, which basically we scratch down to the dirt and we have a border around the entire fire," he said. "When we're talking about percentage of containment, we're talking about the area of which we separated those fuels... around that fire."

Firefighters working in teams — called hand crews — use tools like axes, hoe picks and chainsaws to create a brushless border, typically three to five feet wide, Lester said.

Chasing the fire

Firefighters build a fire break during the May 2013 Springs Fire near Camarillo.

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David McNew

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Getty Images

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For those big, wind-driven fires we're used to here in California, containment work doesn't actually happen on what most people would think of as the front lines, Lester explained.

"We're essentially heeling in on that fire and chasing it down... going around it, and trying to make sure that it won't jump back the other way," he said. "[Crews are] going to slowly go all the way around and in a big circle -- depending on the perimeter of the fire -- and they're going to do containment all the way around until there's 100%."

Bulldozers have made scraping out lines faster, and planes that drop Phos-Chek (that pink fire retardant) help firefighters carve out buffer zones in steep terrain they can't safely get to on foot. But while the science and technology of fighting fires has improved, Lester said the centuries-old work of hand crews remains the "tried-and-true tradition."

The image below shows a glimpse of operations during the Tick Fire, which burned in late October and early November of 2019 in the hills between Santa Clarita and Agua Dulce. The solid black lines show completed fire lines, while the red lines with ticks indicate where the fire's edge remained uncontained.

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(Courtesy National Interagency Fire Center

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Containment ≠ fire's out

It's important to note that containment does not mean a fire is out — or that the danger from it has passed. Lester noted that just because local residents might not see active flames or smoke-heavy skies, it doesn't mean firefighters aren't hard at work to keep a fire from starting up again.

The second part of the containment process is another phrase you may see in media reports: "mopping up."

That's when firefighters patrol through the burn zone to root out hot spots still smoldering — sometimes literally among roots.

"A big bush burns, it goes all the way down to the ground... All the heat from the fire goes down, and basically just makes charcoal and it's essentially like hot spots underneath the surface of the ground," Lester said. "If the wind were to pick up and change direction, it could potentially blow some of that ... up in the air and create spot fires on the outside of that containment line."

That means a containment percentage could decrease as firefighters work to re-carve a new section of line on the outside of a spot fire.

"The public doesn't see flames and they think the fire's out and they think the work is done," Lester said. "And really what's going on is there's a lot of days of hard labor that goes in with these fire crews."

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