How Do We Contain Wildfires? The Fascinating (and Frightening) Facts

How many days can it take to control a large wildfire? (More than you think.) Who fights wildfires? (You may be surprised!) What tools and methods do firefighters use to control, extinguish, and prevent them?

Sabrina Stierwalt, PhD
5-minute read
Episode #387
The Quick And Dirty
  • Firefighters contain wildfires by setting a control line, which eliminates vegetation available to the fire to use as fuel and spread.
  • Fighting a single fire can require more than 1,000 people, more than 2.5 million gallons of flame retardant, and as long as 45 days.
  • Human carelessness causes 85% of wildfires, so the best form of fire prevention is for us to take better care of our surroundings.

Like many other residents of Los Angeles, I woke up this morning to the smell of campfire in the air. The sun, normally bold and bright and unforgiving, was barely visible, like the beam of a small flashlight trying to make its way through the thick, smoky clouds. When I stepped outside to walk the dog, tiny white flakes of ash rained down on my sleeve. 

As I write this, the El Dorado Fire has burned more than 14,000 acres in San Bernardino and Riverside Counties in Southern California and is 31% contained. Not too far away, the Bobcat Fire in the Angeles National Forest has spread over 32,000 acres and is only at 6% containment. Right now, 500,000 people are under evacuation orders in Oregon. 

When discussing the fires in a recent class, one of my students mentioned that the U.S. Forest Service doesn’t expect the Bobcat Fire to be contained for another 45 days (which means mid-October). Forty-five days of firefighting is mindboggling to me, so I had to investigate. We’ve discussed the causes of wildfires in previous episodes. But once a wildfire happens, how do we contain it?

How do you contain a wildfire?

The main tool in a firefighter’s arsenal is a control line. Also called firebreaks, these are natural barriers that stop the fire from spreading. Sometimes these control lines occur naturally, thanks to a break in the vegetation like a river or a rock outcrop. In fact, part of the containment of the Bobcat Fire came from the fire running up against the previously burned site from a widespread fire the year before. 

Part of the containment of the Bobcat Fire came from the fire running up against the previously burned site from a widespread fire the year before.

Firefighters may also create control lines by digging ditches and using yet more fire and even explosives to break up existing brush. If the creeping fire hits a spot that has already been burned, the fire has no fuel to continue and thus has nowhere to go. In some cases, firefighters may lay down a flame retardant along the control line— that's known as a “wet line.” 

Firefighters usually report the percentage of containment, as in “containment is at 70%.” A wildfire that is 70% contained has complete control lines around 70% of its perimeter. A fire is officially contained if it is fully enclosed by those control lines, and that includes any related fire spots. So the fire may still be burning within the boundaries of the perimeter set by the control line, but opportunities for the fire to spread have been cut off. 

Even fire beaters, a simple rubber flap on the end of a pole, may be used by hand to beat back a fire.

In direct attacks on the ground, bulldozers may be brought in to clear vegetation in order to form a control line. Machinery can also be used to help dig ditches parallel to the flames to force the fire to burn in a certain direction. Even fire beaters, a simple rubber flap on the end of a pole, may be used by hand to beat back a fire. 

When a fire grows so large that it’s not safe for people to be on the ground close to it, we fight from the air. Aircraft drop water and flame retardant across the fire. In recent years, CalFire has reported dropping more than 15 million gallons of flame retardant each year, with sometimes more than 2.5 million gallons working to douse a single fire. 

What's the difference between a contained wildfire and a controlled one?

When you hear that a fire has been contained, you may wonder whether it's been put out altogether. Let's talk about the differences between contained and controlled fires.

Contained wildfires

Once a control line has been set, and a fire is considered 100% contained, it's usually still burning. Although unlikely, it can still jump the control line and reignite. For example, in dry conditions, even a slight wind could pick up a still-smoldering ember and carry it to fresh vegetation to refuel the flames. So contained fires are still monitored very closely at their perimeter, and the fight continues against hot spots—more active and dangerous spots toward the interior that could later reignite.

Controlled wildfires

A fire is considered controlled when it has been contained and it no longer burns along the interior of the control line and any potential hot spots have been snuffed out. 

Who are the firefighters?

540 people have been assigned, according to the Incident Information System, to fight the Bobcat Fire. Over 1,200 are battling the nearby El Dorado Fire. In the United States, those fighting wildfires come from federal agencies, including the National Park Service, the U.S. Forest Service, the Bureau of Land Management, the U.S. Fish and Wildlife Service, the Bureau of Indian Affairs, and from more local organizations at the state level like the Department of Forestry. According to the National Wildfire Coordinating Group, firefighters must meet rigorous fitness standards as well as specific educational requirements set by each agency. Typical areas of study could include agriculture, plant science, natural resources, or civil engineering.

In California, a large fraction of those sent to extinguish fires and save communities from burning are prison inmates.

But a large fraction, at least here in California, of those sent to extinguish fires and save our communities from burning are prison inmates. They are usually set with the laborious tasks of clearing brush by hand with saws and axes in support of creating the ever-important control line. In fact, in one of the more devastating headlines I’ve read recently, NPR reported that the coronavirus pandemic has sidelined more than 1,000 prison inmates who normally help fight wildfires in the States. We owe a tremendous debt of gratitude to these inmates for their efforts.

How do we prevent wildfires?

To keep wildfires in check, firefighters literally fight fire with fire by setting controlled burns before a fire even starts. By choosing strategic locations for the vegetation to be burned out, firefighters set limits on how large a future fire could grow. On smaller scales, residents in at-risk cities and towns are often required to pare back any overgrowth on their properties. 

Firefighters literally fight fire with fire by setting controlled burns before a fire even starts.

Obviously, we don’t want to fell all of our forests, and luckily we don’t need to. 

A big cause of wildfires—as in 85% of wildfires according to the National Park Service—is human carelessness. So far, the city of Monrovia cites a precise time for the start of the Bobcat Fire (12:21 on September 6, 2020) but the cause of the fire is still “under investigation.” The El Dorado Fire, on the other hand, is known to have started by a pyrotechnic set off at the edge of the El Dorado Ranch Park in Yucaipa at a party meant to reveal whether a pregnant woman would be having a boy or a girl.  

Although commonly called gender reveal parties, these parties actually reveal the sex of the baby, not its gender. Sex and gender are not the same.

If that sounds familiar, a large wildfire burned more than 47,000 acres and caused more than $8 million in damage when someone shot a gun at an exploding target at a gender reveal party two years ago in 2018. 

So clearly, a major wildfire prevention mechanism is for humans to take better care of their environment.

Oregonians and Californians aren’t the only ones facing massive wildfires. Thanks to climate change, they are becoming more common across the globe and have raged in Greece, Brazil, Australia, and Indonesia, just for starters. You can check the air quality near you (in the United States) at AirNow and CalFire tracks the status of fire spread and evacuation zone coverage in California. And to all my fellow West Coasters, stay safe!

Please note that archive episodes of this podcast may include references to Ask Science. Rights of Albert Einstein are used with permission of The Hebrew University of Jerusalem. Represented exclusively by Greenlight.

About the Author

Sabrina Stierwalt, PhD

Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.