By Karin HeinemanInside Science - The 2017 California wildfire season was one of the most destructive on record. By the end of December, an area three times the size of Chicago had gone up in flames.

Fire retardant used to smother fires -- also called slurry -- is a mixture of water, fertilizer and chemicals. It’s dropped from planes and is usually a bright red color, so air crews can track what they’ve used. But this powerful way to help slow down fires also comes with a big price tag and some environmental concerns.

The damage to lives and land from the California wildfires this season was enormous; firefighters fought long and hard to put the flames out -- but sometimes they needed a little help.

“A big fire, a gigantic fire with flames 200-300 feet tall -- you can't even get close to it, and so in those cases the approach is to do what's called 'indirect attack,' where you anticipate where the fire is going to move and then you try to reduce the fire potential for when it gets there,” said Don Falk from the University of Arizona. “You can treat those fuels in some ways so when the fire gets there that fuel won't be so flammable. This is why we see the use of these big tankers dropping retardant. They don't drop retardant on the burning point itself -- that would just evaporate, it would be pointless. So, fire retardant, it's a kind of slurry. It's this thick mixture -- it has phosphorous and nitrogen and other retardants in it that help it to stick to vegetation and to suppress combustion when the fire gets there and so it helps to prevent those fuels from preheating,” said Falk.

“Fire retardant is expensive and there are a lot of limitations to its use. You don't really want to be standing under it and get drenched by it, [and] it's not good for aquatic organisms, and so it's used -- we try to use it as judiciously as possible,” said Falk.

The retardants used are considered nontoxic to humans and most plants and animals. But they can be toxic to fish, so drops aren’t allowed within 300 feet of water sources.

But as western wildfires start to move out of forests and into more developed areas, firefighters have less room to target their drops. And the effects of climate change aren’t helping.

“Things like high winds and high temperatures and low humidity are especially prone to help fires spread. The projections for global climate are going in exactly that direction -- higher temperatures, often lower humidity, erratic precipitation. And so, in many parts of the world what we're seeing is a very strong indication that fires are going to be burning larger and larger areas and becoming more and more frequent,” concluded Falk.

Trees, land and animals are all very resilient and can usually bounce back from wildfires given enough time. Recovery for humans, however, can take a lifetime.

Reprinted with permission from Inside Science, an editorially independent news product of the American Institute of Physics, a nonprofit organization dedicated to advancing, promoting and serving the physical sciences.