It’s one of the most obvious and dramatic signs of wildland firefighting, a bright red slurry raining down from the bellies of large planes that roar through the mountains like fighter jets. Fire retardant. 

For years, the U.S. Forest Service used the same ammonium phosphate retardant on wildfires large and small across the country. Last year it authorized a new formula, pioneered by a company that was partially based in Montana, that was supposed to be more environmentally friendly. But while the new retardant may be kinder to waterways it sometimes gets dropped into, it seems to be dangerous to the planes that carry it.

The result is grounded planes, a federal investigation, and questions around how the Forest Service determines what’s safe to use on fires. 

With me today is Joshua Murdock, outdoors and natural resources reporter at the Missoulian, who regularly covers wildfires and the Forest Service.