Every summer, wildland firefighters across the West gear up for a monumental task, aiming to stop fires that are burning hotter and moving faster with climate change. They accomplish this in two ways: on the ground and out of the sky. From above, helicopters sling buckets of water, while airplanes dump fire retardant — a thick red solution made mostly of fertilizer. The United States Forest Service uses millions of gallons of retardant each year.  But there have long been concerns about what happens when that mix of ammonium phosphate, emulsifiers, and colorants finds its way into water. Some environmentalists worry spraying the stuff on forests does more harm than good.

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