Federal and state agencies dropped 31.3 million gallons of fire retardant on U.S. wildfires last year, coating 26,600 acres of burning forest with bright red substance that is mostly water laced with a somewhat toxic chemical brew.

Just how toxic has been the subject of a federal review of the environmental impact of fire retardants dropped from the air.

Despite retardant’s fertilizer-based chemicals, the U.S. Forest Service disclosed last month that it will continue to drop the substance to slow down imminent flames and assist on-the-ground wildfire defense.

The U.S. Forest Service released an environmental assessment in October that analyzed the impacts of fire retardant on soils, air quality, aquatic environments, alpine ecosystems, cultural artifacts and public health.

After a year’s worth of general analysis on 193 million acres of land, the Forest Service declared that fire retardant poses no significant impact to natural and human environments — except for bodies of water.

In most cases, eliminating the use of retardant would likely increase the severity of a fire, which would in turn worsen the fire’s environmental impact, the study concluded.

“Because a limited number of effective firefighting tools exist, it is essential that firefighters are able to utilize every available means — including retardant — to fight wildland fires,” wrote Chief Abigail Kimbell of the U.S. Forest Service in a decision notice.

The makeup of retardant consists of mostly water, but also contains diammonium phosphate, ammonium phosphate and ammonium sulfate — chemicals that can be toxic to fish and may also affect water quality, according to the environmental report.

Guidelines in effect since 2000 prohibit the dropping of fire retardant within 300 feet of any body of water. Exceptions to the rule apply when terrain characteristics inhibit alternative means of defense, or when human life and property are at stake.

“Clearly, that rises above anything else,” said Rose Davis, spokesperson for the U.S. Forest Service. “You’ve got to consider the trade-off. And often that is a decision that is based on the experience of the people on the ground.”

The Forest Service’s decision to continue using retardant included the permanent adoption of the waterway guidelines.

Supervising Engineer Lauri Kemper of the Lahontan Regional Water Quality Board said that the Forest Service guidelines are an effective protective measure.

“We’re aware that [retardant] can cause impacts on fish,” Kemper said in a phone interview. “They’re using [retardant] in a fire. So we’ve got other issues that are more pressing at that point.”

From a water quality standpoint, Kemper said the priority lies in containing the fire.