State and federal use of fire-retardant chemicals has grown significantly in recent years. Yet recent studies have shown the toxins are deadly to fish and may contribute to permanent changes in plant communities.

Chemical fire retardants are considered a vital wildland firefighting tool, helping to slow the spread of flames while ground crews move into position. But as their use increases, the harmful side effects of these chemicals are coming under increasing scrutiny.

The chemicals, usually dropped from low-flying aircraft, largely consist of ammonia compounds, which are known toxins to fish and other aquatic life. Studies have shown retardants can kill fish, alter soil chemistry, feed harmful algae blooms and even encourage the spread of invasive plants. Yet there is little regulation of their use, and no safer alternatives on the market.

In California, state firefighting crews have applied 15.3 million gallons of chemical fire retardants so far this year, according to data provided by CalFire, the state’s wildland firefighting agency. That’s a new record, and double the amount used just three years ago.

CalFire applied 2.7 million gallons of retardant in a single one-week period starting October 9 – also a record. Of that amount, about 2 million gallons were used on the North Bay wildfires, which killed 43 people and burned more than 8,000 structures in October as they swept across several counties north of the San Francisco Bay Area, including Sonoma and Napa.

The growth of retardant use in California has outpaced federal firefighting: In 2016, the United States Forest Service applied 19 million gallons of retardant on all National Forest system lands in the nation, an increase of 55 percent compared to three years earlier.

Timothy Ingalsbee, executive director of Firefighters United for Safety, Ethics and Ecology, said chemical fire retardants are overused. They are intended only to slow a fire down so that ground crews have time to reach a fire front and build containment lines. Instead, retardants are being used in place of ground crews, he said.

In many cases where people and structures are not threatened, Ingalsbee said, fires should be allowed to burn rather than spending money on retardants and putting pilots at risk.

“It looks good on TV – ‘CNN Drops’ is a term firefighters use,” Ingalsbee said. “Eventually, the fire burns through that stuff and keeps on trucking. The worst concern is, they cause some significant impacts, particularly to water quality.”

In 2014, scientists at the National Marine Fisheries Service published a study showing that two fire-retardant formulations are deadly to Chinook salmon, even when heavily diluted in streams.

The chemicals killed salmon at concentrations well below 1 percent of the strength at which the retardants are applied. And the study found the chemicals had lasting effects: Even salmon that survived initially could be killed weeks later when they migrated to salt water, apparently because the chemicals damaged their gills.

“They are sensitive at low amounts compared to what is applied,” said Joseph Dietrich, an ecotoxicologist with NOAA’s Northwest Fisheries Science Center and lead author of the study. “It’s a very low concentration compared to what is in the stock solution that’s in a helicopter during a firefighting event.”

The retardants evaluated in the study, known by the trade names 259-F and LC-95A, are made by Phos-Chek, the U.S. subsidiary of an Israel-based company called ICL Performance Products. Both retardants are still in use by both the Forest Service and CalFire.

However, CalFire spokesperson Scott McLean said the agency this year has mainly used a different Phos-Chek formulation known as MVP-F. It is advertised as being a safer product, though it has not yet been subjected to similar studies.

CalFire takes its lead in retardant selection from the U.S. Forest Service, which produces a so-called “qualified products list” after testing the chemicals.

Most retardants are delivered as a powder, then mixed with water before being loaded onto aircraft. The water is merely a delivery mechanism.

The primary ingredient in retardants is an ammonium phosphate or sulfate solution – essentially a type of fertilizer. Other ingredients are exempt from public disclosure under federal law as trade secrets. But one is a gelling agent that allows the ammonia solution to cling to plants, temporarily insulating them from heat and flame to deprive an advancing fire of fuel.

McLean said retardants are a vital firefighting tool, and defended their use.

“Aerial application of retardant provides support to firefighters on the fire line that could not be otherwise immediately supported by a vehicle, [a] fire engine,” McLean said. “At present, there is no other long-term fire retardant that has met the requirements of the [Forest Service] qualified products list.”

Dietrich said it’s likely that MVP-F will also be deadly to salmon. The concentration known to be toxic to fish, as reported by the manufacturer, is still less than 1 percent of the concentration at which the retardant is released by aircraft. This means, again, that it can be heavily diluted in the environment and still remain deadly to fish.

The unknown proprietary ingredients in retardants may play a role in this, he said.

“They’re definitely enhancing toxicity,” Dietrich said. “Whether they are working with ammonia or just causing separate effects that are accumulating, I couldn’t say. There is some other source of toxicity that comes into play.”

Not much is known about how long retardants linger in the environment. But the compounds most often used are known as “long-term” retardants, because they are designed to cling to plants and remain effective for weeks – until they are either burned off or washed away by heavy rain. In the latter case, the chemicals will then flow into streams and lakes.

A team at the U.S. Geological Survey is engaged in a large research effort to analyze the longevity of these chemicals once applied. The results have not yet been published, and the team leader did not respond to a request for comment.

The Forest Service used fire retardants for decades without analyzing their environmental effects. Then, in 2009, Forest Service Employees for Environmental Ethics, a nonprofit group, sued the agency, blaming retardants for killing 50 endangered steelhead during a fire near Santa Barbara, California.

The lawsuit resulted in a court order requiring the Forest Service to prepare an environmental impact study, which was completed in 2011. The study concluded retardants are potentially harmful to wildlife and water quality, and set parameters on their use to minimize these effects. These included mapping sensitive exclusion zones where retardants may not be used.

Also, fire crews are now trained not to drop retardant within 300ft of water bodies if using an airplane, and within 100ft if applied from a helicopter or fire engine. Fire crews can, however, deviate from these guidelines when life or property are threatened.

Earlier this year, the Forest Service prepared a risk assessment to evaluate the ecological effects of the latest retardant chemicals. It reviewed the same products analyzed in Dietrich’s study, as well as others, and acknowledged that toxic effects to fish and plants are possible. But it minimized these concerns by reporting, in the case of fish, that long-term exposure is unlikely in flowing-water environments, and stated that plants would be worse off if they were allowed to burn.

Ingalsbee said even a 300ft buffer may not be enough to protect water and wildlife. Unpredictable fire-fed winds may spread retardant into a stream or lake from much greater distances, he said. Pilot error is also common.

In addition, retardants can alter plant communities long after a fire is extinguished.

Invasive plants are known to aggressively colonize burned areas after a fire. The ammonia in retardants is a potent fertilizer: It can become a kind of junk food for invasive plants, which can quickly crowd out native plants after a fire. As a result, in 2008 the U.S. Fish and Wildlife Service recommended limiting the use of retardants.

Officials say retardants are made from “food grade” ingredients and pose no danger to people or animals if ingested.

But in the wake of the North Bay fires, Napa County issued an advisory urging residents not to consume fruits and vegetables from backyard gardens that may have been sprayed by fire retardant. Sonoma County offered similar advice on its website. Both urged residents to wash retardants off plants and buildings “as soon as possible.” But targeting areas hit by retardant may be difficult in some cases, because a coloring agent in the compound is designed to vanish after exposure to sunlight.

Fortunately for California’s wine industry, as much as 90 percent of Napa and Sonoma county grapes had been harvested before the fires swept through. But some vineyards did get hit by retardant drops, and winery owners have said those grapes won’t be harvested.

“The bottom line is, it’s really not benign at all,” Ingalsbee said. “It does have negative effects on native flora and fauna. It does have impacts on water quality and soil.”