As intense wildfires become more frequent and extensive globally, should we pay more attention to the chemicals routinely used as retardants?

2020 seems to have been a particularly bad year for wildfires globally. Major fires that commenced in Australia in 2019 continued into the first few months of 2020. They were followed by wild (or biomass) fires in many other parts of the world, from the Amazon to the Arctic and from central Africa to California, and other states in western/northwestern US. Wildfires have a multiple of causes, of course, but climate conditions are often a major contributing factor. Perhaps unsurprisingly, an August 2020 paper in the journal Environmental Research Letters, available here, finds climate change to be important in increasing the likelihood of extreme late summer/autumn wildfire conditions across California. The authors note that a cocktail of environmental conditions (high temperatures, low rainfall, strong winds) have enabled the ignition and spread of wildfires. They use climate models to project future conditions, and find that continued climate change will further lengthen the period during which extreme fire weather occurs, and hence the most devastating wildfires (assuming there is still biomass left to burn!).

Given that major wildfires are perhaps even more likely, rather than less, in coming years – as a result of climate change but also as humans and their sources of ignition penetrate deeper into forested and peat lands, now is probably a good time to evaluate the effectiveness of our response. Here I would like to focus, in particular, on the chemicals that are used to limit the spread or reduce the intensive of wildfires ~ fire suppressant and retardant chemicals, in other words. These are the often brightly coloured substances that get air-dropped on and around wildfires – they are to fighting wildfires what chemical dispersants are to dealing with oil spills at sea. Just as we have begun to question the efficacy of chemical dispersants when dealing with a major oil spill it is probably useful to enquire about fire suppressant and retardant chemicals, and in particular whether they do more harm than good.

There are several types of fire suppressant (generally applied to a fire to reduce its intensity) and retardant (used to reduce the chances of a fire starting or spreading once started) chemicals. Fire suppressants generally reduce the temperature of the fire – water sprayed onto burning vegetation is an example, as are chemical foams. Foams are wetting agents that allow water to penetrate a surface more effectively, thereby allowing its more efficient use in suppressing a fire.

Fire retardants (e.g. Phos-Chek) are most often used in the creation of fire breaks – their application is aimed at reducing the combustibility of vegetation ahead (i.e. in front) of an advancing fire. They are therefore designed to persist in the environment rather than to degrade immediately. The most common retardants are a combination of water and either ammonium sulfate [(NH4)2SO4] or diammonium phosphate [(NH4)2HPO4] mixed with additives to thicken, reduce spoilage during storage and colour the liquid. The latter give fire retardants their bright colours – so that it is possible to see which parts of the landscape have already been treated, but also (I guess) to look good on the TV …. Retardants also include other elements to help reduce combustibility, including antimony, chlorine, bromine, boron. Chlorine and bromine are used in what are known as “halogenated retardants”.

Generally retardants are regarded as being non-toxic to humans and large mammals, at least in low concentrations. However, their toxicity to aquatic life, even at low concentrations, has long been known (e.g. Dietrich, 2013). Although retardants are meant for terrestrial use (spraying on forest and other combustible biomass) they can end up being washed into or inadvertently sprayed over aquatic ecosystems. Further, the toxicity of some of the chemicals used may be enhanced by the heat of, or through mixing with emissions from, the wildfire. At high temperatures, some halogenated retardants can form extremely toxic dioxins, for example.

And then there are the environmental effects of spraying ammonium sulfate and diammonium phosphate over large areas of vegetation and soil. Both are ammonium-based fertlizers, and therefore likely to result in environmental eutrophication and acidification. Ironically, depositing large amounts of fertilizer is likely to boost productivity in vegetation that survives the fire – resulting in increased loadings of highly-combustible litter, potentially adding to the intensity of any burn during the next fire-season ….

Given the current extent and intensity of wildfires and the likelihood that conditions that predispose biomass to burning will become more extensive, frequent and intense in coming years, chucking chemicals that only add to environmental pollution and its harmful effects at the problem seems neither desirable nor sustainable.