More-intense wildfires release more arsenic

When wildfires reach sites contaminated with high levels of arsenic, the extreme heat can cause any biomass or soil to emit that arsenic to the wider environment. But just how much arsenic gets released during these wildfires remains tough to predict

In a new study, researchers led by Marek Tuhý at Charles University have determined at which temperatures certain minerals and compounds release their arsenic (Appl. Geochem. 2025, DOI: 10.1016/j.apgeochem.2025.106318). The researchers say understanding these mechanisms is becoming more important as wildfires grow in size and intensity.

To understand arsenic release, the researchers first selected arsenic-containing compounds and minerals that are commonly found in contaminated or naturally arsenic-rich soil, such as sulfides, sulfosalts, arsenic oxides, and arsenates. The selection also included a dried and pulverized sample of an arsenic-rich mushroom to represent biological samples.

Using a cylindrical furnace, the scientists then slowly heated the samples from 25 °C to around 800 °C, raising the temperature by 10 °C every minute. As the temperature climbed, they used a spectrometer to detect when the arsenic and other elements were emitted. They found that organic arsenic-bearing materials like fungi started releasing their arsenic compounds at around 275 °C. Minerals such as enargite and arsenopyrite released their arsenic at much higher temperatures (625 °C and more than 725 °C, respectively).

The results suggest that typical, low-intensity wildland fires would affect only organically bound arsenic and arsenolite, but higher-intensity wildland fires could cause arsenic release from all the types of materials studied. It’s these latter fires that pose a huge risk of releasing arsenic beyond contaminated areas, the team says.

Environmental and agricultural chemist Thomas Borch of Colorado State University, who was not involved in the study, calls the observations “intriguing.” But since no real soil was heated in field trials, he remains cautious about extrapolating these laboratory findings to actual wildfires. He says the results should nonetheless stimulate further studies to verify the results in field settings, since arsenic is toxic (to humans) at almost any concentration.

The researchers also say that future experiments in real-world settings could help researchers understand the complexity of how real wildfires and soils interact—for example, how other released gases interact with arsenic-contaminated soil.