Biochar Is Not Always Chemically Inert
Biochar is often described as a natural and harmless soil amendment. While that can be true when it is produced well, the reality is more complicated. Like many materials created through high-temperature processing, biochar can contain compounds that require careful management during production, handling, and application.
One group of compounds that can appear in biochar are volatile organic compounds (VOCs). More than 80 different VOCs have been detected in some biochar samples, particularly when the material is poorly carbonized or produced under uncontrolled conditions. These include compounds such as benzene, toluene, ethylbenzene, xylenes, phenols, and volatile fatty acids - chemicals associated with incomplete combustion. Their presence highlights the importance of controlling feedstock quality, pyrolysis temperature, and post-processing conditions when producing biochar.
Production Conditions Matter: Reactor Design and Gas Treatment
Another pollutant class of concern are polycyclic aromatic hydrocarbons (PAHs), which belong to a broader group of semi-volatile organic compounds. PAHs form whenever wood, crop residues, or other biomass is burned. They are present in smoke, soot, tar, creosote, and char, and some are known or suspected carcinogens. Because biochar is produced through thermal decomposition of biomass, it can contain PAHs in small or sometimes elevated concentrations depending on how it is made.
A systematic evaluation of 73 biochars found that most products were dominated by relatively low-toxicity naphthalene, but some samples contained elevated levels of higher-ring PAHs. These higher-toxicity compounds were not primarily linked to feedstock type but to production conditions. In particular, poorly designed reactors or cooling zones that allow tar vapors to condense onto the char surface can increase PAH levels. These conditions are most common in traditional soil-mound kilns, barrel kilns, or homemade systems that lack hot afterburning of pyrolysis gases.
Field observations reinforce this concern. Soils around traditional ring kilns have been reported to accumulate significant concentrations of the sixteen commonly monitored PAHs. These compounds can bind tightly to soil particles and persist for long periods. When present at elevated levels, they may present occupational or environmental risks if dust is inhaled, ingested, or repeatedly contacted through the skin. These findings highlight why poorly controlled production and the handling of dusty biochar materials conflict with basic health and safety principles. The risks are particularly relevant when biochar is produced on farms using rudimentary kilns that lack gas filtration or high-temperature oxidation of pyrolysis vapors. If char cools in the presence of these vapors, condensation can occur within the material’s porous structure, which is why routine product testing is recommended.
Another emerging concern involves PFAS (per- and polyfluoroalkyl substances) contamination. Recent work suggests that pyrolysis must reach sufficiently high temperatures—approximately 600–650 °C - followed by high-temperature oxidation of gases to safely treat PFAS-containing materials. This requirement again emphasizes that production conditions strongly influence whether a biochar product is safe for environmental use.
Certification Standards for Safe Biochar Production
To address these concerns, a number of certification systems have been developed to guide responsible production and use of biochar. These standards are intended to protect users, soils, and surrounding ecosystems from contaminants and other hazards. The European Biochar Certificate (EBC) and the World Biochar Certificate (WBC) provide testing protocols and quality benchmarks for biochar products. For agricultural use, the recommended designation is WBC-Agro, which includes limits on contaminants and requirements intended to reduce environmental and human health risks. However, these certifications remain voluntary. Their effectiveness depends on consumer awareness and responsible production practices, which may not fully address risks where biochar is produced informally or outside regulated supply chains.
For these reasons, biochar should not be treated as a completely inert soil input. Like other industrial carbon materials, it requires thoughtful production practices, quality testing, and careful handling to ensure that it is used safely and responsibly.
