<p>The multifaceted utility of biochar in environmental applications stems from its porous structure, ample surface area, and rich oxygen-containing functional groups. However, interactions between biochar and its surroundings can lead to the release of potentially harmful components, necessitating a comprehensive understanding of environmental impacts. This review categorizes adverse biochar effects on their detrimental components, surface attributes, structure, and size, delving on water, soil, plants, animals and atmospheric ecosystems. It also presents different methodologies for detecting environmental risks associated with biochar application, offering guidance for future toxicity assessment and avoidance strategies. Biochar created via high-temperature pyrolysis under limited oxygen can harbor various known contaminants and emerging threats (persistent free radicals and metal cyanides), posing risks like phytotoxicity, cytotoxicity and neurotoxicity. The ecotoxic potential of biochar concerning specific contaminants, comprehensive strategies to mitigate this entire spectrum of contaminants within biochar are lacking. This review comprehensively explores the formation mechanisms of these contaminants and their potential risks to ecosystems and underscores the need for effective contamination control strategies during biochar production. It emphasizes the significance of designing pyrolysis units that ensure separation of pyrolysis liquids from solids, minimizing organic contaminant condensation onto biochar. Reducing total levels of PTE holds promise through strategies such as co-pyrolysis of biomass containing both metal-rich and metal-free components, complemented by the inherent decrease in PTE levels with higher pyrolysis temperatures. With these recommended strategies, there is potential to produce biochar posing minimal environmental risks, empowering sustainable applications in diverse environmental contexts.</p> Graphical Abstract <p></p>