https://www.sciencedirect.com/science/article/pii/S0160412019313273

Polyaromatic hydrocarbons (PAHs) are a group of carcinogenic chemicals that occur naturally in coal and oil, and occur from the burning of certain organic matter. As an environmental remediation scientist, I often remediate them from soils impacted by coal storage or burning of organic matter, like burn pits at farm, among other sources. Turns out that biochar contains PAHs as well, although based on this research, the contamination to the produce and the soil can be kept low and low-risk to health by keeping biochar applications within a certain limit. It concludes that "Biochar application ≥20 tonne/hectare should be avoided to minimize human cancer risks."

You can read the article to get more info, link posted above!