That is nice biochar researchers want to investigate PAHs before committing to the massive scale production of biochar required for sequestration of carbon to offset emissions. For individual people though when dealing with brush it is probably much better to turn it into biochar with a clean burn instead of a bonfire that produces more smoke (and PAHs). So in some cases it is a pretty obvious improvement.
Coal tar asphalt sealer is about 15% PAHs, something to think about there for sure. Most likely we are all taking part in a full scale uncontrolled experiment on the impact of PAHs already!