I believe the risk is from carbon monoxide (CO) due to incomplete combustion. Not sure what the best summary of this topic is but here's a paper that provides

I believe the risk is from carbon monoxide (CO) due to incomplete combustion. Not sure what the best summary of this topic is but here's a paper that provides an example: https://onlinelibrary.wiley.com/doi/full/10.1002/ccr3.2173 https://onlinelibrary.wiley.com/doi/full/10.1002/ccr3.2173