University of York study shows that scented candles can be deadly

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4. More general rule: don't breathe smoke or exhaust from fires.

It's the stuff that doesn't burn completely that does the most harm. Almost all fires have areas where there is insufficient oxygen for complete combustion, and products of pyrolysis (heat-induced decomposition) form instead. Many different organic materials, including cellulose (paper, cotton, etc.) form some formaldehyde on pyrolysis. Lots of other nasties, such as acrolein, are formed as well, and any nitrogenous material is likely to form traces of hydrogen cyanide.