Formaldehyde

Typically used to preserve dead bodies, this chemical possibly has the most 'deadly' dual purpose of them all. It's a colourless, odourless gas that is generally released into the environment by the burning of fuels and waste. Please note the emphasis on the word waste!

It's also otherwise used in glues, adhesives and disinfectants! Care to roll up any of those and smoke?! Incidentally, formaldehyde is also directly responsible for any throat, eye or nose irritation you experience when you smoke.