A lot of the carcinogens are produced by burning the leaves rather than just existing within the leaf on its own.
You’ll get things like tar and soot which are carcinogenic if you burn other natural materials like wood or tree leaves too – it’s not unique to tobacco.
The main differences with tobacco are the frequency with which people burn it (multiple times daily whereas most people wouldn’t have a bonfire even weekly); and the way people deliberately inhale the smoke deeply to get the effects of the nicotine in their body.
Bringing the smoke into the lungs on purpose means that carcinogens which you’d usually need to be exposed to in much larger quantities on, say, your skin for them to cause cancer can act more directly on the delicate lung tissue.
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