When do PAHs occur?
I'm assuming you mean Polycyclic Aromatic Hydrocarbons.
Some PAHs are toxic, some are not. An example of a toxic one is
This compound is found in coal tar, automobile exhaust, and cigarettes, among other sources. If you inhale it, eventually, after you continually breathe in oxygen, some of the benzene rings in here eventually get epoxidized to potentially get:
If that happens, when enough water reacts with this in the presence of a catalyst, cytochrome P450, you could get cancer. This is represented when you get this happening: