Carcinogens, how do they work?
There are at least two different radiationless pathways to wreak havoc on the DNA:


intercalaction: compounds like benzo[a]yrene or ethidium molecules slide between DNA strands and cause transcription errors
alkylation: compounds like N-Nitroso-N-methylurea may transfer alkyl groups to nucleobases


But the list is by no means complete.