3-nitrobenzanthrone is (or was when last I checked) the most mutagenic chemical ever measured. The test used for such things is called the Ames Test, which measures the mutagenic properties of compounds in salmonella typhimurium. 3-nitrobenzanthrone racks up a score of more than six million mutations per nanomole. The compound is formed when the exhaust from heavily loaded diesel engines reacts with airborne nitrogen oxides.