Many tests for carcinogenity use the MTD, half the MTD, a quarter of the MTD and no product, to do life time tests of animals to check for carcinogenity of a product. In fact at the MTD, the animal is continuously poisoned, that can result in a lot of excess cancers, caused by the poisoning, not the carcinogenity of the product. In such cases, lower doses give much lower cancer rates and at a certain level, no excess cancers are seen anymore. Polycyclic aromatic hydrocarbons are naturally present in crude petroleum and are formed by any incomplete combustion. A few microgram per liter seawater is enough to give problems in the growth of plankton.
Several members are highly potent carcinogens.
See dioxins and PAH emissions compared . They are persistent and liable to accumulate in the food chain. These are nitrated variants of the PAH's . Until now the most potent carcinogens and mutagens found. They are mainly products of combustion.