Before each of these chemicals was added to the list, its relevant scientific literature was thoroughly evaluated. Tobacco smoke contains both cumene and ortho-toluidine, and we know that the latter causes bladder cancer in humans. Exposure to pentachlorophenol, a wood preservative, is highly associated with an increased risk for developing non-Hodgkin lymphoma. And, 1-bromopropane is a colorless liquid often used in dry cleaning as an alternative to the environmentally hazardous perchloroethylene; it's more likely to be a risk to those actually working with the chemical. No studies have shown 1-bromopropane to cause cancer in humans, but in rodents, it was associated with the growth of tumors in the large intestine, lungs, and skin.
But, we should point out that just being exposed to a carcinogen doesn't mean you will definitely develop cancer. The American Cancer Society states that, because carcinogens may have different levels of cancer-causing potential (and because other factors in our lives may decrease our risk), "carcinogens do not cause cancer in every case, all the time." So, while you should be aware of these substances, that doesn't mean you need to immediately freak out. After all, we are pretty much surrounded by potential carcinogens, anyways.