Some chemicals are so dangerous that even experienced chemists hate to use them. Here's a real doozy. Diazomethane is an explosive, highly toxic gas, which is a carcinogen. It is made from another toxic carcinogen and the chemical that makes Drano work. Aside from that, it's just fine.
The good news: Aromatherapy won't give you cancer. The bad news: It won't do anything else either. But it is full of chemicals, some of which are considered to be carcinogens. Especially in California.