Thanks largely to the press and some radical environmental groups most people are terrified of chemicals because they can give us cancer. Almost all of these scares are bogus, but one that isn't is a beloved chemical – alcohol. Unlike a diet soda, a glass of alcohol poses a legitimate risk.
There are thousands of chemicals, mostly dyes, used to create tattoos. Some of them are known carcinogens. Although the tattoo-cancer link is weak, people who are worried can get them removed. But the lasers that remove tattoos react with some inks and produce a different set of carcinogens—a strange but interesting problem.
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.