You do it to cover greys, give hair a little pick-me-up or go for a drastic new look, but do you really know what's in your hair dye? Some studies link hair dye to cancer and birth defects, and most dyes don't go through the rigorous safety testing of other cosmetic colorants before going out on the shelves. Much ado is made about using dyes during pregnancy, but experts say there have been no pregnancy problems linked to dyeing hair during pregnancy. It may be a better idea to save hair treatments until the second trimester or stick with less chemical-laden semi-permanent color, but again, the jury is out. Sure, you can try and sound out all those multisyllabic words on the back of your box, but it won't do a lot of good. The dyes of yesteryear used coal-tar dyes, which have been replaced with other petroleum-based products. Cosmetic makers have removed the ingredients that have been proven to cause cancer in animals, but experts argue they've simply replaced them with other carcinogenic products. Many salons have switched to kinder, gentler dyes that are more natural and even herbal.