The same toxic preservative used in embalming is added to most nail polishes and mascaras to inhibit bacteria growth. Whether it's applied to wounded or broken skin (think cracked cuticles) or inhaled as a gas (wet nail polish), formaldehyde is responsible for the largest proportion — more than 25 percent — of cosmetic-instigated contact dermatitis and allergic reactions, according to a recently published study. Skin reactions can occur on eyelids, neck, and face, while fumes irritate eyes, nose, and throat.

How to avoid it: Choose nail polishes and removers that use gentler polyester resins instead of formaldehyde-releasing preservatives such as tosylamide formaldehyde, quaternium-15, diazolidinyl urea, and DMDM hydantoin.