Recently companies have begun removing three elements from their formulas due to health concerns raised from prolonged exposure to these chemicals. The resulting Big 3 Free nail polishes do not contain formaldehyde, toluene and dibutyl phthalate (DBP).
Formaldehyde acts as an adhesive polymer that hardens the nails. It is important to note that formaldehyde is found in nail hardeners not in nail polish. Formaldehyde resin, however, is often used in nail polishes to ensure bonding to the nails surface. Formaldehyde is listed in the 12th Report on Carcinogens by the US National Toxicology Program (NTP) as " known to be a human carcinogen". The main problem with formaldehyde resin is allergies which can cause skin abrasions and respiratory irritation in sensitive individuals. Some nail polish companies have removed formaldehyde resin from their formula also.
Toulene is used as a solvent keeping the nail polish as a liquid. Inhaling toluene can cause severe harm and that is why it is always recommended when using nail polish and nail polish remover that you are in a well ventilated area. According to the US National Pollutant Inventory (NPI) short-term exposure to high levels results first in "light-headedness and euphoria, followed by dizziness, sleepiness, unconsciousness, and in some cases death"
DBP is a plasticiser that makes the nail polish flexible after drying and helps reduce chipping. The use of DBP in cosmetics, including nail polish, has been banned in the European Union under Directive 76/768/EEC. DBP was added to the California Proposition 65 as a suspected teratogen (causes abnormalities in physiological development). Shortly after, most major nail polish companies began removing DBP from their nail polishes. For more information I suggest you read this article from the American Chemistry Council to find out about the studies that have been undertaken in DBP.
Michele from Alllacqueredup has a very informative post on B3F here and another post outlining which nail polishes are free of different chemicals depending on how "green" you want to be found here!
If you wish to know what exactly is in your favourite nail polishes you should check out EWG's Skin Deep Cosmetic Database. Skin Deep is a cosmetic database that has online safety profiles for cosmetics and personal care products.According to Skin Deep for each product there are two ratings available - the Hazard Score and the Data Availability Rating. The Hazard Score has a scale of 0-10 from low to high hazard, reflecting known and suspected hazards. The Data Availability Rating - none, limited, fair, good or robust - reflects how much scientists know about an ingredients safety. If you're concerned I suggest you try EWG's Skin Deep and even if you're not take a look around you, might be surprised by what you find!
Some companies have also removed camphor (a plasticiser) and formaldehyde resin (adhesive polymer) from their nail polishes.