Chemicals, chemicals, chemicals. One of the major offenders in most nail polishes is formaldehyde. Ever have Biology in High School and have the specimens in a jar of liquid? Yep. That chemical is in many, many things. Read your labels and make an informed decision. I only use nail polish on very special occasions.
When you have nail polish on all of the time, studies show that it is not good because the vitamins from the sun cannot get to your nails and they will not grow as fast, and will not be as good of a quality.