Brittle splitting nails is a condition marked by thin or weak fingernails or toenails that are prone to chipping, splitting, pealing and breaking. Also known as onychoschizia, the condition can occur if nails are either dry and brittle from too little moisture, or soft and brittle from too much moisture.

Nails consist of layers of a protein called keratin. Each nail is made up of several parts, including the following:

• Nail plate. Part of the nail that is most visible.
  
  
• Nail folds. The skin that borders the nail on three sides.
  
  
• Nail bed. Skin beneath the nail plate.
  
  
• Cuticle. Tissue that overlaps the nail plate at the base of the nail.
  
  
• Lunula. White shape that resembles a half–moon at the bottom of the nail.

As new cells grow, older cells harden and are pushed up toward the end of the fingertip. Nails also are permeable, or let in liquids that come into contact with them. These liquids can cause nails to become either too moist or too dry. Either state can cause nails to become brittle. When this happens, they often split – either vertically (onychorrhexis) or horizontally (lamellar onychoschizia) – or soften.

Repeated wetting and drying of nails is the chief cause of brittle splitting nails. In addition, use of some nail polish removers (particularly those with acetone), nail hardeners and other chemicals that may come in contact with nails also may contribute to the problem. Detergents and other harsh cleaning products can damage several layers of the nail.  Low humidity and dry weather and excessive sun exposure can exacerbate the condition as well. However, contrary to common belief, nail polishes do not harm nails. In fact, they actually prevent detergents from damaging the nails while also locking moisture into the nails.  

Repetitive trauma to the fingernail, such as from typing and other activities, can also cause a person to develop brittle splitting nails.

In rare cases, internal disease or vitamin deficiencies – such as a lack of iron or zinc – may be the cause of brittle splitting nails. Examining the nails of both the fingers and the toes may offer clues as to whether or not an internal disease is responsible for brittleness. If both the fingernails and toenails are split, an internal disease, such as thyroid problems, or low levels of zinc and iron may be responsible. If the fingernails are split, but the toenails remain strong, external factors are likely contributing to the problem. Brittle nails may also result from underlying skin conditions such as lichen planus.

More than 20 percent of the United States population has brittle nails, according to the American Academy of Dermatology. Brittle splitting nails most commonly affect women. The condition is also more common in older adults. This is due to the fact that nails grow more slowly as people age. The nails of older people have also undergone more exposure to sun and other conditions that cause brittle splitting nails.