White lines in toenails and fingernails are called Leukonychia striata and can occur after trauma to the nail or as a response to systemic illnesses. Vigorous manicuring can also cause these white lines, explains WebMD.
Beau's lines are another nail abnormality. These depressions across the fingernail may occur after injury to the nail, chemotherapy or poor nutrition, according to MedlinePlus. Fungus or yeast can also change the appearance of the nail. Ridges are raised lines that develop horizontally or vertically across the nail. Arsenic poisoning may cause horizontal ridges or white lines in the nail. Other aggravating factors include biting the nails, chronically rubbing fingers or toes, using nail polish and consistent exposure to moisture.
MedlinePlus recommends contacting a doctor if a person develops widening dark streaks in nails, white lines, white color under the nails or pits. Blue nails indicate silver ingestion. People who develop splinter hemorrhages should immediately contact a doctor. Doctors may perform X-rays or blood tests to determine the cause of nail abnormalities.
People should not bite, pick or tear at their nails to prevent white lines and other nail abnormalities, notes MedlinePlus. They should also keep hangnails clipped, wear shoes that do not squeeze the toes together, cut nails straight across the top and not use nail polish. Taking biotin and applying clear nail polish with protein in it can strengthen nails and prevent abnormalities.