White lines in fingernails can be caused by protein deficiency, leukonychia, and arsenic poisoning.
One of the medical terms used to refer to white, transverse lines that appear across the width of nails is "Muehrcke's lines." It is certainly one of the milder forms of human discoloration, because as soon as a person applies pressure to the nail or nails in question, the vascular nail bed aberration disappears.
Along with the pressure-disappear trick, the root cause of Muehrcke's lines is pretty simple: protein deficiency. When a person ups their intake of protein, the lines tend to quickly disappear.
Another condition leading to white nail lines, "leukonychia," is even more negligible. That's because the lines it causes are deemed by doctors to be completely "normal", part of a healthy childhood growth curve or, in the case of adults, an active daily lifestyle.
Unlike Muehrcke's, the lines in this case are less uniform and usually do not span the width of the nail. They also more typically hopscotch, affecting random nails. Random, light blunt force can often be the reason for these lines, which are again much less clean and straight that Muehrcke's or another similar condition, Mee's lines.
In the classic Cary Grant movie Arsenic and Old Lace, a pair of very eccentric women use the poison to take out tenants. Were the film to be remade again today, chances are a CSI-like character would get wise to their scheme by noticing the white lines in the fingernails of victims,
Arsenic poisoning is indeed another cause of the abnormal cutaneous striping. In large amounts, arsenic can kill; but unless someone is deliberately targeted or exposed in some toxic work environment, the closest they will come to this nail threat is the small quantities of arsenic that are all around us. Sadly, in countries like Bangladesh, millions of citizens are being exposed to the higher risk of arsenic-contaminated drinking water.