There are two types of white spots: “total leukonychia” (spots on the nail itself) and “apparent leukonychia” (spots that appear to be on the nail, but are actually on the skin under the nail).
Total leukonychia could be a symptom of arsenic or heavy metal poisoning, although this is quite rare. See those dangerous things you eat without knowing it. Apparent leukonychia, on the other hand, can be due to kidney, liver or heart problems. Do the test by pressing on the nail: if the white spots remain, it means that they are on the nail itself and there is generally nothing to worry about. This sometimes happens when banging your finger against a door.
If the white spots disappear or change color, then it’s coming from the skin on the nail bed and could mean a more serious health issue, says Dr. Ivy Lee, a dermatologist in Pasadena, Calif., and assistant clinical professor in dermatology at the University of California at Los Angeles (UCLA).