Freshly-produced human tears are isotonic with blood serum [the same concentration of salt], which is the equivalent of 0.9% sodium chloride or about 150 millimolar (although the electrolytes in tears are slightly different). If you hold the eye open so that there is significant evaporative loss from the surface layer of fluid over the eyeball, the concentration can rise to about the equivalent of 1.0% sodium chloride. It is thought that this rise in salt helps to stimulate the corneal nerves and trigger a blink.

Usually we measure the osmolarity of tears. Although this includes non-electrolytes too, the contribution from tear proteins is very small compared to that from the electrolytes [salts]. The normal value is about 305 mOsmoles/kg. In ocular surface disorders such as aqueous-tear-deficient "dry eye" (or keratoconjunctivitis) the value is always above 312 mOsm/kg.