Ideally, no mercury should be present in drinking water, or, for that matter, in any food. Unfortunately, some mercury may be found in some drinking water or foods. The sources of the mercury are industrial products such as batteries, their manufacturing processes and the way they have been disposed of, or natural water flowing over mercury deposits such as Cinnabar.

The most harmful form of mercury is ionic mercury, which appears as Hg²⁺, a doubly charged positive ion. A negligible amount of the ion, Hg₂²⁺ may be stipulated in some cases, but it quickly decomposes to metallic mercury and the ion Hg²⁺.

All forms of mercury should be avoided, because they damage the kidneys, liver and nervous system. However, ionic mercury is much more dangerous than metallic or water insoluble mercury compounds such as mercury-sulfur compounds. Mercury vapors are also dangerous and must also be avoided. Therefore, people who handle mercury amalgam or collect spills of liquid mercury have to exercise caution. Mercury amalgams are mixtures of mercury and other metals such as silver, and are commonly used by dentists to make fillings for teeth.

The primary standards relevant to exposure to mercury in air are:

Liquid metallic mercury evaporates slowly and its vapor concentration may exceed allowable concentrations.

Exposure may occur where small droplets of liquid mercury hide after a spill which was not collected or decontaminated properly. Mercury may hid in carpets or in nooks and crannies in dental offices, in factories which make tilt switches, in manufacturing plants which use mercury or mercury amalgams and even in homes where thermometers were broken and mercury was spilled.

Ionic mercury does not get into the air frequently but may be found in plants which manufacture batteries, electrodes, certain UV equipment as well as in certain plants that extract or purify gold and other precious metals.