Coal burning. This releases mercury into the atmosphere.

Fish. Fish may contain varying amounts of mercury. Ocean bacteria, algae, and small fish may all contain some; mercury concentrations usually increase with the size of the fish. An excessive intake of fish foods may lead to increased body levels of mercury.

Other sources of mercury are mirrors, latex paints, fabric softeners, felt, floor waxes and polishes, sewage sludge, laxatives containing calomel, cinnabar jewelry, tatoo dyes, and many others. Most of these are not specifically mercury toxic, as they do not give off high amounts of volatile mercury. Fungicides are the most widely used and probably the most potentially toxic.

Methods of toxicity: Mercury has no known essential functions, though it was at one time used to treat syphilis, with some success. Mercury probably affects the inherent protein structure which may interfere with functions relating to protein production. Mercury has a strong affinity for sulfhydryl, amine, phosphoryl, and carboxyl groups, and inactivates a wide range of enzyme systems, as well as causing injury to cell membranes. However, none of mercury's specific body interactions are clearly defined, though the main problems seem to result from its attack on the nervous system. Mercury may also interfere with some functions of selenium, and can be an immunosuppressant.

Symptoms of toxicity: There are many processes and symptoms of mercury toxicity. Poisoning can come from four categories of mercury: metallic or elemental mercury, which is relatively mild; inorganic mercury, such as mercury chlorides, which primarily affect the kidneys; organo-mercurials, such as mercury salts in diuretics or fungicides, which convert to inorganic mercury; and short chain alkyl mercury compounds, of which methyl mercury is the most toxic, more so than ethyl or diethyl mercury.

Acute symptoms are caused mainly by mercuric chloride or methyl mercury exposure. Chronic, lower level exposure may lead to specific acute symptoms or to subtle renal and nervous system problems. Inhaled mercury has a different effect differently than ingested mercury, for which most symptoms are related to the gastrointestinal tract and the nervous system. Inhaling high levels of metallic mercury (in an industrial setting or a dentist's office) can cause acute symptoms, such as fever, chills, coughing, and chest pain. With low, long-term exposure, more subtle symptoms such as fatigue, headache, insomnia, nervousness, impaired judgment and coordination, emotional liability, and loss of sex drive, may be experienced. Ingested mercury may cause stomatitis and gastrointestinal inflammation, with nausea, vomiting, abdominal pain, and bloody diarrhea, progressing to neurological problems. These symptoms, which are often confused with psychogenic causes, are referred to as "micromercurialism."

Mild or early symptoms of mercury intoxication include fatigue, insomnia, irritability, anorexia, loss of sex drive, headache, and forgetfulness or poor memory. This may lead to other nervous system symptoms, such as dizziness, tremors, incoordination, and depression; then progress to numbness and tingling, most commonly of the hands, feet, or lips; and to further weakness, worse memory and coordination, reduced hearing and speech, paralysis, and psychosis. Mercury toxicity may be a factor in multiple sclerosis. Other problems of severe mercury intoxication are kidney and brain damage, as well as birth defects in pregnant women. Luckily, these extreme symptoms are unusual. However, the subtle and nervous system symptoms from low-level chronic exposure may be more common than we realize.