Some science behind the scenes
Types of mercury poisoning
A person can be poisoned not just by mercury in its ‘pure’ state, but by a number of the compounds of mercury. The symptoms tend to be very similar although not all cause brain damage as we will see. Mercury is found in the environment in three basic states:
- elemental mercury or mercury vapor
- inorganic mercury
- organic mercury (ethyl-, methyl-, alkyl-, or phenylmercury).
Each form has an individual toxicological profile and metabolic process.
Quicksilver (liquid metallic mercury) is poorly absorbed by ingestion and skin contact. It is hazardous due to its potential to release mercury vapor.
It is found in thermometers, thermostats, dental amalgams, and mercury added to latex paint.
Animal data indicate that less than 0.01% of ingested mercury is absorbed through the intact gastrointestinal tract; though it may not be true for individuals suffering from ileus. Some mercury vapor is absorbed dermally but uptake by this route is only approximately 1% of that by inhalation.
In humans, approximately 80% of inhaled mercury vapor is absorbed via the respiratory tract, where it enters the circulatory system and is distributed throughout the body. Chronic exposure by inhalation, even at low concentrations in the range 0.7–42 ?g/m3, has been shown in case control studies to cause effects such as tremors, impaired cognitive skills, and sleep disturbance.
Acute inhalation of high concentrations causes a wide variety of cognitive, personality, sensory, and motor disturbances. The most prominent symptoms include tremors (initially affecting the hands and sometimes spreading to other parts of the body), emotional lability (characterized by irritability, confidence loss, and nervousness), insomnia, memory loss, neuromuscular changes (weakness, muscle atrophy, muscle twitching), headaches, polyneuropathy, and performance deficits in tests of cognitive function. And hallucinations.
Inorganic mercury (mercury salts) is found in cosmetic products, laxatives, teething powders, diuretics, and antiseptics.
It can be formed from the metabolism of elemental mercury vapor or methylmercury.
Two oxidation states of mercury form salts (Hg22+ and Hg2+), mercury salts occur in both mercury(I) (or mercurous) and mercury(II) (mercuric) forms. Mercury(II) salts are usually more toxic than their mercury(I) counterparts because their solubility in water is greater; thus, they are more readily absorbed from the gastrointestinal tract. An example of a Mercury II salt is mercury(II) chloride.
Calomel is the common name for Mercury(I) chloride a chemical compound with the formula Hg2Cl2. It is also known as mercurous chloride. [Note that Calomel is not the same as Calamine, which is a mixture of zinc oxide (ZnO) with about 0.5% ferric oxide (Fe2O3) and used to soothe and calm skin conditions].
Mercury salts affect primarily the gastro-intestinal tract and the kidneys, and can cause severe kidney damage; however, as they cannot cross the blood–brain barrier easily, mercury salts inflict little neurological damage without continuous or heavy exposure.
Chronic exposure to inorganic mercury salts primarily affects the renal cortex and may manifest as renal failure (dysuria, proteinuria, hematuria, oliguria, and uremia) or gastrointestinal problems (colitis, gingivitis, stomatitis, and excessive salivation). Irritability and occasionally acrodynia can occur.
Compounds of mercury tend to be much more toxic than the element itself, and organic compounds of mercury are often extremely toxic and have been implicated in causing both brain and liver damage.
Organic mercury exposure is also the most frequent form of mercury exposure, organic mercury is found in fish, poultry that has been fed fishmeal, pesticides, fungicides, insecticides, and thimerosal-containing vaccines.
The most dangerous mercury compound, dimethylmercury, is so toxic that even a few microliters spilled on the skin, or even a latex glove, can cause death, [as in the case of Karen Wetterhahn]. It however is usually only found in laboratories.
Methylmercury is the most common source of organic mercury poisoning. It works its way up the food chain through bioaccumulation in the environment, reaching high concentrations among populations of some species. Larger species of fish, such as tuna or swordfish, shark, king mackerel and tilefish all present a risk.
There is a long latent period between exposure to methylmercury and the appearance of symptoms in adult poisoning cases. The longest recorded latent period is five months after a single exposure; other latent periods in the range of weeks to months have also been reported. No explanation for this long latent period is known.
When the first symptom appears, typically paresthesia (a tingling or numbness in the skin), it is followed rapidly by more severe effects, sometimes ending in coma and death. The toxic damage appears to be determined by the peak value of mercury, not the length of the exposure.
Methylmercury exposure also appears to increase risk for cardiovascular disease. In a longterm prospective study, both intake of nonfatty freshwater fish and hair mercury content demonstrated a statistically significant correlation with increased risk for acute myocardial infarction. Men with the highest hair mercury had a 2.9-fold increased risk for cardiovascular death. An examination of the same cohort found a significant correlation between hair mercury and increased risk for progression of carotid atherosclerosis.
Thimerosal, is a form of organic mercury. It is very toxic by inhalation, ingestion, and in contact with skin, with a danger of cumulative effects. It is also very toxic to aquatic organisms and may cause long-term adverse effects in aquatic environments In the body, it is metabolized or degraded to ethylmercury (C2H5Hg+) and thiosalicylate. Animal experiments suggest that thiomersal rapidly dissociates to release ethylmercury after injection; that the disposition patterns of mercury are similar to those after exposure to equivalent doses of ethylmercury chloride; and that the central nervous system and the kidneys are targets, with lack of motor coordination being a common sign. Similar signs and symptoms have been observed in accidental human poisonings.
Thiomersal is known to cause allergic reactions. In a recent study of Polish children and adolescents with chronic/recurrent eczema, positive reactions to thiomersal were found in 11.7% of children (7-8 y.o.) and 37.6% of adolescents (16-17 y.o.). This difference in the sensitization rates can be explained by changing exposure patterns: The adolescents have received six thiomersal-preserved vaccines during their life course, with the last immunization taking place 2–3 years before the mentioned study, younger children received only four thiomersal-preserved vaccines, with the last one applied 5 years before the study, while further immunizations were performed with new thiomersal-free vaccines.
Ethylmercury is a breakdown product of the antibacteriological agent ethylmercurithiosalicylate, which has been used as a topical antiseptic and a vaccine preservative. Its characteristics have not been studied as extensively as those of methylmercury. It is cleared from the blood much more rapidly, with a half-life of 7 to 10 days, and it is metabolized much more quickly than methylmercury.
Other exposure sources of organic mercury include phenylmercuric acetate and phenylmercuric nitrate. These were used in indoor latex paints for their anti-mildew properties, but were removed in 1990 because of cases of toxicity.