1. Methyl mercury
Among adults and children, following ingestion, through bloodstream, methyl mercury target organs are the Central Nervous System - especially the cerebellum and the visual cortex, and the Peripheral Nervous System - targeting the spinal dorsal root ganglia. Disruptive mechanisms are related to the high affinity of methyl mercury cations to the sulfhydryl group and the ability of methyl mercury to mimic methionine, an essential amino acid. Due to this affinity of methyl mercury to sulfhydryl proteins, hair can be used as a biological indicator. Since methyl mercury cations are bound to the hair strands at the time of hair formation, it is possible to estimate methyl mercury exposure over time (1).
With a blood half-life of around 45 — 60 days after crossing the blood brain barrier, methyl mercury stays trapped within the CNS. On dose dependent bases, clinical symptoms and signs associated with methyl mercury exposure include paresthesia (loss of sensation in the extremities and around the mouth), ataxia (impairment of the gait), dysarthria (impairment of speech), hearing impediments tremor in the limbs and constriction of the visual fields. Research from the Iraq epidemic reveals that paresthesia was correlated with hair mercury levels in the range of 50 to 120 ppm, whereas ataxia, dysarthria, deafness and death were seen at hair mercury concentrations of 200, 350, 750 and 1,500 ppm, respectively (1).
During prenatal life, high maternal exposures can cause neurological disorders resembling cerebral palsy — microcephaly, hyper-reflexia, gross motor and mental impairments, blindness and deadness. In the developing brain, cell migration and cell division are affected by methyl mercury due to disruptions on cell substances synthesis, mitotic arrest, changes in synaptic formations and abnormal cellular-architecture, as well as other damages (1).
However, current evidence shows that delays on neuro and motor development during the first 6 — 8 years of life are associated with maternal exposure levels in the range of 5 — 30 ppm hair mercury concentrations. These conclusions are derived from cohort studies conducted where people rely on consumption of whales or marine fish. In the Faroe Islands, the non-observable effect levels (NOELs) were found in the range of 10 — 20 ppm; Seychelles Islands, with NOELS in the range of 20 — 30 ppm, and New Zealand (with NOELs of 7 — 8 or 20 — 25 depending on the inclusion of peak levels). These are relatively low exposures for NOELs related to the nervous system (2).
However, one must not neglect the benefits of fish consumption, especially omega 3 fatty acids, and selenium that counteracts methyl mercury toxicity (2). Fish advisories based on the evidence of each local context is required to ensure methyl mercury protection during prenatal and early postnatal life.
2. Ethyl mercury
Ethyl mercury is also organic mercury; it is used in thiomersal as a vaccine preservative. The similarities and differences between ethyl and methyl mercury have been under attention (7). Ethyl mercury has a blood half-life (3 to 7 days), much shorter than methyl mercury (45 — 60 days), and therefore it is mostly excreted from the body and does not accumulate in the central nervous system. Epidemiological studies investigating associations of neurobehavioral disorders and use of thiomersal vaccines from the UK and Denmark did not challenge the safety of vaccines in infants. Additionally, there appeared to be methodological problems and uncertainties in the US significant associations between neurodevelopment disorders and thiomersal vaccines were found (8). During a meeting in June of 2012, based on an extensive review, the Global Advisory Committee on Vaccine Safety concluded that the amount of ethyl mercury in the blood and brain from cumulative doses of vaccines does not reach toxic levels; thus, it is biologically implausible that any correlation between thiomersal in vaccines and neurological toxicity is existent (9).
3. Metallic mercury
Metallic mercury is converted into inorganic mercury in the central nervous systems. Clinical effects of metallic mercury exposure are oral disturbances (metallic taste), tremor (in fingers, eyelids, lips or tongue) and erethism — psychogenic manifestations with irritability and outburst of temper. The mad hatter persona of Lewis Carol's Alice in Wonderland is considered to be a tentative depiction of occupational exposure to metallic mercury during fur hat production.
Current exposures to metallic mercury occur in the context of gold mining, when miners burn the gold mercury amalgam. Dental personnel are exposed to mercury on the amalgam manipulation. In the health sector, mercury thermometers are easily broken, especially in the pediatrics or emergency departments of hospitals, and leaked mercury can be easily volatilized and inhaled by people nearby. Blood and urine mercury levels can be used to evaluate these exposures (10).
4. Inorganic mercury
Inorganic mercury has been found as an ingredient in skin lightening products due to its property of preventing melanin formation. Kidney damage is one major consequence of inorganic mercury. Nephrotic syndrome with high levels of protein in the urine was found among people using mercury-containing skin lightening creams. Skin rashes, discoloration and scarring may also occur (10, 11).
The United Nations Environment Programme's Global Mercury Assessment 2013: Sources, Emissions, Releases, and Environmental Transport.