Mercury Toxicity

Mercury toxicityMercury toxicity

Dr shabeel pn

ObjectivesObjectives

Review the basic chemistry and biology of mercury in fish and humans

Review attempts to determine “safe” level of consumption of mercury (reference dose)

Present current risk advisories and discuss their impacts

Toxic PathwayToxic Pathway

ExposureAbsorptionDistributionBiotransformationElimination +/- Clinical Illness

Absorption of Toxic AgentAbsorption of Toxic Agent

Through IngestionThrough InhalationThrough SkinFrom the mother –

“transplacental”

Fetal ToxicityFetal Toxicity

Birth defects may be due to a brief exposure during critical periods of fetal development

Affected fetuses may spontaneously abortRelationship between exposure and

outcome is difficult to establish

Potential for Mercury ToxicityPotential for Mercury Toxicity

Elemental Mercury is “quicksilver”Mercury occurs naturally in soil and in the

atmosphere from volcanic emissionsMercury is extracted and used in industry,

then enters air or water from pollutionMercury was formerly used in some

medications causing direct exposure

Elemental MercuryElemental Mercury

Also referred to as “inorganic” mercury along with mercury salts

Very toxic to the nervous system, also to kidneys

But….very poorly absorbed by the GI tract so ingestion poses little risk

Inhalation route gives higher exposureMercury in fillings is inorganic

Toxicity of Organic MercuryToxicity of Organic Mercury

Mercury can be formulated as an organic compound with strong anti-microbial properties

Poisoning from diaper creams demonstrated dermal absorption and toxicity

the form of mercury with the most toxicity concerns

Organic Mercury PoisoningOrganic Mercury Poisoning

Minimata, Japan, 50 years ago…Seafood from the bay was polluted with mercury from an industrial source, many cases of neurotoxcity were seen, directly related to seafood consumption

Most striking was the vulnerability of the fetal brain to mercury toxicity shown by the high rate of cerebral palsy in children born during this period

Iraq mercury contaminated seed grain – similar toxicity

MethylmercuryMethylmercury

Methylmercury (organic) is far more toxic than other forms and is well absorbed when ingested

It can be measured in blood and hairIt is very slowly eliminated, ½ life of 2 to

3 months

MethylmercuryMethylmercurySources of ExposureSources of Exposure

Elemental mercury is biotransformed by bacteria into methyl mercury and then the bacteria are eaten by mollusks, crustaceans etc.

Poorly eliminated so it concentrates up the food chain… Biggest and oldest predators at the top of the ecosystem have the highest concentrations

Methylmercury is distributed evenly throughout the fish and is not changed by cooking

Sources of MethylmercurySources of Methylmercury

Levels are higher in long-lived predator fishSome fish may have higher levels based on

regional environmental pollutionMany health departments issue health

advisories on consumption of “sport” fish Small fish, mollusks and crustaceans have

low levels unless in polluted waters

What Fish are Low in What Fish are Low in Mercury?Mercury?

Ocean fish are less likely to have industrial contamination than lake fish

Fish that are not predators Smaller, “Pan-sized” fishSalmon – (except large, lake salmon)Data is lacking on many species

Mercury Mercury How Much is Toxic?How Much is Toxic?

Two large prospective cohort studies of high fish eating populations

Both assessed mother/infant/child mercury levels from hair and blood and performed neurological testing over time

Mercury from Fish and Neurologic Mercury from Fish and Neurologic OutcomesOutcomes

Faroe Islands, Grandjean P, et al. Cognitive deficit in 7-year-old children with prenatal exposure to methylmercury. Neurotoxicology and Teratology. 1997;19:417-428.

Adverse outcomes on some tests correlate with degree of mercury exposure


Davidson PW, Effects of prenatal and postnatal mercury exposure from fish consumption on neurodevelopment: Seychelles Child Development Study. JAMA. 1998;280:701-707

No adverse neurologic effects on offspring related to level of mercury exposure


Steurwald U, et al. Maternal seafood diet, methyl mercury exposure, and neonatal neurologic function. The Journal of Pediatrics. 2000;136:5:599-605.

Abnormalities in newborn exams correlated with high maternal mercury levels


Conclusion: No “bright line” for safety Some believe risk is overstated:

Commercial Fish: Eat Up: despite low levels of mercury. http://www.rochester.edu/pr/releases/med/mercury.htm (9/2001)

Some believe government advisories are not strong enough

http://www.rochester.edu/pr/releases/med/mercury.htm

http://www.rochester.edu/pr/releases/med/mercury.htm


National Academy of Sciences. Toxicologic effects of methylmercury. Washington, DC: National Research Council, 2000.

Reference Dose recommended to EPA is 0.1micrograms/kg/day

(“safe” daily intake to avoid toxicity)


To follow EPA reference dose:– Fish with levels of 1 part per million

or greater should not be eaten at all– Fish with levels greater than 0.2 ppm

need to be limited to about once per week

Mercury Consumer WarningsMercury Consumer Warnings

January 2001, FDA recommends pregnant women, those who may be pregnant and children less than 5, not eat swordfish, shark, king mackerel and tilefish

Pregnant women should limit intake of all other types of fish to 12oz./week and eat a variety of fish

What Do We Know About Fish What Do We Know About Fish Consumption in Hawaii?Consumption in Hawaii?

Several epidemiologic dietary studies for cardiovascular and cancer risk have established that many people in Hawaii have high fish consumption

Pilot testing of risk advisories through WIC program show many Hawaii women eat fish more than once a week, and eat varieties of fish with higher levels of mercury

What do we know about MM What do we know about MM Levels in Hawaii Fish?Levels in Hawaii Fish?

DOH has data from 1991 demonstrating expected high levels in shark and swordfish

Other sampled fish included large tuna, marlin a few other species, levels often greater than 0.2ppm

National data is scarce or absent for many Hawaii fish

Hawaii FishHawaii Fish

Swordfish in Hawaii - Broadbill Swordfish – (Shutome, Au)

Shark Other Billfish – Blue and Stripped Marlin (Au,

Kajiki, Nairagi) Tuna – Big Eye, Yellow Fin, Skipjack (ahi, aku) Other predators Ono, Moonfish, Mahimahi,

Snappers, Groupers

Importance of Mercury Importance of Mercury ToxicityToxicity

Animal evidence and studies of special populations show reason for concern regarding methylmercury

Brain injury is still being demonstrated related to Minimata exposure

The source of many developmental deficits in children are unknown

Mercury Toxicity ControversyMercury Toxicity ControversyEating fish confers health benefitsIndividuals may manage mercury risk by

eating less healthy foodsRisk advisories aimed at pregnant women

affect other family membersDietary changes initiated during

pregnancy will have little effect on outcome

What are we Doing?What are we Doing?

WIC Fish Advisory BrochureLooking to define levels of commonly eaten

fish and establish fish with low levels“Translate” conservative RD approach for

Hawaii populationPromote “smarter fish eating”, not switch to

other foods


Working with Fishing IndustryGather better data on mercury content of

fish eaten in HawaiiSampling of Hawaii fish underwayLooking at data sources for wholesale and

retail fish volumes


Promote research:– Studies of women of child-bearing age

and cord blood to determine extent of exposure

– Outcome studies Looking for funding for studies

Other Issues for HawaiiOther Issues for Hawaii

Volcano’s contribution unknown – while volcanic emissions are an important natural source of mercury, ocean levels are not reported to be rising

Many biology questions related to how MM concentrates up the “food chain” remain unanswered

ResourcesResources

Federal Drug Administration: An important message for pregnant women and women of childbearing age who may become pregnant about the risk of mercury in fish. March 2001. http://www.cfsan.fda.gov/~dms/admehg.html (9/2001)

Environmental Protection Agency: Mercury Update, Impact on Fish Advisories, June 2001. http://www.epa.gov/ost/fish/ (9/2001)

http://www.cfsan.fda.gov/~dms/admehg.html

http://www.epa.gov/ost/fish/

ResourcesResources

American Academy of Pediatrics. Technical Report: Mercury in the Environment: Implications for Pediatricians. Committee on Environmental Health. Pediatrics. 2001;108:1;197-205.

Agency for Toxic Substances and Disease Registry (ATSDR) – http://atsdr1.atsdr.cdc.gov:8080/child

http://atsdr1.atsdr.cdc.gov:8080/child

Education

Mercury Toxicity