Sierra Nevada Mercury Assessment and Education Project

Introduction

Mercury has been well known as an environmental pollutant for several decades. As early as the 1950šs it was established that emissions of mercury to the environment could have serious effects on human health, primarily damaging the brain and the rest of the nervous system. These early studies demonstrated that fish and other wildlife from various ecosystems commonly attain mercury levels of toxicological concern when directly affected by mercury-containing emissions from human-related activities. Human health concerns arise when fish and wildlife from these ecosystems are consumed by humans.

Mercury was introduced into the upper watersheds of the Northern Sierra during the gold rush days of the mid- to late 1800’s. By using mercury in the sluice systems of hydraulic and dredge mining operations, it was possible to gather fine particles of gold that otherwise could not have been recovered by gravity separation alone. However, a significant amount of mercury is lost during this process, usually running off into the soil or rivers. The Central Valley Regional Water Quality Control Board estimates that over 3 billion grams of mercury were lost into Sierra Nevada streams. For reference, a gram of mercury is about the amount used in a thermometer. (CVRWQCB, 1987)

Two studies of significance document mercury contamination in the upper watersheds of the Northern Sierra. In December, 1996 the Sacramento Sanitation District and the University of California/Davis released their "Gold Mining Impacts on Food Chain Mercury in Northwestern Sierra Nevada Streams." This report documents significant accumulation of mercury in the reservoirs, rivers and tributaries of the Yuba River watershed, the Bear River watershed, the American River watershed, the Cosumnes River watershed, and the Feather River watershed. In particular, this study strongly suggests that accumulation of mercury in rainbow trout – particularly behind dam structures and in reservoirs – has reached levels regarded as unhealthful by the California Department of Health. The Yuba River, at Englebright Reservoir, is the cause for significant concern with mercury levels of .51 to .89 ppm in rainbow trout, at or well above Department of Health safety guidelines of .50 ppm.

In 1998, Forest Service Archeologist Hank Meals documented extensive mercury contamination and potential mercury "hotspots" throughout the Yuba River Watershed in "The Historic Use of Mercury for Gold Mining in the Yuba River Watershed." This study strongly suggests that mercury hotspots throughout the watershed continue to contaminate the rivers and reservoirs of the Yuba.

Immediate Necessity of Sierra Nevada Mercury Containment Project

It is now imperative to begin the process of assessment and education regarding mercury contamination in the Northern Sierra Nevada. This process is particularly important given the significant financial resources committed to watershed restoration and water quality enhancements being directed towards Sierra Nevada watersheds. Projects being funded by local, state and federal agencies may have the adverse negative impact of compounding mercury contamination and accumulation:

Proposal to begin implementation and education regarding mercury contamination in Northern Sierra Watersheds (See Chart A and Chart B – attached)

This proposal seeks to begin the process of education and assessment regarding mercury contamination in the upper watersheds of the Northern Sierra Nevada. Specifically, this proposal will implement:

The total cost of this Phase I Implementation and Planning process is $55,000. A request of $35,000 is being made from funds available under Proposition 204 for the Yuba River Restoration Projects. Matching and in-kind efforts are committed and valued at $20,000 and pledged by Nevada County RCD, the South Yuba River Citizens League (SYRCL), and the Yuba RiverKeeper. In addition, the Bear River Watershed Group CRMP will create protocols for mercury assessment and monitoring for use as a demonstration project within the Sierra Nevada Mercury Assessment and Education Project. The Bear River CRMP has requested $20,000 from Proposition 204 in a separate proposal to implement this plan.

Background: How does mercury become a toxicological problem?

Like many environmental contaminants, mercury undergoes bioaccumulation. Bioaccumulation is the process by which organisms (including humans) can take up contaminants more rapidly than their bodies can eliminate them, thus the amount of mercury in their body accumulates over time. If for a period of time an organism does not ingest mercury, its body burden of mercury will decline. If, however, an organism continually ingests mercury, its body burden can reach toxic levels. The rate of increase or decline in body burden is specific to each organism. For humans, about half the body

burden of mercury can be eliminated in 70 days if no mercury is ingested during that time.

Biomagnification is the incremental increase in concentration of a contaminant at each level of a food chain. This phenomenon occurs because the food source for organisms higher on the food chain is progressively more concentrated in mercury and other contaminants, thus magnifying bioaccumulation rates at the top of the food chain. The bioaccumulation effect is generally compounded the longer an organism lives, so that larger predatory game fish will likely have the highest mercury levels. Adding to this problem is the fact that mercury concentrates in the muscle tissue of fish. So, unlike organic contaminants (for example PCBs and dioxins) which concentrate in the skin and fat, mercury cannot be filleted or cooked out of consumable game fish.

Background: What are the human health effects of mercury toxicity?

Humans generally uptake mercury in two ways: (1) as methylmercury (CH3Hg+) from fish consumption, or (2) by breathing vaporous mercury (Hg0) emitted from various sources such as metallic mercury, dental amalgams, and ambient air. Our bodies are much more adapted for reducing the potential toxicity effects from vaporous mercury, so health effects from this source are relatively rare. Methylmercury, on the other hand, affects the central nervous system, and in severe cases irreversibly damages areas of the brain.

The most well documented cases of severe methylmercury poisoning are from Minamata Bay, Japan in 1956 (industrial release of methyl-mercury) and in Iraq in 1971 (wheat treated with a methylmercury fungicide). In each case, hundreds of people died, and thousands were affected, many with permanent damage. In milder cases of mercury poisoning, adults complain of reductions in motor skills and dulled senses of touch, taste, and sight. These milder effects are generally reversible if exposure to mercury is halted. Unborn children are at greatest risk from low-level exposure to methylmercury. Recent research suggests that prenatal effects occur at intake levels 5-10 times lower than that of adults. If these results are confirmed, a substantial fraction of unborn children would be at risk.

Background: Mercury Cycling in the Environment

Mercury cycling pathways in aquatic environments are very complex. The various forms of mercury can be converted from one to the next; most important is the conversion to methylmercury (CH3Hg+), the most toxic form. Ultimately, mercury ends up in the sediments, fish and wildlife, or evades back to the atmosphere by volatilization. With the exception of isolated cases of known point sources, the ultimate source of mercury to most aquatic ecosystems is deposition from the atmosphere, primarily associated with rainfall. Atmospheric deposition contains the three principal forms of mercury, although the majority is as inorganic mercury (Hg2+, ionic mercury). Once in surface water, mercury enters a complex cycle in which one form can be converted to another. It can be brought to the sediments by particle settling and then later released by diffusion or resuspension. It can enter the food chain, or it can be released back to the atmosphere by volatilization. The concentration of dissolved organic carbon (DOC) and pH have a strong effect on the ultimate fate of mercury in an ecosystem. Studies have shown that for

the same species of fish taken from the same region, increasing the acidity of the water (decreasing pH) and/or the DOC content generally results in higher body burdens in fish.

Many scientists currently think that higher acidity and DOC levels enhance the mobility of mercury in the environment, thus making it more likely to enter the food chain. Many of the details of the aquatic mercury cycle are still unknown, however, and remain areas of active research.

Background: How does mercury enter the food chain?

The exact mechanism(s) by which mercury enters the food chain remain largely unknown, and probably vary among ecosystems. However, it is understood that certain bacteria play an important early role. Studies have shown that bacteria that process sulfate (SO4=) in the environment take up mercury in its inorganic form, and through metabolic processes convert it to methylmercury. The conversion of inorganic mercury to methylmercury is important for two reasons: (1) methylmercury is much more toxic than inorganic mercury, and (2) organisms require considerably longer to eliminate methylmercury. At this point, the methylmercury-containing bacteria may be consumed by the next higher level in the food chain, or the bacteria may release the methylmercury to the water where it can quickly adsorb to plankton, which are also consumed by the next level in the food chain.

Background: If human-related emissions could be eliminated or reduced, how long would it take for ecosystems to recover?

The only way to attempt to answer this question is to incorporate all the best information currently available on how mercury behaves in the environment into a computer model. Such a model was constructed as part of the research effort on northern Wisconsin lakes. Modeled scenarios predict that if emissions could be reduced by 5 percent, it would take 8 years before any change in fish concentrations would be observed, and the decrease would be small.

Additional Sources of Assistance with Mercury Contamination in Upper Sierra Watersheds

Several federal and state agencies are most prepared to assist in the development of assessments and response to mercury contamination in the upper watersheds of the Northern Sierra Nevada:

As a national agency with a mission to describe the nationšs water resources, the USGS is uniquely positioned to provide a leadership role in aquatic mercury investigations. The USGS was a prominent participant in studies conducted in northern Wisconsin, which largely form the basis of current knowledge about mercury in aquatic ecosystems. With offices in every state, staffed with scientists trained in the collection of water samples, the USGS can conduct studies on mercury contamination throughout the country.

The ongoing National Water-Quality Assessment (NAWQA) program provides additional infrastructure and expertise to gain a national perspective on mercury

contamination. A recently established mercury research laboratory in Madison, Wisconsin gives USGS scientists the necessary analytical capability to conduct state-of-the-art contamination studies.

Options for Containment

MERCURY FACTSHEET

Common Name: Mercury

CAS Number: 7439-97-6

DOT Number: UN 2809

Date: October, 1986

_________________________________________

IDENTIFICATION

Mercury is a silvery heavy liquid. It is used in thermometers, barometers, vapor lamps, mirror coating, and in making chemicals and electrical equipment.

_________________________________________

WORKPLACE EXPOSURE LIMITS

OSHA: The legal airborne permissible exposure limit (PEL) is 0.1 mg/m3, not to be exceeded at any time.

NIOSH: The recommended airborne exposure limit is 0.05 mg/m3 averaged over an 8-hour workshift.

ACGIH: The recommended airborne exposure limit for Mercury Vapor is 0.05 mg/m3, averaged over an 8-hour workshift.

* The above exposure limits are for air levels only. When skin contact also occurs, you may be overexposed, even though air levels are less than the limits listed above.

WAYS OF REDUCING EXPOSURE

* Where possible, enclose operations and use local exhaust ventilation at the site of chemical release. If local exhaust ventilation or enclosure is not used, respirators should be worn.

* Wear protective work clothing.

* Wash thoroughly immediately after exposure to Mercury and at the end of the workshift.

* Post hazard and warning information in the work area. In addition, as part of an ongoing education and training effort, communicate all information on the health and safety hazards of Mercury to potentially exposed workers.

This Fact Sheet is a summary source of information of all potential and most severe health hazards that may result from exposure.  Duration of exposure, concentration of the substance and other factors will affect your susceptibility to any of the potential effects described below.

__________________________________________

HEALTH HAZARD INFORMATION

Acute Health Effects

The following acute (short-term) health effects may occur

immediately or shortly after exposure to Mercury:

* Exposure to high levels of Mercury vapor can irritate the

lungs, causing cough, chest tightness, shortness of breath and

fever. This usually begins one to four hours after exposure

and can go on to fluid in the lungs (pulmonary edema) and

death.

Chronic Health Effects

The following chronic (long-term) health effects can occur at some

time after exposure to Mercury and can last for months or years:

Cancer Hazard

* According to the information presently available to the New

Jersey Department of Health, Mercury has been tested and has

not been shown to cause cancer in animals.

Reproductive Hazard

* There is limited evidence that Mercury may cause an increase

in spontaneous abortions in exposed women.

* Organic Mercury substances (organic substances are those which

contain carbon) have been identified as human teratogens.

While inorganic Mercury substances (those without carbon) have

not been shown to be human teratogens, they still should be

handled with caution as they may cause reproductive problems

in males and females.

Other Long-Term Effects

* Repeated low exposure or a very high single exposure can cause

Mercury poisoning. Symptoms include tremors (shaking), trouble

remembering and concentrating, gum problems, increased

salivation, loss of appetite and weight, and changes in mood

and personality. These can be severe and cause hallucinating

and psychosis.

* Repeated vapor exposures (usually more than five years) can

cause clouding of the eye lens.

* Mercury may cause a skin allergy. If allergy develops, very

low future exposures can cause itching and a skin rash.

* Exposure can cause kidney damage.

* Mercury may lower sex drive.

Medical Testing

For those with frequent or potentially high exposure (half the TLV

or greater, or significant skin contact), the following are

recommended before beginning work and at regular times after that:

* Exam of the nervous system (including handwriting test to

detect early hand tremor).

* Urine Mercury level (usually less than 0.02 mg/Liter).

* Kidney function tests.

If symptoms develop or overexposure is suspected, the following may

be useful:

* Consider chest x-ray after acute over-exposure.

* Evaluation by a qualified allergist, including careful

exposure history and special testing, may help diagnose skin

allergy.

Any evaluation should include a careful history of past and present

symptoms with an exam. Medical tests that look for damage already

done are not a substitute for controlling exposure.

Request copies of your medical testing. You have a legal right to

this information under OSHA 1910.20.

WORKPLACE CONTROLS AND PRACTICES

Unless a less toxic chemical can be substituted for a hazardous

substance, ENGINEERING CONTROLS are the most effective way of

reducing exposure. The best protection is to enclose operations

and/or provide local exhaust ventilation at the site of chemical

release. Isolating operations can also reduce exposure. Using

respirators or protective equipment is less effective than the

controls mentioned above, but is sometimes necessary.

In evaluating the controls present in your workplace, consider: (1)

how hazardous the substance is, (2) how much of the substance is

released into the workplace and (3) whether harmful skin or eye

contact could occur. Special controls should be in place for

highly toxic chemicals or when significant skin, eye, or breathing

exposures are possible.

In addition, the following controls are recommended:

* Vigorous periodic cleaning of all work surfaces.

* Where possible, automatically pump liquid Mercury from drums

or other storage containers to process containers.

* Specific engineering controls are recommended for this

chemical by NIOSH. Refer to the NIOSH criteria document:

Occupational Exposure to Mercury #73-11024.

Good WORK PRACTICES can help to reduce hazardous exposures. The

following work practices are recommended:

* Workers whose clothing has been contaminated by Mercury should

change into clean clothing promptly.

* Do not take contaminated work clothes home. Family members

could be exposed.

* Contaminated work clothes should be laundered by individuals

who have been informed of the hazards of exposure to Mercury.

* On skin contact with Mercury, immediately wash or shower to

remove the chemical. At the end of the workshift, wash any

areas of the body that may have contacted Mercury, whether or

not known skin contact has occurred.

* Do not eat, smoke, or drink where Mercury is handled,

processed, or stored, since the chemical can be swallowed.

Wash hands carefully before eating or smoking.

* If there is the possibility of skin exposure, emergency shower

facilities should be provided.

* For clean-up use a specialized charcoal-filtered vacuum or

suction pump to avoid generating Mercury vapor. Care should

be taken not to disturb spilled material.

PERSONAL PROTECTIVE EQUIPMENT

WORKPLACE CONTROLS ARE BETTER THAN PERSONAL PROTECTIVE EQUIPMENT.

However, for some jobs (such as outside work, confined space entry,

jobs done only once in a while, or jobs done while workplace

controls are being installed), personal protective equipment may be

appropriate.

The following recommendations are only guidelines and may not apply

to every situation.

Clothing

* Avoid skin contact with Mercury. Wear protective gloves and

clothing. Safety equipment suppliers/manufacturers can

provide recommendations on the most protective glove/clothing

material for your operation.

* Non absorbent materials are recommended.

* All protective clothing (suits, gloves, footwear, headgear)

should be clean, available each day, and put on before work.

Eye Protection

* Wear chemical goggles and face shield when working with liquid

mercury, unless full facepiece respiratory protection is worn.

Respiratory Protection

IMPROPER USE OF RESPIRATORS IS DANGEROUS. Such equipment should

only be used if the employer has a written program that takes into

account workplace conditions, requirements for worker training,

respirator fit testing and medical exams, as described in OSHA

1910.134.

* Where potential exists for exposure to Mercury vapor over 0.05

mg/m3 8-hour average airborne exposure, but less than 0.5

mg/m3, use an MSA half-mask facepiece with belt-mounted

"Mersorb" cartridges. These cartridges have end of service

life indicators which visually indicate when filters must be

changed. For this reason filters are belt-mounted for

visibility. This is the only commercially available air

filtering respirator approved by MSHA/ NIOSH for Mercury.

* Be sure to consider all potential exposures in your workplace.

You may need a combination of filters, prefilters, cartridges,

or canisters to protect against different forms of a chemical

(such as vapor and mist) or against a mixture of chemicals.

* Where the potential for exposures above 0.5 mg/m3 exists, use

a MSHA/NIOSH approved supplied-air respirator with a full

facepiece operated in the positive pressure mode or with a

full facepiece, hood, or helmet in the continuous flow mode.

* Exposure to 28 mg/m3 is immediately dangerous to life and

health. If the possibility of exposures above 28 mg/m3 exists

use a MSHA/NIOSH approved self-contained breathing apparatus

with a full facepiece operated in continuous flow or other

positive pressure mode.

Common Name: Mercury

DOT Number: UN 2809

DOT Emergency Guide code: 60

CAS Number: 7439-97-6

________________________________________

NJ DOH Hazard rating

FLAMMABILITY Not Found

REACTIVITY Not Found

_________________________________________

POISONOUS GAS IS PRODUCED IN FIRE

CORROSIVE

________________________________________

Hazard Rating Key: 0=minimal; 1=slight; 2=moderate; 3=serious;

4=severe

FIRE HAZARDS

* Mercury may burn, but does not readily ignite.

* Use dry chemical, CO2, water spray, or foam extinguishers.

* POISONOUS GAS IS PRODUCED IN FIRE.

* Use water to keep fire exposed containers cool.

* If employees are expected to fight fires, they must be trained

and equipped as stated in OSHA 1910.156.

SPILLS AND EMERGENCIES

If Mercury is spilled or leaked, take the following steps:

* Restrict persons not wearing protective equipment from area of

spill or leak until clean-up is complete.

* Spills should be collected with special Mercury vapor

suppressants or special vacuums. Kits specific for clean-up

of Mercury spills are available.

* It may be necessary to contain and dispose of Mercury as a

HAZARDOUS WASTE. Contact your Department of Environmental

Protection (DEP) or your regional office of the federal

Environmental Protection Agency (EPA) for specific

recommendations.

==========================================

FOR LARGE SPILLS AND FIRES immediately call your fire department.

==========================================

HANDLING AND STORAGE

* Prior to working with Mercury you should be trained on its

proper handling and storage.

* Mercury must be stored to avoid contact with CHLORINE DIOXIDE,

NITRIC ACID, NITRATES, ETHYLENE OXIDE, CHLORINE and

METHYLAZIDE since violent reactions occur.

* Store in tightly closed containers in a cool, well-ventilated

area away from ACETYLENE, AMMONIA and NICKEL.

* Mercury may initiate fires of other combustible materials.

FIRST AID

Eye Contact

* Immediately flush with large amounts of water for at least 15

minutes, occasionally lifting upper and lower lids.

Skin Contact

* Quickly remove contaminated clothing. Immediately wash

contaminated skin with large amounts of water.

Breathing

* Remove the person from exposure.

* Begin rescue breathing if breathing has stopped and CPR if

heart action has stopped.

* Transfer promptly to a medical facility.

* Medical observation is recommended for 24 to 48 hours after

breathing overexposure, as pulmonary edema may be delayed.

PHYSICAL DATA

Vapor Pressure: 0.0012 mm Hg at 68oF

Water Solubility: Insoluble

Other Names and Formulations:

Colloidal Mercury; Quick Silver.

__________________________________________

Not intended to be copied and sold for commercial purposes.

__________________________________________

NEW JERSEY DEPARTMENT OF HEALTH

Right to Know Program

CN 368, Trenton, NJ 08625-0368

__________________________________________

ECOLOGICAL INFORMATION

Elemental mercury is a heavy and relatively inert liquid which is

oxidized to inorganic mercury (II) under natural conditions.

Mercury (II) may combine with an organic fraction to from

methylmercury. Both mercury (II) and methylmercury are of

environmental concern. Mercury (II) may enter the environment in

industrial or municipal waste treatment discharges, from previously

contaminated sediments, and from the weathering of natural rocks.

Bacteria may then convert it into methylmercury. The concentration

of mercury (II) in bodies of water may be elevated with acid rain

due to the scouring of mercury from the air and increased

partitioning from the sediment into the water.

ACUTE (SHORT TERM) ECOLOGICAL EFFECTS

Acute toxic effects may include the death of animals, birds, or

fish, and death or low growth rate in plants. Acute effects are

seen two to four days after animals or plants come in contact with

a toxic chemical substance.

Mercury(II) and methylmercury has high acute toxicity to aquatic

life. Insufficient data are available to evaluate or predict the

short term effects of mercury (II) or methylmercury to plants,

birds, or land animals.

CHRONIC (LONG-TERM) ECOLOGICAL EFFECTS

Chronic toxic effects may include shortened lifespan, reproductive

problems, lower fertility, and changes in appearance or behavior.

Chronic effects can be seen long after first exposure(s) to a

toxic chemical.

Mercury (II) and methylmercury have high chronic toxicity to

aquatic life. Eating fish contaminated with mercury residues has

caused secondary poisoning in humans: birds or land animals

similarly exposed to mercury and its compounds could also be

subject to such effects. Insufficient data are available to

evaluate or predict the long-term effects of mercury and its

compounds to plants.

DISTRIBUTION AND PERSISTENCE IN THE ENVIRONMENT

Mercury is highly persistent in water, with a half-life greater

than 200 days. The half-life of a pollutant is the amount of time

it takes for one-half of the chemical to be degraded.

BIOACCUMULATION IN AQUATIC ORGANISMS

Some substances increase in concentration, or bioaccumulate, in

living organisms as they breathe contaminated air, drink

contaminated water, or eat contaminated food. These chemicals can

become concentrated in the tissues and internal organs of animals

and humans.

The concentration of mercury(II) and methylmercury found in fish

tissues is expected to be considerably higher than the average

concentration of mercury(II) or methylmercury in the water from

which the fish was taken.

SUPPORT DOCUMENT: AQUIRE Database, ERL, Duluth, U.S.EPA,

Phytotox.

SEPTEMBER, 1994

Mercury In Fish:

Cause For Concern?

Swordfish and shark taste great--especially grilled or broiled. But reports that these and some other large predatory fish may contain methyl mercury levels in excess of the Food and Drug Administration's 1 part per million (ppm) limit has dampened some fish lovers' appetites.

FDA scientists responsible for seafood safety are also concerned about the safety of the eating these types of fish, but they agree that the fish are safe, provided they are eaten infrequently (no more than once a week) as part of a balanced diet.

Mercury Is Everywhere

Mercury occurs naturally in the environment. According to FDA toxicologist Mike Bolger, Ph.D., approximately 2,700 to 6,000 tons of mercury are released annually into the atmosphere naturally by degassing from the Earth's crust and oceans. Another 2,000 to 3,000 tons are released annually into the atmosphere by human activities, primarily from burning household and industrial wastes, and especially from fossil fuels such as coal.

Mercury vapor is easily transported in the atmosphere, deposited on land and water, and then, in part, released again to the atmosphere. trace amounts of mercury are soluble in bodies of water, where bacteria can cause chemical changes that transform mercury to methyl mercury, a more toxic form.

Fish absorb methyl mercury from water as it passes over their gills and as they feed on aquatic organisms. Larger predator fish are exposed to higher levels of methyl mercury from their prey.

Methyl mercury binds tightly to the proteins in fish tissue, including muscle. Cooking does not appreciably reduce the methyl mercury content of the fish.

Nearly all fish contain trace amounts of methyl mercury, some more than others. In areas where there is industrial mercury pollution, the levels in the fish can be quite elevated. In general, however, methyl mercury levels for most fish range from less than 0.01 ppm to 0.5 ppm. It's only in a few species of fish that methyl mercury levels reach FDA limit for human consumption of 1 ppm. This most frequently occurs in some large predator fish, such as shark and swordfish. Certain species of very large tuna, typically sold as fresh steaks or sushi, can have levels over 1 ppm. (Canned tuna, composed of smaller species of tuna such as skipjack and albacore, has much lower levels of methyl mercury, averaging only about 0.17 ppm.) The average concentration of methyl mercury for

commercially important species (mostly marine in origin) is less than 0.3 ppm.

FDA works with state regulators when commercial fish, caught and sold locally, are found to contain methyl mercury levels exceeding 1 ppm. The agency also checks imported fish at ports and refuses entry if methyl mercury levels exceed the FDA limit.

Spot-caught predator fresh-water species like pike and walleye sometimes have methyl mercury levels in the 1 ppm range. Other fresh-water species also have elevated levels, particularly in areas where mercury levels in the local environment are elevated.

FDA suggests sports fishers check with state or local governments for advisories about water bodies or fish species. These advisories provide up-to-date public health information on local areas and warn of areas or species where mercury (or other contamination) is of concern.

Safety Studies

Eating commercially available fish should not be a problem, day FDA toxicologists. The 1 ppm limit FDA had set for commercial fish is considerably lower than levels of methyl mercury in fish that have caused illness.

For information about the likely outcome of eating fish with low levels of methyl mercury, scientists look to studies of persons exposed to high levels; in particular, studies of two poisoning episodes from highly contaminated fish in Japan in the 1960's, and another poisoning incident in Iraq in the 1970's involving contaminated grain.

In the first episode, which occurred in Minimata, Japan, 111 people died or became very ill (mostly from nervous system damage) from eating fish (often daily over extended periods) from waters that were severely polluted with mercury from local industrial discharge.

Following a similar incident in Nigata, Japan, where 120 person were poisoned, studies showed that the harm caused by methyl mercury poisoning, particularly the neurological symptoms, can progress over a period of years after exposure has ended. The average mercury content of fish samples from both areas ranged from 9 to 24 ppm, though in Minimata, some fish were found to have levels as high as 40 ppm. Fortunately, no similar incidents have occurred in the United States.

The best indexes of exposure to methyl mercury are concentrations in hair and blood. The average concentrations of total mercury in non-exposed people is about 8 parts per billion (ppb) in blood and 2 ppm in hair. From the Japanese studies, toxicologists have learned that the lowest mercury level in adults associated with toxic effects (paresthesia) was 200 ppb in blood and 50 ppm in hair, accumulated over months to years of eating contaminated food.

The Japanese studies did not, however, provide information on what levels of methyl mercury might adversely affect the fetus and infant.

"There is no doubt that when humans are exposed to high levels of methyl mercury, poisoning and problems in the nervous system can occur," Bolger says.

The types of symptoms reflect the degree of exposure. Paresthesia (numbness and tingling sensations around the lips, fingers and toes) usually is the first symptom. A stumbling gait and difficulty in articulating words is the next progressive symptom, along with a constriction of the visual fields, ultimately leading to tunnel vision and impaired hearing. Generalized muscle weakness, fatigue, headache, irritability, and inability to concentrate often occur. In severe cases, tremors or jerks are present. These neurological problems frequently lead to coma and death.

"During prenatal life, human are susceptible to the toxic effects of high methyl mercury exposure levels because of the sensitivity of the developing nervous system," Bolger explains. Methyl mercury easily crosses the placenta, and the mercury concentration rises to 30 percent higher in fetal red blood cells than in those of the mother.

"But none of the studies of methyl mercury poisoning victims have clearly shown the level at which newborns can tolerate exposure," Bolger says. "It is clear that at exposure levels that affect the fetus, adults are also susceptible to adverse effects. What is not clear the effect, if any, on fetuses at much lower levels--those that approach current exposure levels through normal fish consumption."

Studies of the poisoning incident in Iraq have provided limited data about what effects low levels of methyl mercury exposures to the fetus have on the infant. One possible effect, for example, is lateness in walking. In the fall and winter of 1971-72, wheat seed intended for planting--and which had therefore been treated with an alkyl mercury fungicide--was mistakenly used to prepare bread; more than 6,500 Iraqis were hospitalized with neurological symptoms and 459 died. The vast majority of the mothers experienced exposures that resulted in hair levels greater then the lowest levels associated with effects in adults. But there was no clear evidence that the fetus was more sensitive that the adult to methyl mercury.

Another study on methyl mercury toxicity was published by the World Health Organization in 1990. It concluded, "the general population does not face a significant health risk from methyl mercury." Bolger says there is a consensus among scientists on all the results of this study except for the findings related to the relationship between low exposure levels and fetal toxicity.

Searching for More Information

FDA and the National Institute of Environmental Health Sciences are supporting a study by the University of Rochester to gather conclusive data on the effects of long-term exposure to low levels of methyl mercury in the fetus and infant. The study is being conducted in the Seychelles Islands, off the coast of East Africa in the Indian Ocean.

Fish is the major source of protein for people in the Seychelles Islands,. Begun about 10 years ago, the study focuses on the approximately 700 pregnancies that occur on the islands each year.

"That's more significant database than we had in the Iraqi study," says Bolger. "Also, the population is mostly Muslim," he says, a religion that prohibits smoking and drinking, behaviors that could affect the prenatal health of fetuses (and interfere with efforts to understand the subtle effects of methyl mercury).

The study tracks women from pregnancy to childbirth, and monitors the babies' consumption of breast milk. As children grow older, they are followed for any signs of nervous system disorders. Reports from the Seychelles study are not ready for publication, but Bolger expects the results to make a significant contribution to the consideration of whether further controls or other actions may be needed.

FDA Advice for Consumers

Fish is an important source of high-quality protein, vitamins and minerals. FDA seafood specialists say that eating a variety of types of fish, the normal pattern of consumption, does not put any one in danger of mercury poisoning. It is when people eat fad diets - frequently eating only one type of food or a particular species of fish - that they put themselves at risk.

Pregnant women and women of childbearing age, who may become pregnant, however, are advised by FDA experts, to limit their consumption of shark and swordfish to no more than once a month. These fish have much higher levels of methyl mercury than other commonly consumed fish. Since the fetus may be more susceptible than the mother to the adverse effects of methyl mercury, FDA experts say that it is prudent to minimize the consumption of fish that have higher levels of methyl mercury, like shark and swordfish. This advice covers both pregnant women and women of childbearing age who might become pregnant, since the first trimester of pregnancy appears to be the critical period of exposure for the fetus. Dietary practices immediately before pregnancy would have a direct bearing on fetal exposure during the first trimester, the period of greatest concern.

FDA toxicologists have determined that for persons other than pregnant women and women of childbearing age who may become pregnant, regular consumption of fish species with methyl mercury levels around 1 part per million (ppm)--such as shark and swordfish--should be limited to about 7 ounces per week (about one serving) to stay below the acceptable daily intake for methyl mercury. For fish with levels averaging 0.5 ppm, regular consumption should be limited to about 14 ounces per week. Current evidence indicates that nursing women who follow this advice do not expose their infants to increased risk from methyl mercury.

Consumption advice is unnecessary for the top 10 seafood species, making up about 80 percent of the seafood market--canned tuna, shrimp, pollock, salmon, cod, catfish, clams, flatfish, crabs, and scallops. This is because the methyl mercury levels in these species are all less than 0.2 ppm and few people eat more than the suggested weekly limit of fish (2.2 pounds) for this level of methyl mercury contamination.

FDA's action level of 1 ppm for methyl mercury in fish was established to limit consumers' methyl mercury exposure to levels 10 times lower than the lowest levels associated with adverse effects (paresthesia) observed in the poisoning incidents. FDA based its action level on the lowest level at which adverse effects were found to occur in adults. This is because the level of exposure was actually lower than the lowest level found to affect fetuses, affording them greater protection.

FDA toxicologists are developing a more complete database for addressing low-level methyl mercury exposures from fish; however, they consider the 1 ppm limit to provide an adequate margin of safety. This doesn't mean that it is safe to regularly and frequently eat fish that contain 1 ppm methyl mercury. The limit was established taking into consideration the types of fish people eat, the levels of methyl mercury present in each species, and the amounts of fish that are normally consumed.

Not everyone agrees, however, about what advice to provide to consumers. This is particularly evident in sport fish advisories provided by states around the country. Because states often use different criteria for their fish advisories, adjoining states may provide different advice about fish from the same bodies of water. Some states have adopted a zero risk approach and have advised consumers not to eat certain species, while others have advocated a limit on intake that is more consistent with the FDA approach.