mercury links to learning disabilities
Mercury Exposure During Pregnancy

Links to Learning Disabilities, ADD and Behavior Disorders

The following is one chapter from a 1997 Graduate Student Project conducted at the
University of South Florida.  The project involved locating published peer reviewed medical journal articles which have shown various environmental and chemical exposure factors can cause learning disabilities, hyperactivity and other disorders by damaging the delicate brain growth process in the unborn child during pregnancy.

Author: Richard W. Pressinger (M.Ed.)
Project Supervisor:
Kofi Marfo (Ph.D.) University of South Florida, Special Education Dept.
email correspondence:
Learning Disability Research Web Site:


The toxic effects of mercury on human behavior and mood have been recognized as early as the 1860’s as a clinical disorder among "felters" who made hats (hence the phrase "mad as a hatter"). Over the past few years there have been numerous re-evaluations on the so-called safety of mercury exposure. Background mercury levels among the general population are associated with seafood intake, environmental exposures at work and at home, and also from the silver fillings in a person’s mouth. Seafood concentrates mercury because of water contamination caused by trash incinerator fallout, runoff from mercury fungicides used in farming, or industry wastes such as in the manufacture of chlorine. Occupational mercury exposure can come from working in jobs such as fluorescent light manufacturing. Home exposures can occur as mercury evaporates into the air from broken thermometers, broken fluorescent lights or prior to the early 1990’s, from using household latex paints which routinely contained mercury as an antifungal agent. Individuals are also exposed to higher levels of mercury as the number of "silver" fillings increases in the mouths since silver fillings contain approximately 52% mercury.

Behavioral weaknesses have been well documented among industrial workers chronically exposed to mercury. In 1990, workers from a fluorescent lamp manufacturing plant employed for an average of 9 years were found to exhibit deficits in short-term memory and mood. Their average urinary mercury levels were 27.7 ugs/l (micrograms per liter).

A 1993 Chinese study of fluorescent lamp manufactures with an average urinary mercury level of 24 ugs/l (corresponding to air mercury levels of 0.033 mg/m3) were tested on a variety of subtle neurological tasks and compared to 70 controls recruited from an embroidery factory. Results found the fluorescent workers scored more poorly on mental arithmetic, 2-digit search, switching attention, visual choice reaction time and finger tapping (1).

One of the largest studies was conducted in Singapore (23) which evaluated 98 dentists and 54 people who were not dentists. Poorer scores in mood, finger tapping symbol-digit, digit span, logical memory and visual reproduction were found in dentists whose offices were found to contain over 16.7 ugs/m3 of mercury in the air.

The following Associated Press article summary discusses the potential concerns for the general population regarding mercury exposure:

Associated Press, November 8, 1990

"Mercury Poses Health Risk"

Mercury is spewing unregulated from the nation’s smokestacks and reaching dangerous levels in soil, plants and animals. One of the highest mercury concentrations in the country is contaminating the once-pristine Everglades, and has been blamed for the death of at least one endangered Florida panther.

Scientists and environmental groups are pointing the finger at two mercury emission sources left unregulated by the Environmental Protection Agency - coal fired energy plants and the fastest-growing threat, trash burning municipal incinerators.


"In the past, we would take our mercury wastes and bury them so they would contaminate the groundwater in just one place," said Gary Glass, an EPA mercury researcher in Duluth, Minn. "Now we've gotten smarter - we’re blowing it into the air so it can go everywhere."

Washington-based Clean Water Action, using data supplied by industry and the EPA, reported last month that 187 incinerators nationwide emit almost 75,000 pounds of mercury each year. Ninety more incinerators are planned. Massachusetts, New York, Florida and Ohio led the list, but all but four states have incinerators emitting mercury; those four are planning facilities.

"Mercury emissions have doubled in the last decade and will probably double again if all planned incinerators are brought on line," warned Henry Cole, who helped write Clean Water Action’s study.

Mercury comes from a variety of sources in trash, including batteries, paints, dyes electronics, fluorescent lights, and plastics.

Municipal garbage-burning plants spread the indestructible mercury by heating it into a vapor that can travel 200 miles, said Paul Connett, a chemistry professor at St. Lawrence University in Canton, N.Y.

Concentrations found in fish at one Minnesota lake have increased 5 percent a year since 1970, according to a Minnesota Pollution Control Agency.

As EPA’s mercury researcher points out, incinerators were initially hailed as a solution to keep mercury and other toxic wastes out of landfills, where they often leached into groundwater. But only after the fact have studies shown that the scrubber and filter technology that cleans smoke of toxic gasses fails to work on mercury. Ray Morrison, mercury project officer for the EPA, said a recent check of California incinerators showed that instead of position control devices collecting 70% of the mercury as expected, "the collection rate is dropping off to zero."

Although some companies are attempting to remove mercury from their products (such as in batteries and household paints), there are no plans to limit mercury from coal-burning energy plants, believed to be the biggest source of contamination in some parts of the country.

Current safety standards for mercury exposure are based upon clearly visible signs of neurological problems resulting from mercury exposure. Researchers Joan M. Spyker and colleagues at the Minnesota Medical School and Johns Hopkins School of Public Health reported their concerns for this outdated practice in the August 1972 journal Science,

"Obvious neurological dysfunction is the only criterion used in officially recognizing a case of methylmercury poisoning and in setting standards for the Allowable Daily Intake (ADI) of methylmercury. No consideration is given to possible behavioral, biochemical, carcinogenic, or other subtle effects such as lowered intellectual capacity and premature senility (G. Lofroth, Methylmercury (Swedish Natural Science Research Council, Stockholm ed. 2, 1970), pp. 1-17). We think subtle alterations are important and should be considered. Furthermore, attention should be directed to the fetus since the unborn organism is much more susceptible to the toxic effects of methylmercury than the adult. Methylmercury easily crosses the placental barrier and is concentrated in fetal tissues; it has a greater affinity for the embryonic central nervous system than for that of the adult."


Low Level Mercury Causes Behavior Problems During Pregnancy

Subtle behavior problems were observed in mice offspring exposed to a single low level mercury dose. Working as a team, researchers from several universities discovered that fetal mice exposed to mercury on day 7 or day 9 of pregnancy at levels of approximately one-fourth the mercury LD-50 (the amount needed to kill 50% of the test animals) resulted in problems in what is called open field testing as well as abnormalities in swimming behaviors. No unusual findings in brain weight or other physiological parameters measured were found.

In the open field testing, the animals were placed in the center of the field under a light-tight chamber for 15 seconds. The chamber is then removed and eight behavioral measures are recorded during the following 2-minute interval. In comparing the non-exposed control animals with the mercury exposed animals, researchers found the mercury exposed offspring took a significantly longer time to begin exploring their new surroundings, and when they did, a significant number took three or more backward steps initially or during the test period. According to the researchers, this backing phenomenon is "highly unusual" and only 1 out of 19 (5%) control animals exhibited this "strange behavior," whereas backing was observed in 10 out of 20 mercury treated animals (50%).

Swimming behaviors were also significantly different in the mercury exposed animals (see photograph). When placed in a small water holding tank, the 19 control offspring quickly stopped struggling and began to swim back and forth, 12 of the 20 mercury exposed offspring showed one or more signs of "neuromuscular impairment while swimming." Often observed behaviors were "freezing" in the water and floating motionless with all four legs extended (see pictures b and d). Other unusual swimming behaviors included sometimes floating in a vertical position with only the head above water (picture c). Other typical behaviors seen included excessive churning and splaying of all four legs with the tail whipping above and below water until the animal was able to maintain its body position.

In summary, the researchers stated,

"We found that in the absence of any overt sign, offspring from methylmercury-exposed mice behaved significantly different from controls when tested for subtle deviations during postnatal development. We believe there is need to examine subtle parameters when assessing the risks from methylmercury exposure. It seems likely that many people with minor symptoms or subclinical damage have gone undetected."

Drs. Joan M. Spyker, Sheldon B. Sparber and Alan M. Goldber
Departments of Anatomy, Pharm. and Psychiatry, University of Minnesotal Medical School
Department of Environmental Medicine, Johns Hopkins School of Public Health
SCIENCE August 18, 1972 pp. 621-623


Neurological Problems Among Dentists - New Concerns for Mercury

Dentists who tested as having higher levels of mercury in their blood were also found to have more subtle neurological problems including poor mental concentration, emotional lability, somatosensory irritation and lower mood scores. The study, headed by the Battelle Center for Public Health Research and Evaluation and the Department of Environmental Health, University of Washington, was designed to investigate whether sensitive measures of exposure such as symptoms, mood and behavior could be determined below the so-called "safe" levels of urinary mercury of 50 ugs/l (micrograms per liter). Only dentists were used in the study to generate similar educational, social and lifestyle backgrounds.

In 1991, 9,566 dentists attended the annual American Dental Association meeting in Seattle, WA. 1,502 dentists at the convention had their urine analyzed for mercury concentrations. 29 dentists (2%) were classified as "exposed" because their urine mercury levels were above 19 ugs/l. Nineteen of these dentists agreed to participate in the study as those exposed to mercury. There were 150 dentists (10%) with nondetectable levels of mercury in their urine, of which 20 were chosen to be behaviorally tested. A job related questionnaire found the "exposed" dentists placed over 50% more filling installations per week, had worked an average of 19 years in their present office, compared to 12 years for the "unexposed" group and averaged 2.3 mercury spills the previous year compared to only .1 for the "unexposed" dentists.

Each group was then asked to complete symptom and medical questionnaires including a mood assessment rating scale which is based on 65 mood descriptors rated on a 5 point scale ranging from none at all to extreme. The items comprise six mood scales: tension, depression, anger, fatigue, confusion, and vigor. Other areas tested were memory, attention, digit span, simple reaction time, verbal skill and a manual dexterity test which measures the amount of time it takes to grasp, move, position, and reach while transferring small pins from a large target to a small target hole. This last test has been reported to be very sensitive for measuring other neurotoxicants such as toluene and ethanol.

Results for the total mood score were very significantly linked to urine mercury levels. The strongest association with mood and mercury exposure was in the categories of tension, fatigue and confusion. Severe mercury exposure was associated with reductions in vigor and increased depression.

Although the behavioral measures were not as strongly associated with mercury exposure as the mood scores, the average time it took to "move" a pin in the one-hole test was significantly slower than dentists who had lower exposure levels to mercury.

In summarizing the results of their study, the researchers stated,

"This hypothesis generating study showed highly significant correlations between mood scores and most measures of low level Hg (mercury) exposure. The specific mood measures most associated with Hg exposure were tension, fatigue, and confusion. The strength of these associations suggests that mood should be included for investigation in future hypothesis-testing studies..... Among the cognitive and motor function tests, only the digit span and simple reaction time (nondominant hand) scores were associated with any measure of exposure...... Overall, and despite the small size of the study population, this investigation found some evidence of adverse preclinical effects at mercury doses averaging 36 ugs/l in urine. The mood and symptom results of this study agree with prior evaluations of both high and low urinary mercury doses. These preliminary survey findings support a critical evaluation of the adequacy of the 50 ug/g creatinine biologic threshold for mercury proposed by the World Health Organization This is the first U.S. dental study to detect potential behavioral deficits at such a low level of exposure. A larger and more comprehensive study is required to accurately determine a biologic threshold of adverse central and peripheral nervous system effects for elemental mercury."

Drs. Diana Echeverria, Nicholas J. Heyer, Michael D. Martin, Conrad A Naleway
Depart. of Environ. Health, Univ. of Washington, School of Dentistry, Univ. of Washington
Neurotoxicology and Teratology, Vol. 17(2):161-168, 1995


Dopamine Uptake in Brain Cells Changed By Methylmercury

Attention Deficit Disorder children were reported in other headings in this book as having altered dopamine levels. Several compounds, including alcohol, have been shown to alter dopamine levels in test animals. Now, researchers at Duke University Medical School have shown that even very low levels of methylmercury result in dopamine and norepinephrine brain neurotransmitter changes.

Litters of rat pups were exposed to methylmercury via drinking water for 21 days beginning the day after birth. Levels of mercury exposure were 1 mg/kg, 2.5 mg/kg and 5 mg/kg.

Results showed that the uptake of dopamine in the animals brains was 58% under normal when the animals were 5 days of age when exposed to the 5.0 mg/kg dose, and approximately 15% under normal at 10 days of age at the 2.5 mg/kg dose. Interestingly, by the time the animals were 32 days old and 11 days after mercury exposure terminated, the animals’ dopamine uptake levels had now jumped 50-70% above normal levels and were considered statistically significant. Similar significant changes were also seen with the neurotransmitter norepinephrine.

In summary, the researchers stated,

"These studies show that neonatal methylmercury exposure produces both acute and long-lasting effects on maturation of central catecholamine neurotransmitter systems which may be transmitter specific. In keeping with the behavioral teratology of this substance, abnormalities of neurotransmitter biochemistry can be detected at doses well below those which suppress growth of the organism (only the 5 mg/kg dose altered growth parameters, i.e. brain weight).

Dr. George Bartolome, Patricia Trepanier
Department of Pharmacology, Duke University, North Carolina
Toxicology And Applied Pharmacology 65: 92-99, 1982

Additional References

1)  Environmental Research, 60:320-327, 1993