being recognized unless specific examinations are performed by pathologists. 

 Excessive exposures to other toxic substances that can damage brain tissue 

 can produce interactive effects and potentially reduce the ability of the 

 human body to tolerate subclinical exposures to methylmercury. 



Also of interest in Minimata was the observation by public health officials 

 that a number of teenage children in the village who were born around the 

 time of the original episode are now experiencing difficulty in coordination 

 when they attempt to play baseball and basketball. Others have visual im- 

 pairment and more obvious signs. It would appear that subclinical brain 

 damage had occurred earlier in their lives, probably before they were born, 

 but signs of brain damage were delayed and are now beginning to be seen. 



In Niigata, the lowest blood methylmercury level associated with toxic 

 symptoms was 0.2 ppm. This level has been exceeded by certain Swedish 

 fishermen eating freshwater pike from streams contaminated by mercury 

 effluents from pulp-paper operations. So far, they have not shown any ob- 

 vious symptoms but further investigation is indicated and we hope autopsies 

 are obtainable in the future. 



However, the blood level at 0.2 ppm mercury has been made the reference 

 point that both Swedish and American health authorities use as the 

 threshold of toxicity. Using the biologic half-life data to perform steady-state 

 calculations, it has been determined that a daily intake not to exceed 0.3 mg 

 would permit a 70-kg (150-pound) individual to remain at or below a blood 



level of 0.2 ppm. Both the Swedes and the Americans determined that a 10- 

 fold safety factor was necessary and appropriate to protect individuals with 

 unusual susceptibilities and infants from subclinic brain damage. 



Accordingly, to maintain blood methylmercury levels at or below 0.02 

 ppm, average total dietary intake of methylmercury should not exceed 30 ^ g 

 per day. This permits an individual to eat 60 g offish at maximum permis- 

 sible mercury levels (approximately 0.5 ppm) each day over a long period of 

 time, without invading the safety factor and accumulating methylmercury in 

 the body such that blood levels would exceed 0.02 ppm. We know that 

 Americans eat less fish than do the Japanese, consequently since the average 

 serving of fish in America approximates 210 g, which is a little less than Vi 

 pound, this means that two meals of fish at guideline would use up 1 week's 

 "ration" of methylmercury (additional exposure to inorganic mercury). Or 

 said another way, one meal of swordfish with average mercury content at 1 

 ppm uses up a week's ration of methylmercury. Fortunately, most fish from 

 both fresh and salt waters are well below guideline. Since many people who 

 eat fish eat several meals per week, especially "weight-watching" women 

 during their child-bearing years, the FDA guideline exists to protect people 

 who like to eat fish from the potential hazards of accumulating excessive 

 methylmercury in their bodies, and especially to protect children from in 

 utero exposures to methylmercury that could cause clinical or subclinical 

 brain damage. 



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