revised final - Agency for Toxic Substances and Disease Registry ...

MERCURY 127
2. HEALTH EFFECTS
occurred is frequently limited because of difficulties in retracing prior exposure and uncertainties in
estimating dose levels based on assumed food intake and contamination levels.
Although isolated instances of alkyl mercury poisoning have been reported (Cinca et al. 1979; Engleson and
Herner 1952), the epidemic poisonings in Japan and Iraq focused attention on the neurotoxicity of these
compounds. The first reported widespread outbreak of neurological disorders associated with the ingestion
of methylmercury-contaminated fish occurred in the Minamata area of Japan (Kutsuna 1968). The
neurological syndrome was characterized by a long list of symptoms including prickling, tingling sensation
in the extremities (paresthesia); impaired peripheral vision, hearing, taste, and smell; slurred speech;
unsteadiness of gait and limbs; muscle weakness; irritability; memory loss; depression; and sleeping
difficulties (Kutsuna 1968; Tsubaki and Takahashi 1986). Elevated concentrations of methylmercury were
observed in the hair and brains of victims (see Section 2.5). Epidemics of similar neurological disorders
were reported in Iraq in 1956 and 1960 (Bakir et al. 1973; Jalili and Abbasi 1961) as the result of eating
flour made from seed grain treated with ethylmercury p-toluene sulfonanilide. Affected individuals had an
inability to walk, cerebellar ataxia, speech difficulties, paraplegia, spasticity, abnormal reflexes, restriction
of visual fields or blindness, tremors, paresthesia, insomnia, confusion, hallucinations, excitement, and loss
of consciousness. In the winter of 1971–1972 in Iraq, more than 6,530 patients required hospitalization and
459 deaths occurred, usually due to central nervous system damage, after the ingestion of contaminated
bread prepared from wheat and other cereals treated with a methylmercury fungicide (Bakir et al. 1973).
Al-Mufti et al. (1976) attempted to correlate symptoms of the poisoning incident with an estimate of
methylmercury intake based on average levels found in grain and self-reported estimates of the number of
loaves ingested. A number of assumptions were made in the estimates, and there were logistical constraints
in surveying the widely spread rural population in Iraq. Moreover, only a total mercury intake was derived
and compared with the results of a clinical evaluation and a survey for symptoms. Nonetheless, interesting
and useful results were reported based on the 2,147 people surveyed. The mean period of exposure for the
Iraqi population exposed to contaminated bread was 32 days, with some people consuming the bread for as
long as 3 months. A mean of 121 loaves per person was eaten; the maximum was 480 loaves. Based on the
mean number of loaves, the total intake of methylmercury was estimated at between 80 mg and 250 mg.
However, those who had consumed the most loaves may have ingested up to 1,000 mg of methylmercury
over a 3-month period. Of those with symptoms of alkylmercury poisoning at the time of the survey
(October 1972–May 1973), 80% had eaten more than 100 loaves. Of the 75 people who had reported eating
more than 200 loaves, 53 (71%) presented with some evidence of poisoning. The incidence rate for