Secondary sex ratio in regions severely exposed to methylmercury "Minamata disease".
OBJECTIVE
Secondary sex ratio (i.e., male proportion at birth) is considered to function as a sentinel health indicator. Thus, examining this ratio spatially and temporally in regions with severe environmental exposure to compounds such as methylmercury may provide insight into the evolution of exposure.


METHODS
We evaluated spatial and temporal distributions of the secondary sex ratio in Minamata, Japan, and neighboring areas, where severe methylmercury poisoning occurred in the 1950s and 1960s. We selected four areas exposed to methylmercury: Minamata, Ashikita, Goshonoura, and Izumi. After obtaining the number of live births, we conducted descriptive analyses by study area.


RESULTS
We observed a reduction in male births in the exposed areas. In particular, a decline in the sex ratio of the Minamata area, where the first patient was officially identified in 1956, was seen around 1955. The ratio during 1955-1959 around Minamata was 0.496 [95% confidence interval (CI) 0.481-0.511]; the 95% CI did not include the value of 0.515 (the secondary sex ratio of the entire Japanese population during the study period). Declines in this ratio were also observed in other exposed areas around 1960, when acetaldehyde production (the origin of methylmercury) reached its peak.


CONCLUSIONS
These analyses demonstrate that temporal and spatial distributions of the secondary sex ratio reflect the evolution of methylmercury exposure corresponding with the known history of Minamata disease.
Department of Human Ecology, Okayama University Graduate School of Environmental and Life Science, 3-1-1 Tsushima-naka, Kita-ku, Okayama, 700-8558, Japan. yorichan@md.okayama-u.ac.jp.
Takashi
T
Yorifuji
Department of Public Health and Public Policy, Institute of Biomedical and Health Sciences, Hiroshima University, 1-2-3 Kasumi, Minami-ku, Hiroshima, 734-8551, Japan.
Saori
S
Kashima
D005260:Female; D006801:Humans; D007231:Infant, Newborn; D007564:Japan; D050498:Live Birth; D008297:Male; D020262:Mercury Poisoning, Nervous System / Q000209:etiology; D008767:Methylmercury Compounds / Q000633:toxicity*; D016273:Occupational Exposure / Q000009:adverse effects*; D012744:Sex Ratio; D062211:Spatio-Temporal Analysis
D008767:Methylmercury Compounds
Environmental pollution; Food contamination; Methylmercury poisoning; Minamata disease; Sex ratio