Kirk here with an article on Minamata disease. I was surprised by the percentages of people who came to see their symptoms as Minamata disease relatively recently.
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Some respondents said they had only recently become aware that they had the disease, which afflicted residents who consumed marine products tainted with wastewater by chemical manufacturer Chisso Corp.
Asked when they realized that they themselves had Minamata disease, 2.2 percent answered “recently,” 13.7 percent said “10 years ago,” and 32.8 percent said “20 years ago.” In total, 48.7 percent said it was within the past 20 years.
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Even if the harm from mercury came decades ago, I have seen from my interactions with patients that the effects tend to get much worse with age. So, in that sense delayed recognition of the nature of the health problems one experiences probably shouldn't be surprising; it's harder to put everything together when the symptoms are still relatively mild.