A study appearing in the November 22, 2004 issue of the New England Journal of Medicine correlates long term mercury exposure with heart attacks. The researchers tested the toenails of 1408 men in nine countries; 684 of the men had a first diagnosis of myocardial infarction and 724 men were in the control group, with no such diagnosis. Measuring the mercury in toenails is a good way to test for long-term exposure. Overall, the mercury levels in the subjects who had had myocardial infarction were 15% higher than the levels in the control group.