More than a dozen medical groups have agreed to change the guidelines for what constitutes high blood pressure in adults, based on the findings of a major study conducted two years ago. For decades, the upper threshold for high blood pressure has been a top reading of at least 140 or a bottom number of 90; the new guidelines, announced Monday at the American Heart Association's conference in California, drop the numbers to 130 over 80. That means an additional 30 million Americans now have the condition, and it affects half of all adults in the United States. The study found that when people tried to keep their top number at 120, it lowered their risk of having heart problems.

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