The researchers found that the number of emergency visits for headaches would rise by an average of 7.5 percent within 24 hours if the temperature rose by 9 degrees Fahrenheit above the expected temperature.
In a hypothetical example, the hospital would expect to see 7.5 percent more headache patients 24 hours after the temperature was 90 degrees instead of a typical 81 degrees.
High temperatures alone, such as those in the summer, were not as much of a trigger. The most influential factor was whether a particular day was hotter than expected.
"Warmer days were associated with higher risk, even in the winter," Mukamal said.
The researchers also found that drops in barometric pressure made headache visits more likely 48 to 72 hours later. Pollution did not seem to have an effect on headaches.
Why might the weather affect migraines? Barometric pressure could affect the layer of fluid that protects the brain inside the skull, said Dr. Richard Lipton, director of the Montefiore Headache Center at Montefiore Medical Center in New York City. But the effect of temperature, he said, is mystifying.
"If someone knows that they're vulnerable to changes in temperature, what they might do is be particularly cautious about the things they can control," he said. "If you know the temperature is changing, that might be a good day to make sure you get your regular amount of sleep, avoid red wine, chocolate and the other triggers."
The U.S. National Institute of Neurological Disorders and Stroke has more on migraines.
SOURCES: Kenneth J. Mukamal, M.D., internist, Beth Israel Deaconess Medical Center, Boston; and Richard Lipton, M.D., director, Montefiore Headache Center, Montefiore Medical Center, New York City; March 10, 2009,
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