March 9 (HealthDay News) -- If you think changes in the weather bring on migraines, it might not be all in your head.

Harvard researchers report in a new study that people are more likely to visit emergency rooms with migraines if the outside temperature is above normal. Barometric pressure has an effect, too, although it is not as significant.

The findings do not definitively prove that the weather causes migraines. Nor are they "a reason to stay indoors or move to a different part of the country," said study author Dr. Kenneth J. Mukamal, an internist at Beth Israel Deaconess Medical Center in Boston.

"But this does tell us that when we identify migraine triggers, we need to keep temperature in mind," he said. "Before, we might say it must be that ice cream that gave you a headache. Now, maybe it's the temperature that made you want to eat the ice cream."

An estimated 28 million Americans suffer from migraine headaches, perhaps as many as 17 percent of women and 6 percent of men. The headaches can disable sufferers, forcing some to flee to quiet, darkened rooms for relief.

Treatments include painkillers, biofeedback and newer drugs that relieve swelling in the brain.

Many people report "triggers" that cause their migraines, including red wine, chocolate, menstrual cycles and lack of sleep. Others blame changes in the weather, and previous studies have suggested they're on to something.

In the new study, researchers examined the records of 7,054 emergency room patients who were treated for migraines at Beth Israel Deaconess between 2000 and 2007.

The researchers tried to find links between the number of headache cases and levels of temperature, barometric pressure and humidity. They also looked at air pollution levels.

The study results appear in the March 10 issue of Neurology.

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."

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, Neurology