Many people swear that weather is a potent trigger for headaches. Recent research shows that they’re right—at least when the weather is hot. Researchers analyzed temperature, barometric pressure, humidity and pollutants for the three days before patients with severe headache went to the emergency room and compared those levels to those seen on other days of the month. After reviewing data for over 7,000 patients, they found a strong correlation with higher temperatures—there was a 7.5% higher risk of severe headache for every increase of 9 degrees Fahrenheit. The increased risk for migraine was 11%.

Lower barometric pressure also increased the risk of headache, though not of migraine. No significant link between pollutants and headache was found, though the researchers noted a slightly increased risk from exposure to nitrogen dioxide.

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