Warmer weather leads to an increase in severe headaches, such as migraines, that send people to hospital emergency rooms 24 hours later, according to a Harvard University study.

Patients' risk of getting a severe headache increased 7.5 percent for every 9-degree Fahrenheit rise in outdoor temperature in the preceding two to three days, according to the study, published in the journal Neurology. Lower barometric pressure, a reading that reflects humidity as well as temperature, also increased headaches to a lesser extent.

The finding could add outdoor temperatures to the list of factors suspected to trigger the headaches, which includes certain foods, fatigue, emotions and flickering lights, said study author Kenneth Mukamal, an assistant professor of medicine at Harvard. The costs from migraine, which is three times more common in women than in men, total about $17 billion every year in the U.S. for medical treatment and lost job productivity, according to background information in the study.

"One of the next steps is to ask if there is medication that can prevent this kind of triggering," said Mukamal, in a telephone interview on March 6.

The research also showed that migraines were more common in the summer, he said. It's not clear if migraines that weren't serious enough to warrant emergency room evaluation were also affected, the authors wrote in the study.

Pollution Not a Factor

Warmer weather and barometric air pressure, which measures humidity as well as temperature, have been cited previously as potential causes for migraine, although this is the first time there's been any documentation for the claims, Mukamal said. Air pollution didn't increase the likelihood of getting a severe headache.

The study evaluated 7,054 patients who reported to the Beth Israel Deaconess Medical Center in Boston with a diagnosis of headache. The work was funded by the National Institute of Environmental Health Sciences and the U.S. Environmental Protection Agency.