According to U.S. research hot weather conditions may trigger migraine and other severe headaches and increase the risk of an attack the next day.

The study, published in Neurology is one of the largest ever to examine the impact of weather and air pollution on headaches. The researchers looked at 7,054 patients attending the emergency department of a large US hospital for advice about a headache at any point during a seven year period.

The researchers used meteorological and pollutant monitors to compare measurements of a number of environmental factors during the days leading up to and again some weeks after a patient's hospital visit. Overall, 2,250 were diagnosed with migraine and 4,803 with "tension" or "unspecified" headaches.

Although rising temperature was identified as the biggest weather-related headache trigger, the researchers concluded that the impact may not be clinically meaningful.

“This magnitude of excess risk is obviously modest and may not be an important factor in the clinical management of individual patients, given the many other potential triggers of migraine that patients face,” they write.

The researchers reported that irrespective of the time of the year, temperature increase was the biggest weather-related headache trigger and every 9 degree Fahrenheit, or five degree Celsius increase in temperature raised the headache risk by 7.5%.  This held in the case of people who suffer from non-migraine headaches too.

The study found no link between migraines and low-pressure systems although low barometric air pressure is considered by some to be specific to migraines. Lower pressure however the researchers said   was associated with a small increase in risk for non-migraine headaches.

Air pollution did not strongly affect migraine or non-migraine headaches but the automobile exhaust pollutant nitrogen dioxide did show a borderline effect on non-migraine headaches.

Study lead author Kenneth J. Mukamal, MD, of Boston’s Beth Israel Deaconess Medical Center and Harvard School of Public Health said, "These findings help tell us that the environment around us does affect our health and, in terms of headaches, may be impacting many, many people on a daily basis."
 
He said headache patients should see their doctor to identify the triggers that lead to their symptoms, and added that although you can't change the weather, people can watch the forecast and pop a pill to avert attacks. He said that an even bigger study would be needed to understand the impact of air pollution on headaches. “We are not saying that air pollution is not a headache trigger,” he said. “What we can say with some confidence is that the effect is not enormous.”

Dr Brendan Davies, consultant neurologist and spokesman for the Migraine Trust said, "This adds scientific validity to what we have suspected for some time. Up to 50% of patients believe weather may trigger their migraine.

"It looks like it is the change in the air temperature, and to a lesser extent the change in air pressure that is important.

"It would be interesting to see if the same environmental factors apply to workers in an office. It also adds more to our understanding that migraine is a sensitivity problem."

"This does not mean that just because there is a hot day people are going to have a migraine the next day, although it is more likely. We do not want people taking too many painkillers too often because that carries its own risks."

The study was supported by the National Institute of Environmental Health and Sciences and the Environmental Protection Agency.