Warmer weather and changes in atmospheric pressure may trigger headaches and migraines, rather than pollution, researchers said.

A U.S. research team showed that each temperature increase of 5 degrees Celsius — about 9 degrees Fahrenheit — appeared to increase the risk of seven-headaches by nearly 8 percent compared to days when the weather was cooler.

Air temperature, humidity and barometric pressure are often cited as a reason for headaches but until now there has been little concrete evidence to back this, Kenneth Mukamal of Beth Israel Deaconess Medical Center in Boston and colleagues said.

His team studied than 7,000 men and women diagnosed with a headache or migraine at the hospital emergency room between May 2000 and December 2007. “In other words, our study design was able to directly compare weather and air pollution conditions right before an emergency room visit with those same factors measured earlier and later the same month," Mukamal said.

The study found that of all the environmental factors tested, higher air

temperature in the 24 hours before a hospital visit was most closely associated with headache symptoms.

Lower barometric pressure also appeared to be a trigger, though the association was not as strong. There was no evidence that air pollutants played a role in sparking headaches, but bigger studies are needed to exclude this as a problem, the researchers added.

The findings published in the journal Neurology suggest the weekly forecast could help people ready their medication to ward off headaches.

"We wanted to find out if we could verify this 'clinical folklore, " Mukamal said in a statement. "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."