PHOENIX - A new study by the National Oceanic and Atmospheric Administration shows dust storms in the Southwest have more than doubled since the 1990s.

The study also finds a rise in the number of valley fever cases.

But Arizona doctors question whether the two are related.

"We don't see upticks in the weeks or months following dust storms," Dr. John Galgiani, director of the Valley Fever Center for Excellence in Tucson said.

If dust storms were causing more valley fever cases, he said, he would expect to see more patients after large storms. Instead, Galgiani said, the number of cases appears to be dependent on the weather in the offseason. Wetter winters tend to bring more valley fever cases in the summer, he said.

"A dust storm only lasts for a few hours," Galgiani said. "I think dust spores get into the air all the time."

the National Weather Service in Phoenix said the increase in dust storms is likely caused by a shift in temperature and weather patterns, but also in the size of cities and the amount of development in the desert.