Kidney stones, which are formed from dissolved minerals in the urine and can be extremely painful, are often caused by caused by dehydration, either by not drinking enough liquid or losing too much due to high heat conditions.

If global warming trends continue as projected by the UN Intergovernmental Panel on Climate Change in 2007, the United States can expect as much as a 30 percent growth in kidney stone disease in some of its driest areas, said the findings published in Monday's Proceedings of the National Academy of Sciences.

The increased incidence of disease would represent between 1.6 million and 2.2 million cases by 2050, costing the US economy as much as one billion dollars in treatment costs.

"This study is one of the first examples of global warming causing a direct medical consequence for humans," said Margaret Pearle, professor of urology at University of Texas Southwestern and senior author of the paper.

"When people relocate from areas of moderate temperature to areas with warmer climates, a rapid increase in stone risk has been observed. This has been shown in military deployments to the Middle East for instance."

The lead author of the research, Tom Brikowski, compared kidney stone rates with UN forecasts of temperature increases and created two mathematical models to predict the impact on future populations.

One formula showed an increase in the southern half of the country, including the already existing "kidney stone belt" of the southeastern states of Alabama, Arkansas, Florida, Georgia, Louisiana, Mississippi, North Carolina, South Carolina and Tennessee.

The other showed that the increase would be concentrated in the upper Midwest.

"Similar climate-related changes in the prevalence of kidney-stone disease can be expected in other stone belts worldwide," the study said.

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