Researchers have matched an upward curve in temperatures in the last few decades with increases in patient's seeking treatment for kidney stones, which both reflects and foretells a warming planet's impact on human health, they say. Of course, they also found increases in very low temperatures.

60,000 patients were matched up with hot days between 2005 and 2011 in Atlanta, Chicago, Dallas, Los Angeles and Philadelphia. The authors described the risk of stone presentation for the full range of temperatures in each city. As mean daily temperatures rose above 50 F (10 C), the risk of kidney stone presentation increased in all the cities except Los Angeles. The delay between high daily temperatures and kidney stone presentation was short, peaking within three days of exposure to hot days. 

"We found that as daily temperatures rise, there is a rapid increase in the probability of patients presenting over the next 20 days with kidney stones," said study leader Gregory E. Tasian, M.D., M.Sc., M.S.C.E., a pediatric urologist and epidemiologist at The Children's Hospital of Philadelphia (CHOP). "These findings point to potential public health effects associated with global climate change. However, although 11 percent of the U.S. population has had kidney stones, most people have not. It is likely that higher temperatures increase the risk of kidney stones in those people predisposed to stone formation."

Because higher temperatures may contribute to dehydration, which leads to a higher concentration of calcium and other minerals in the urine that promote the growth of kidney stones, though the factors causing kidney stones are currently unknown. They may be influenced by changes in diet and fluid intake. 

The study team also found that very low outdoor temperatures increased the risk of kidney stones in three cities: Atlanta, Chicago and Philadelphia. The authors suggest that as frigid weather keeps people indoors more, higher indoor temperatures, changes in diet and decreased physical activity may raise their risk of kidney stones.

The researchers argue that the number of hot days in a given year may better predict kidney stone risk than the mean annual temperature. Atlanta and Los Angeles share the same annual temperature (63 F, or 17 C), but Atlanta has far more hot days than Los Angeles, along with nearly twice the prevalence of kidney stones.

Tasian added that while the five U.S. cities have climates representative of those found throughout the world, future studies should explore how generalizable the current findings are. Other studies should analyze how risk patterns vary in different populations, including among children, represented by a small sample size in the current study.

The study's broader context is in patterns of global warming. The authors note that other scientists have reported that overall global temperatures between 2000 and 2009 were higher than 82 percent of temperatures over the past 11,300 years. Furthermore, increases in greenhouse gas emissions are projected to raise earth's average temperatures by 2 to 8 F (1 to 4.5 C) by 2100. "Kidney stone prevalence has already been on the rise over the last 30 years, and we can expect this trend to continue, both in greater numbers and over a broader geographic area, as daily temperatures increase," concluded Tasian. "With some experts predicting that extreme temperatures will become the norm in 30 years, children will bear the brunt of climate change."