Following two weeks of surging heat in Philadelphia in 2012, Gregory Tasian, a urologist and a clinical epidemiologist at the Children's Hospital of Philadelphia, saw two patients within 12 hours of each other suffering from the same affliction.

A boy and girl both presented with large kidney stones, but neither had a history of this condition, also known as nephrolithiasis. In fact, a scan of one of the patients from a few months earlier showed no indication of any kidney trouble, so the problem must have struck fast and hard.

The cases intrigued Tasian, who then investigated the links between heat and kidney stones further. He found that there were studies pointing to a relationship between these factors since the 1980s. "It has been something that was suspected and somewhat established for a while," he said.

However, the larger studies' findings looked at temperatures averaged over the calendar year, and Tasian suspected that this didn't tell the whole story. Atlanta and Los Angeles, for example, have roughly the same average annual temperature, but Los Angeles has a steady climate, while hot Atlanta summers balance out its colder winters.

In a study published earlier this month in the journal Environmental Health Perspectives, Tasian and his collaborators reported that higher average daily temperatures increased the relative risk of forming kidney stones. The results indicate that the changing climate can directly affect human physiology.

"We were looking at what is the risk of a patient presenting with stones after daily temperatures rose," he said.

Nephrolithiasis results from dietary minerals building up as solid crystals in the urinary tract downstream from the kidneys, which filter waste and excess fluid from the bloodstream. The resulting stones can be extremely painful as they move through the ureters to be excreted and in some instances require surgery to remove.

The researchers in this case looked at a database of insurance claims from 60,433 patients from Atlanta, Chicago, Dallas, Los Angeles and Philadelphia between 2005 and 2011. They assessed the relative risk of kidney stone formation compared to a base-line reference temperature of 10 degrees Celsius using weather data.

The analysis showed that that there were differences between the cities, but in general, as temperatures went up, so did the risk of kidney stones.

When average daily temperatures rose to 30 degrees Celsius, the relative risk of kidney stone presentation within 20 days compared to the base line increased by 38 percent in Atlanta. Risk increased by 37 percent in Chicago, 36 percent in Dallas and 47 percent in Philadelphia. Increased risk in Los Angeles was not statistically significant.

Heightened risk may go unnoticed
The largest surges of patients reporting kidney stones lagged behind a 30-degree-Celsius day by a few days.

The effects worked in the other direction, as well. Kidney stone risks increased in Chicago and Philadelphia when the temperatures dropped below 10 degrees Celsius, while in Atlanta, risks increased at temperatures below 2 degrees Celsius.

David Goldfarb, chief of nephrology for the Veterans Affairs New York Harbor Healthcare System, who was not involved in the study, explained that climate plays an important role in kidney function.

"The idea is that as ambient temperatures increase, your fluid losses through skin increase," he said. With more water coming out as sweat and less coming out as urine, minerals can build up and form stones. In cold weather, researchers suspect, people dehydrate in warm, dry indoor air.

"And you might have this effect without even realizing the fluid loss through your skin is increasing," he added.

Not realizing you're dehydrated is a big part of the problem. In drier climates, people may not have puddles of sweat to gauge how much water they're losing. This is especially important when people migrate to warmer areas and aren't used to drinking more water.

In addition, just as infections like dengue and Lyme disease creep northward as the climate warms, risk for kidney stones is increasing beyond the "stone belt" in the southeastern United States.

A 2008 study in the Proceedings of the National Academy of Sciences found that by 2095, 70 percent of Americans will live in high-risk zones for kidney stones due to warming, compared to 40 percent of Americans at risk in 2000. By 2050, the number of climate-related kidney stone cases would increase to between 1.6 million and 2.2 million.

Under a new normal of warmer temperatures, some people may be able to adapt. "There's certainly evidence that you can reduce your sweat losses in response to acclimation," Goldfarb said. "Whether that's sufficient to prevent kidney stones is not completely clear."

The researchers said drinking plenty of water is a good way to reduce kidney stone risk. Medications and dietary changes can also mitigate the likelihood of this condition.

Goldfarb said he is now looking into the effects of urban heat islands. Cities tend to be substantially warmer than the surrounding countryside, so as people migrate to metropolitan areas, their risks may increase. While climate change may push global temperatures up by 2 to 4 degrees Celsius on average globally, heat islands are already pushing temperatures up to 12 degrees Celsius over their surroundings.

"That may be more important than global warming to exposing people to significant temperatures," he said.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500