Given the history of climate change science - predictions that, no matter how draconian, are often so vague that the dangers are easily ignored or misinterpreted - the specificity of new research out Thursday from Children's Hospital of Philadelphia is intriguing: measurable rises in the number of kidney-stone cases at hospitals and doctors' offices that can be linked to increases, even small ones, in the average daily temperature.
Their research suggests that both adults and children could be at a higher risk for the painful condition as the world warms.
"Kidney-stone prevalence has already been on the rise over the last 30 years," said Gregory Tasian, a pediatric urologist and epidemiologist at Children's, and lead author of the new paper. "We can expect this trend to continue, both in greater numbers and over a broader geographic area, as daily temperatures increase."
Previous studies have shown that hotter climates in general are associated with a greater prevalence of kidney stones. For instance, the Southeastern United States is known to urologists as the "kidney-stone belt" because of higher case rates. Researchers have also observed that when people relocate from moderate climates to hotter places, their likelihood of developing stones increases.
Tasian and his colleagues wanted to define more precisely the relationship.
Wading into insurance claims databases, the team analyzed records of more than 60,000 patients who sought help for kidney stones at hospitals and outpatient clinics between 2005 and 2011 in five major U.S. cities - Atlanta, Chicago, Dallas, Los Angeles, and Philadelphia.
They then factored in average daily temperatures, and calculated risk factors.
As average daily temperatures rose above 50 degrees - a reference point the researchers chose because it is mild and occurred in each city - the risk of a patient's presenting with a kidney stone over the following 20 days generally increased.
The average temperatures varied from city to city, as did the amount of risk. In Philadelphia, for example, when the average temperature reached 86 degrees, close to the highest average for the city, the likelihood of a patient's seeking medical help for a kidney stone was 47 percent (1.47 times) higher than at 50 degrees.
Interestingly, the relative risk increased at colder daily temperatures as well - a finding that the researchers wrote could be attributed to more time spent in heated indoor spaces.
The researchers also looked at how soon after a summer heat wave people became ill. In general, medical visits peaked within three days.
The study appears Thursday in Environmental Health Perspectives, the journal of the National Institute of Environmental Health Sciences.
The findings are significant, Tasian said, because they show an association not just with average annual temperatures, but with acute exposure to heat waves.
For example, Atlanta and Los Angeles have the same average annual temperature, 63 degrees. But Atlanta has more extremely hot days than the more even climate of Los Angeles. And it turns out that Atlanta has twice the prevalence of kidney stones.
Margaret Pearle, a professor of urology at UT Southwestern Medical Center in Dallas, said the study was well done.
"It's really important because we're seeing increases in temperature due to climate change. It means that there's a big increase in costs and a need for health care resources as temperatures rise," she said.
Pearle, who was not involved with the new research, was lead author of a 2008 study predicting that by 2050, higher temperatures from climate change could cause an additional 1.6 million to 2 million kidney stone cases in the United States - with additional health care costs as high as $1 billion.
Kidney stones prompt a half-million emergency room visits every year. About 11 percent of the U.S. population has had them.
That rate is up about 70 percent for adults in the last two or three decades, according to national data - and is increasing even faster among adolescents, who rarely had kidney stones until recent years.
Higher temperatures are considered one factor because they can cause dehydration. This leads to a higher concentration of calcium and other minerals in urine, which promotes growth of kidney stones.
"The worrisome aspect that climate change brings into this is that temperatures are projected to increase" up to eight degrees over the next 100 years, Tasian said. "What effect will that have on patients at risk for stone formation?"
He said much remains unknown, noting, for example, that his study looked only at patients with health insurance, who might have more access to air-conditioning.
Overall, the health effects of climate change are "an understudied area that will become increasingly important," he said.