As daily temperatures increase, so does the number of patients seeking treatment for kidney stones, according to a new study.
The research team found a link between hot days and kidney stones in 60,000 patients in several US cities with varying climates.
"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, a pediatric urologist and epidemiologist at The Children's Hospital of Philadelphia (CHOP).
The study team analysed medical records of more than 60,000 adults and children with kidney stones between 2005 and 2011 in Atlanta, Chicago, Dallas, Los Angeles and Philadelphia, in connection with weather data.
Tasian and colleagues described the risk of stone presentation for the full range of temperatures in each city. As mean daily temperatures rose above 10 degrees Celsius, 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.
"These findings point to potential public health effects associated with global climate change," said Tasian.
"However, although 11 per cent of the US 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," said Tasian.
Higher temperatures contribute to dehydration, which leads to a higher concentration of calcium and other minerals in the urine that promote the growth of kidney stones.
A painful condition that brings half a million patients a year to US emergency rooms, kidney stones have increased markedly over the world in the past three decades.
While stones remain more common in adults, the numbers of children developing kidney stones have climbed at a dramatically high rate over the last 25 years.
The factors causing the increase in kidney stones are currently unknown, but may be influenced by changes in diet and fluid intake. When stones do not pass on their own, surgery may be necessary.
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 study was published in the journal Environmental Health Perspectives.