Korea's stone patients follow the weather. In 687,833 records, the lag was under five days

That urinary stones are a summer disease is one of the oldest observations in urology, and for most of its history it has been an impression rather than a measurement. The question a health system actually needs answered is narrower and harder: not whether summer is worse, but how quickly a hot week turns into patients.

What the six-city analysis measured#

A study published in the Journal of Korean Medical Science analysed data on 687,833 urolithiasis patients from 2009 to 2013 across six large Korean cities: Seoul, Incheon, Daejeon, Gwangju, Daegu and Busan. It used a time-series design with distributed lag non-linear methods, estimating the relative risk of mean daily urolithiasis incidence against mean daily meteorological factors, including a cumulative relative risk across a twenty-day window. City-specific associations were then pooled with multivariate meta-regression.

The design matters more than it sounds. A seasonal comparison can only say that August differs from February, which leaves open whether the cause is heat, holiday travel, diet, or the way people use hospitals in summer. A distributed lag model asks a different question — how today's temperature relates to presentations over the following days — and it is that question whose answer is operationally useful.

A positive association between mean daily urolithiasis incidence and mean daily temperature was confirmed in all six cities, and a negative association with mean daily relative humidity in all six. The lag effect was within five days.

The cities behaved alike#

Six cities in one country can still differ enough that a pooled figure conceals more than it shows. The study tested for that directly.

Neither test reached significance, and the I-squared values put roughly a third or less of the variability down to differences between cities. The authors attribute what variation there is partly to the different frequencies and ranges of temperature and humidity the six cities actually experience — which is to say the cities differ in their weather more than in how their residents respond to it.

The same pattern in the emergency data#

Korea's National Emergency Department Information System shows the annual shape of this independently. Across 2014 to 2016, urolithiasis visits peaked in August at 35,927 in a single month against 24,008 in February, and clustered at weekends and around 6 a.m. and 8 p.m.

Two datasets, two designs, one pattern. A regional review of urolithiasis epidemiology across Asia places the finding in a wider frame, naming hot temperature and many hours of sun exposure alongside westernised dietary habits and low fluid intake as contributors to stone formation, and reporting recurrence of 21 to 53 percent within three to five years across the region.

What a time-series design cannot say#

This is an ecological study. It relates city-level daily counts to city-level daily weather, and it holds no information on any individual: not how much water a patient drank, not whether they worked outdoors, not whether they had formed a stone before. The mechanism usually proposed — that heat concentrates urine through fluid loss — is entirely consistent with the data and is not demonstrated by them.

The dates are also worth stating plainly. The exposure window runs 2009 to 2013, and Korean summers since have been hotter. Whether the relationship holds its shape at temperatures the cohort rarely saw is a question this study cannot answer, and it is the question that matters most for the decade ahead.

What it does establish is that in six Korean cities, over five years and nearly 690,000 patients, urolithiasis presentations tracked daily temperature upward and daily humidity downward, consistently, with a delay short enough to see coming.