In our study, various urolithiasis IRs and different climatic factors were presented depending on the regions, but the most important factor was MT in the high population density group compared to the low population density group. Among the significant RRs in the urban group, wind speed and RH also affected the rural group and the amount of sunshine did not show a significant difference between the urban and rural groups. Therefore, MT may be responsible for the difference in urolithiasis incidence between the urban and rural groups.
In the urban group, RRs increased gradually with increasing temperature and the RR almost doubled with a twofold increase in the temperature compared to that at 13 °C. Urolithiasis prevalence in the southeastern United States (USA) is almost 50% higher than that in the northeastern USA which has a 8 °C lower MT and an increase in 1 °C will cause a 4.2% increase in the urolithiasis risk (Brikowski et al. 2008). Another study that used the multivariate autoregressive integrated moving average (ARIMA) model showed that temperature has a strong correlation with calculi presentation rate in New York City (Sirohi et al. 2014). Similarly, among various climatic factors, the association between temperature and urinary stone event is well known.
The importance of temperature in resultant urolithiasis implies two things. First, urbanization may affect the prevalence of urolithiasis. The urban population exceeded the rural population all over the world from 2008 and urbanization is a worldwide phenomenon (Desa 2010). A decrease in the green area in city causes elevation of the temperature due to UHIs compared to the rural area, but water in the rural surface absorbs the heat that converts it to cause evaporation (Goldfarb and Hirsch 2015). Elevated temperatures influence various diseases like cardiovascular disease, asthma etc (O’Neill and Ebi 2009). In addition, many studies have demonstrated the association between hotter season or months and urolithiasis events (Boscolo-Berto et al. 2008; Brikowski et al. 2008; Sirohi et al. 2014). In our study, the urban group included cities and the capital area that have a high population density; hence, most of them are composed of urban areas. The temperature in the urban group was higher than that in the rural group, and the average IR in the urban group was higher than the average IR in the rural group. Meanwhile, there are a few studies comparing urolithiasis presentation according to regional differences. In the USA, there was no difference in the pediatric kidney stone incidence between the urban and rural areas (Sas et al. 2010). The authors did not explain the reason for this occurrence. All their subjects were children; hence, their results cannot be directly compared to our results.
Second, global warming can also increase urolithiasis events. The greenhouse gas concentrations increased surface temperatures from 1.5 to 4.5 °C during the twenty-first century (Meinshausen et al. 2011). The effects of global warming are observed even in ROK such as an increase in atmospheric carbon dioxide and temperatures (Oh et al. 2001). In the USA, the prevalence of urolithiasis increased from 3.6 to 5.2% when the annual MT increased by 0.5 °C from 1976 to 1994 (Stamatelou et al. 2003). Brikowshi et al. (2008) predicted that 56% of the USA population would live in high-risk stone areas and there would be additional 1.6–2.2 million cases based on the global warming model until 2050. According to our linear formula between time and temperature, there was an increase of 1.38 °C during 5 years and there were additional 30,918 cases in 2013 compared to those in 2009. This increasing trend was also observed in another Korean nationwide study and the annual increase rate was calculated as 0.3% using the ARIMA model from 2006 to 2010 (Park et al. 2015).
The main finding of our study is that the pattern of occurrence of urolithiasis is affected by the regional difference. In our study, RH and wind speed affected urolithiasis incidence in the urban and rural areas and sunshine affected urolithiasis incidence only in the urban area. In other studies, the associations between humidity and urolithiasis are debatable (Tasian et al. 2014; Sirohi et al. 2014). Humidity does not have an effect on people’s health as much as temperature, but other factors could conceal the effect of humidity (Barnett et al. 2010). In addition, humidity may cause more severe problems in special zones of the city in which people who are vulnerable to heat like senior citizens live (Kenney and Hodgson 1987). Low RH environment induces an increase in transepidermal water loss (Kenney and Hodgson 1987). This can be related to the hypothesis of dehydration and stone formation. Regions with vegetation have higher humidity and the influence of humidity on temperature can differ depending on the urban conditions (Hass et al. 2016). Increased wind speed may cool the people down and influence solar radiation and temperature on humans (Thorsson et al. 2007). Sunlight exposure is correlated with increased urolithiasis in many studies (Prince and Scardino 1960). However, opposite results in our study might be due to the fact that the rainy season is usually in summer in ROK and sunshine has decreased in cities during the past 46 years because of anthropogenic activities and air pollution (Fu et al. 2015).
Interestingly, in our study, urolithiasis incidence increased during the 2009–2013 period. This increasing trend could be explained by the increase in hospital utility in Korea. The mean number of hospital utility episodes per person increased from 16.04 in 2006 to 18.59 in 2010 (Moon 2014). Another explanation is that nonenhanced computed tomography (CT) has replaced intravenous urography (IVU) as an imaging tool for confirming the diagnosis of urolithiasis (Lee et al. 2015). CT has a higher detection rate of urolithiasis including radiolucent stones and small stones compared with IVU (Lee et al. 2015). CT has been described as the best imaging tool for confirming the diagnosis of urolithiasis, and CT may have affected the annual increase rate of urolithiasis in our study.
There are some limitations to this study. First, lag periods do not directly indicate the duration of stone formation. Lag periods indicate the association between daily temperatures and urolithiasis events. We observed an increasing RR of urolithiasis after lag periods of high temperature. High temperature might cause early occurrence of the urolithiasis event. High temperature could cause the previously formed asymptomatic stones to become symptomatic or aggravate new stone nucleation. Second, there may be confounding factors due to the difference in urban and rural areas. The difference may be applicable to age, lifestyle, diet, income, activity and occupation, which may be extrinsic factors of urolithiasis (Ramello et al. 2000; Scales et al. 2012). Unfortunately, no information was provided about dietary habits of the population of different areas in our study and it is very difficult to estimate the lifestyle and diet habits for whole population in South Korea. Diet is the other main determinant of stone formation and it is reasonable to suppose that people living in the rural area could have different dietary habits than people living in the towns who often did not take their meals at home. Also age distribution could be different in different areas and it could be that populations in the urban areas were younger and more prone to stone formation. Air-conditioned indoor condition and heat exposure outdoor condition in the same region can have other effects on urolithiasis (Ramello et al. 2000; O’Neill and Ebi 2009). Lastly, the inclusion and the exclusion criteria could be a source of bias, which are patients with chronic urolithiasis which come at the control after more than 30 days and there are persons with congenital urolithiasis which is not dependent on the climatic area. Unfortunately, no information was provided from Health Insurance Review and Assessment Service (HIRA) in our study. More studies in different populations are needed to confirm the physiology of urolithiasis caused by change in climatic factors under the control of other extrinsic factors.
