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American Journal of Kidney Diseases

Associations Between Ambient Extreme Heat Exposure and Emergency Department Visits Related to Kidney Disease

  • Author Footnotes
    ∗ Y.Q. and W.Z. contributed equally to this work.
    Yanji Qu
    Footnotes
    ∗ Y.Q. and W.Z. contributed equally to this work.
    Affiliations
    Guangdong Cardiovascular Institute, Guangdong Provincial People’s Hospital, Guangdong Academy of Medical Sciences, Guangdong, People’s Republic of China

    Department of Environmental Health Sciences, School of Public Health, University at Albany, State University of New York, Albany, New York
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  • Author Footnotes
    ∗ Y.Q. and W.Z. contributed equally to this work.
    Wangjian Zhang
    Footnotes
    ∗ Y.Q. and W.Z. contributed equally to this work.
    Affiliations
    Department of Environmental Health Sciences, School of Public Health, University at Albany, State University of New York, Albany, New York

    Department of Medical Statistics, School of Public Health, Sun Yat-sen University, Guangzhou, 510080, Guangdong, People's Republic of China.
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  • Asi-Yahola M. Boutelle
    Affiliations
    Department of Environmental Health Sciences, School of Public Health, University at Albany, State University of New York, Albany, New York
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  • Ian Ryan
    Affiliations
    Department of Environmental Health Sciences, School of Public Health, University at Albany, State University of New York, Albany, New York
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  • Xinlei Deng
    Affiliations
    Department of Environmental Health Sciences, School of Public Health, University at Albany, State University of New York, Albany, New York
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  • Xiaoqing Liu
    Affiliations
    Guangdong Cardiovascular Institute, Guangdong Provincial People’s Hospital, Guangdong Academy of Medical Sciences, Guangdong, People’s Republic of China
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  • Shao Lin
    Correspondence
    Address for Correspondence: Shao Lin, MD, PhD, Department of Environmental Health Sciences, School of Public Health, University at Albany, State University of New York, One University Place, Rensselaer, Albany, NY 12144.
    Affiliations
    Department of Environmental Health Sciences, School of Public Health, University at Albany, State University of New York, Albany, New York
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  • Author Footnotes
    ∗ Y.Q. and W.Z. contributed equally to this work.
Published:October 11, 2022DOI:https://doi.org/10.1053/j.ajkd.2022.09.005

      Rationale & Objective

      Extreme heat exposure is associated with multiple diseases. However, our current understanding of the specific impact of extreme heat exposure on kidney disease is limited.

      Study Design

      Case-crossover study.

      Setting & Participants

      1,114,322 emergency department (ED) visits with a principal diagnosis of kidney disease were identified in New York state, 2005-2013.

      Exposure

      Extreme heat exposure was defined as when the daily temperature exceeded the 90th percentile temperature of that month during the study period in the county.

      Outcome

      ED visits with a principal diagnosis of kidney disease and its subtypes (ICD-9 [International Classification of Diseases, Ninth Revision] codes 580-599, 788).

      Analytical Approach

      Extreme heat exposure on the ED visit days was compared with extreme heat exposure on control days using a conditional logistic regression model, controlling for humidity, air pollutants, and holidays. The excess risk of kidney disease was calculated for a week (lag days 0-6) after extreme heat exposure during the warm season (May through September). We also stratified our estimates by sociodemographic characteristics.

      Results

      Extreme heat exposure was associated with a 1.7% (lag day 0) to 3.1% (lag day 2) higher risk of ED visits related to kidney disease; this association was stronger with a greater number of extreme heat exposure days in the previous week. The association with extreme heat exposure lasted for an entire week and was stronger in the transitional months (ie, May and September; excess rates ranged from 1.8% to 5.1%) rather than the summer months (June through August; excess rates ranged from 1.5% to 2.7%). The strength of association was greater among those with ED visits related to acute kidney injury, kidney stones, and urinary tract infections. Age and sex may modify the association between extreme heat exposure and ED visits.

      Limitations

      Individual exposure to heat—how long people were outside or whether they had access to air conditioning—was unknown.

      Conclusions

      Extreme heat exposure was significantly associated with a dose-dependent greater risk of ED visits for kidney disease.

      Graphical abstract

      Index Words

      Given the increasing frequency and intensity of extreme heat exposure due to climate change and the growing prevalence of kidney disease worldwide, we examined their association with one another. We examined the association between extreme heat exposure and kidney disease–related emergency department visits based on seasons and disease subtypes in New York state. Extreme heat exposure was significantly associated with an increased risk of emergency department visits for multiple types of kidney disease (mainly acute kidney injury, kidney stones, and urinary tract infections). Furthermore, the impact of extreme heat exposure lasted a week and was stronger in the transitional months (May and September) than the summer months.
      Excess death and morbidity due to kidney disease pose substantial public health and economic burdens around the globe.
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      International Society of Nephrology (ISN) and the International Federation of Kidney Foundations (IFKF).
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      In fact, 7.6% of the 1.4 million annual cardiovascular disease (CVD) deaths worldwide can be attributed to decreased kidney function.
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      Together, kidney disease, diabetes, hypertension, and CVD profoundly impact global morbidity and mortality trends,
      • Couser W.G.
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      The contribution of chronic kidney disease to the global burden of major noncommunicable diseases.
      but kidney disease receives far less attention in research on the treatment and etiology of NCDs. While many important NCDs have declined in recent years, kidney disease has experienced a slower rate of decline and even increased in some regions.
      • Couser W.G.
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      • Mendis S.
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      Thus, the prevention of kidney disease in high-risk groups warrants greater prominence on the global public health agenda.
      • Couser W.G.
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      • Mendis S.
      • Tonelli M.
      The contribution of chronic kidney disease to the global burden of major noncommunicable diseases.
      In recent decades, climate change has played an increasing role in the world’s growing burden of kidney disease.
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      • Purcell C.A.
      • Levey A.S.
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      Global, regional, and national burden of chronic kidney disease, 1990–2017: a systematic analysis for the Global Burden of Disease Study 2017.
      The unprecedented elevation of ambient temperatures worldwide substantially impacts human health.
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      Previous studies have demonstrated associations between extreme heat exposure and increased incidence,
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      The impact of daily temperature on renal disease incidence: an ecological study.
      morbidity,
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      and hospital admission rates for kidney disease.
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      • Bobb J.F.
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      Vulnerability to extreme-heat-associated hospitalization in three counties in Michigan, USA, 2000-2009.
      • Isaksen T.B.
      • Yost M.G.
      • Hom E.K.
      • Ren Y.
      • Lyons H.
      • Fenske R.A.
      Increased hospital admissions associated with extreme-heat exposure in King County, Washington, 1990-2010.
      • Gronlund C.J.
      • Zanobetti A.
      • Schwartz J.D.
      • Wellenius G.A.
      • O’Neill M.S.
      Heat, heat waves, and hospital admissions among the elderly in the United States, 1992-2006.
      • Bobb J.F.
      • Obermeyer Z.
      • Wang Y.
      • Dominici F.
      Cause-specific risk of hospital admission related to extreme heat in older adults.
      Various studies have assessed the associations between extreme heat exposure and emergency department (ED) visits for kidney disease overall
      • Kim E.
      • Kim H.
      • Kim Y.C.
      • Lee J.P.
      Association between extreme temperature and kidney disease in South Korea, 2003-2013: Stratified by sex and age groups.
      • Kim S.E.
      • Lee H.
      • Kim J.
      • et al.
      Temperature as a risk factor of emergency department visits for acute kidney injury: a case-crossover study in Seoul, South Korea.
      • Winquist A.
      • Grundstein A.
      • Chang H.H.
      • Hess J.
      • Sarnat S.E.
      Warm season temperatures and emergency department visits in Atlanta, Georgia.
      • Heidari L.
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      • Klein M.
      • O’Lenick C.
      • Grundstein A.
      • Ebelt Sarnat S.
      Susceptibility to heat-related fluid and electrolyte imbalance emergency department visits in Atlanta, Georgia, USA.
      • Chen T.
      • Sarnat S.E.
      • Grundstein A.J.
      • Winquist A.
      • Chang H.H.
      Time-series analysis of heat waves and emergency department visits in Atlanta, 1993 to 2012.
      • Cervellin G.
      • Comelli I.
      • Comelli D.
      • Meschi T.
      • Lippi G.
      • Borghi L.
      Mean temperature and humidity variations, along with patient age, predict the number of visits for renal colic in a large urban emergency department: results of a 9-year survey.
      and various subtypes, including acute kidney injury (AKI),
      • Kim E.
      • Kim H.
      • Kim Y.C.
      • Lee J.P.
      Association between extreme temperature and kidney disease in South Korea, 2003-2013: Stratified by sex and age groups.
      ,
      • Kim S.E.
      • Lee H.
      • Kim J.
      • et al.
      Temperature as a risk factor of emergency department visits for acute kidney injury: a case-crossover study in Seoul, South Korea.
      ,
      • Chen T.
      • Sarnat S.E.
      • Grundstein A.J.
      • Winquist A.
      • Chang H.H.
      Time-series analysis of heat waves and emergency department visits in Atlanta, 1993 to 2012.
      ,
      • Honda T.
      • Manjourides J.
      • Suh H.
      Daily ambient temperature is associated with biomarkers of kidney injury in older Americans.
      • Johnson R.J.
      • Sánchez-Lozada L.G.
      • Newman L.S.
      • et al.
      Climate change and the kidney.
      • McTavish R.K.
      • Richard L.
      • McArthur E.
      • et al.
      Association between high environmental heat and risk of acute kidney injury among older adults in a northern climate: a matched case-control study.
      chronic kidney disease (CKD),
      • Kim E.
      • Kim H.
      • Kim Y.C.
      • Lee J.P.
      Association between extreme temperature and kidney disease in South Korea, 2003-2013: Stratified by sex and age groups.
      ,
      • Johnson R.J.
      • Sánchez-Lozada L.G.
      • Newman L.S.
      • et al.
      Climate change and the kidney.
      kidney stones,
      • Ross M.E.
      • Vicedo-Cabrera A.M.
      • Kopp R.E.
      • et al.
      Assessment of the combination of temperature and relative humidity on kidney stone presentations.
      ,
      • Tasian G.E.
      • Pulido J.E.
      • Gasparrini A.
      • et al.
      Urologic Diseases in America Project. Daily mean temperature and clinical kidney stone presentation in five U.S. metropolitan areas: a time-series analysis.
      urinary tract infections (UTIs), and renal colic.
      • Cervellin G.
      • Comelli I.
      • Comelli D.
      • Meschi T.
      • Lippi G.
      • Borghi L.
      Mean temperature and humidity variations, along with patient age, predict the number of visits for renal colic in a large urban emergency department: results of a 9-year survey.
      However, the collective results are inconclusive. There are still multiple knowledge gaps in substantiating the associations between extreme heat exposure and renal ED visits, including a lack of studies assessing (1) lag and dose-dependent effects, (2) impacts of extreme heat exposure in transitional months before and after summer (May and September), (3) associations between extreme heat exposure and kidney disease subtypes, and (4) risks among populations with existing kidney disease who are highly vulnerable to extreme heat exposure.
      Our study fills existing knowledge gaps by assessing the associations between extreme heat exposure and ED visits related to kidney disease overall and by subtype in summer and transitional months in New York state. In addition, we stratified our data by sociodemographic characteristics to further uncover associations between extreme heat exposure and renal ED visits. Given the increasing frequency and intensity of extreme heat in weather patterns due to climate change and the growing prevalence of kidney disease worldwide, it is imperative to elucidate the association between extreme heat exposure and the various subtypes of kidney disease.

      Methods

      Study Design and Health Outcome

      We used a time-stratified case-crossover design to study the association of extreme heat exposure with the transient acute and severe exacerbation of kidney diseases by using ED visit data.
      • Maclure M.
      The case-crossover design: a method for studying transient effects on the risk of acute events.
      • Zhang W.
      • Lin S.
      • Hopke P.K.
      • et al.
      Triggering of cardiovascular hospital admissions by fine particle concentrations in New York state: before, during, and after implementation of multiple environmental policies and a recession.
      • Rich D.Q.
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      Triggering of cardiovascular hospital admissions by source specific fine particle concentrations in urban centers of New York State.
      The case-crossover design contrasts extreme heat exposure immediately before the ED visits related to kidney disease (case days) to other times when the individual did not go to the ED (control days). We defined control days as the same days of the week in the same calendar month as the corresponding case day, resulting in 3 or 4 control days for each case day. We also examined the associations on lag days 0-6 (the 7 days after the extreme heat exposure).
      Compared with more conventional study designs, cases serve as their own controls in a case-crossover analysis. Therefore, inherited confounders, such as age, sex, unchanged lifestyle, and some time-varying factors, were automatically controlled.
      • Lin S.
      • Bell E.M.
      • Liu W.
      • Walker R.J.
      • Kim N.K.
      • Hwang S.A.
      Ambient ozone concentration and hospital admissions due to childhood respiratory diseases in New York State, 1991-2001.
      We used health data from the New York Statewide Planning and Research Cooperative System (SPARCS), a legislatively mandated database covering over 95% of hospital records in New York state.
      • Zhang W.
      • Lin S.
      • Hopke P.K.
      • et al.
      Triggering of cardiovascular hospital admissions by fine particle concentrations in New York state: before, during, and after implementation of multiple environmental policies and a recession.
      We obtained ED visits reported with a principal diagnosis of acute and chronic kidney diseases—International Classification of Diseases, Ninth Revision (ICD-9) codes 580-599, 788—from January 1, 2005, through December 31, 2013. As in our previous research,
      • Fletcher B.A.
      • Lin S.
      • Fitzgerald E.F.
      • Hwang S.A.
      Association of summer temperatures with hospital admissions for renal diseases in New York state: a case-crossover study.
      we included the following 8 major kidney disease subtypes: AKI (code 584), CKD (code 585), UTI (code 599), kidney stone (code 592), lower urinary tract calculi (code 594), nephritis and nephrosis (codes 580-583, 590, 591), other kidney and ureter disorders (codes 586-589, 593), and other lower urinary tract disorders (codes 595-598, 788). We included acute exacerbations of all kidney diseases to properly assess the acute and transient effects according to the assumption of a case-crossover design. In addition, previous literature had found that acute onset or exacerbation of many chronic conditions such as CKD
      • Kim E.
      • Kim H.
      • Kim Y.C.
      • Lee J.P.
      Association between extreme temperature and kidney disease in South Korea, 2003-2013: Stratified by sex and age groups.
      ,
      • Johnson R.J.
      • Sánchez-Lozada L.G.
      • Newman L.S.
      • et al.
      Climate change and the kidney.
      and kidney stones
      • Ross M.E.
      • Vicedo-Cabrera A.M.
      • Kopp R.E.
      • et al.
      Assessment of the combination of temperature and relative humidity on kidney stone presentations.
      ,
      • Tasian G.E.
      • Pulido J.E.
      • Gasparrini A.
      • et al.
      Urologic Diseases in America Project. Daily mean temperature and clinical kidney stone presentation in five U.S. metropolitan areas: a time-series analysis.
      are significantly related to extreme heat exposure. To ensure a valid outcome definition, we used the principal diagnosis (or discharge diagnosis) confirmed by laboratory testing and clinical examination. Using the principal diagnosis allowed us to identify situations where kidney disease was the leading cause of the ED visit rather than being a morbidity noted in the medical record of uncertain origin and relevance. Furthermore, using ED data for acute kidney disease outcomes was appropriate because these conditions require immediate medical attention. Patients with multiple ED visits were included and treated as independent individual cases. However, we assumed that readmission did not affect the results because the percentage of patients with multiple renal ED visits occurring within 30 days was low (approximately 5%), and we focused on acute exacerbation events rather than incident cases.
      This study was approved by the institutional review board (IRB) at the University at Albany, State University of New York (approval number 17X189). The requirement for individual-level informed consent was waived by the IRB because the data were statistically deidentified except for residential address, which was deleted after being geocoded to the longitude and latitude coordinates and linked with the meteorological data.

      Exposure Data and Definition

      Daily weather data were obtained from the US Environmental Protection Agency (EPA)
      US Environmental Protection Agency (EPA)
      Daily summary data.
      and National Oceanic and Atmospheric Administration (NOAA).
      US National Oceanic and Atmospheric Administration (NOAA), National Centers for Environmental Information. Global Hourly—Integrated Surface Database (ISD).
      We assigned the county-level (62 counties in New York state) temperature and humidity exposure to the individual level because ambient temperature and humidity were not likely to change significantly within a county. We used the 90th percentile of daily temperature in each month over the entire study period in each county as the threshold to define extreme heat exposure (a binary variable) based on previous extreme heat exposure health research.
      • Gronlund C.J.
      • Zanobetti A.
      • Schwartz J.D.
      • Wellenius G.A.
      • O’Neill M.S.
      Heat, heat waves, and hospital admissions among the elderly in the United States, 1992-2006.
      ,
      • D’Ippoliti D.
      • Michelozzi P.
      • Marino C.
      • et al.
      The impact of heat waves on mortality in 9 European cities: results from the EuroHEAT project.
      • Ha S.
      • Liu D.
      • Zhu Y.
      • et al.
      Ambient temperature and stillbirth: a multi-center retrospective cohort study.
      • Ha S.
      • Liu D.
      • Zhu Y.
      • Kim S.S.
      • Sherman S.
      • Mendola P.
      Ambient temperature and early delivery of singleton pregnancies.
      • Heo S.
      • Bell M.L.
      • Lee J.T.
      Comparison of health risks by heat wave definition: applicability of wet-bulb globe temperature for heat wave criteria.
      • Tobías A.
      • Armstrong B.
      • Gasparrini A.
      • Diaz J.
      Effects of high summer temperatures on mortality in 50 Spanish cities.
      Air pollution is related to both extreme heat exposure and kidney disease,
      • Remigio R.V.
      • He H.
      • Raimann J.G.
      • et al.
      Combined effects of air pollution and extreme heat events among ESKD patients within the Northeastern United States.
      and previous studies have identified specific pollutants as confounders,
      • Bekkar B.
      • Pacheco S.
      • Basu R.
      • DeNicola N.
      Association of air pollution and heat exposure with preterm birth, low birth weight, and stillbirth in the US: a systematic review.
      • Chiu P.F.
      • Chang C.H.
      • Wu C.L.
      • et al.
      High particulate matter 2.5 levels and ambient temperature are associated with acute lung edema in patients with nondialysis stage 5 chronic kidney disease.
      • Hoffmann B.
      • Luttmann-Gibson H.
      • Cohen A.
      • et al.
      Opposing effects of particle pollution, ozone, and ambient temperature on arterial blood pressure.
      so we added PM2.5 (airborne particulate matter that is ≤2.5 microns in width) and ozone, the 2 pollutants commonly occurring during the warm season, to our analysis as confounders. In addition, our data showed a 6% to 35% difference in the excess rate when running the regression models with and without adjustments for these 2 pollutants. We found more than a 15% difference in the risk estimation for most lag days (Table S1).
      We used daily PM2.5 and ozone concentrations simulations from the EPA’s Community Multiscale Air Quality Modeling System (CMAQ).
      US Environmental Protection Agency (EPA)
      CMAQ: The Community Multiscale Air Quality Modeling System.
      CMAQ output represents volume-average pollutant concentrations for every 1-mile by 1-mile grid cell and vertical layer in the model domain. For each census tract, we determined the simulation grids of CMAQ data located in the area using the FIPS.names function in the rSPARCS package. We then computed the average air pollution concentration across the simulation grids within the census tract. By geocoding the individual residential address of each case to the street level and assigning the exposures to each case, we obtained individual daily PM2.5 and ozone exposure estimates.

      Statistical Analysis

      Our analysis focused on ED visits due to kidney disease in summer (June through August) and in the transitional months (May and September), as prior studies found stronger effects of extreme heat exposure on many health outcomes during these transitional months.
      • Lin S.
      • Lin Z.
      • Ou Y.
      • et al.
      National Birth Defects Prevention Study. Maternal ambient heat exposure during early pregnancy in summer and spring and congenital heart defects—a large US population-based, case-control study.
      ,
      • Qu Y.
      • Zhang W.
      • Ryan I.
      • et al.
      Ambient extreme heat exposure in summer and transitional months and emergency department visits and hospital admissions due to pregnancy complications.
      Other studies on associations between extreme heat exposure and kidney disease included May and September as part of the warm season.
      • Winquist A.
      • Grundstein A.
      • Chang H.H.
      • Hess J.
      • Sarnat S.E.
      Warm season temperatures and emergency department visits in Atlanta, Georgia.
      ,
      • Sun S.
      • Weinberger K.R.
      • Nori-Sarma A.
      • et al.
      Ambient heat and risks of emergency department visits among adults in the United States: time stratified case crossover study.
      We first drew a plot to present the overall relationship between continuous temperature, extreme heat exposure, and the number of renal ED visits over time. We also conducted a sensitivity analysis to fit the relationship between continuous temperature levels and risks of kidney disease (ORs) using the distributed lag nonlinear model (DLNM). We then used a conditional logistic model to regress the outcome (ie, the case vs control indicator) against the extreme heat exposure while controlling for multiple confounders on lag days 0-6. Specifically, we controlled for humidity, holiday, and the concentration of PM2.5 and ozone on the same lag day as the extreme heat exposure. Odds ratios (ORs) were first estimated, then excess rate was calculated as (OR − 1) × 100%. We used excess rate because it removes the baseline change and was commonly used in previous studies.
      • Sun S.
      • Weinberger K.R.
      • Nori-Sarma A.
      • et al.
      Ambient heat and risks of emergency department visits among adults in the United States: time stratified case crossover study.
      We first ran an overall model including all cases in different seasons (ie, May vs June-August vs September) and then a separate model for each kidney disease subtype in the warm season. Afterward, we examined the modifying effects of sociodemographic factors such as age, sex, race, ethnicity, and insurance status on the associations by calculating the P value for the factor-exposure interaction term.
      • Zhang W.
      • Lin S.
      • Hopke P.K.
      • et al.
      Triggering of cardiovascular hospital admissions by fine particle concentrations in New York state: before, during, and after implementation of multiple environmental policies and a recession.
      Finally, we conducted stratified analyses among those factors with significant modification effects (P for interaction < 0.05). In addition, we calculated the number of extreme heat exposure days within the week before the admission day for each participant and evaluated the potential dose-dependent effect on ED visits.

      Results

      We identified 1,114,322 ED visits related to kidney disease in New York state during the study period. The basic characteristics of our study population are described in Table 1. Figure S1 shows a plot over time of continuous temperature and the number of renal ED visits; overall, temperature and ED visit volume were highly correlated. As shown in Figure S2, we observed a monotonically increasing risk of renal ED visits as temperatures increase during the warm season (May through September). Figure 1 displays the excess rate of renal ED visits attributable to extreme heat exposure among New York residents. Depending on the day, we found that extreme heat exposure was associated with a 1.7% (95% CI, 0.9%-2.5%) to 3.1% (95% CI, 2.3%-4.0%) excess rate of a renal ED visit during the week after the exposure (ie, lag days 0-6). The impact of extreme heat exposure on renal ED visits increased from lag day 0 to lag day 2, weakening after the strongest effect on lag day 2.
      Table 1Basic Characteristics of the Study Population in New York State
      CharacteristicNo. (%)
      Age
       <5 y91,581 (8.2%)
       5-18 y444,034 (39.8%)
       18-65 y267,641 (24.0%)
       >65 y311,066 (27.9%)
      Sex
       Female679,307 (61.0%)
       Male435,003 (39.0%)
       Unknown12 (0.0%)
      Race
       African American216,301 (19.4%)
       Non–African American898,021 (80.6%)
      Ethnicity
       Hispanic185,815 (18.0%)
       Non-Hispanic843,920 (82.0%)
      Insurance
       Medicaid109,845 (9.9%)
       Self-pay120,696 (10.8%)
       Other insurance883,781 (79.3%)
      Kidney diseases subtype
       Acute kidney injury86,194 (7.7%)
       Chronic kidney disease9,053 (0.8%)
       Lower urinary tract calculi3,189 (0.3%)
       Nephritis and nephrosis79,754 (7.2%)
       Kidney stone182,779 (16.4%)
       Urinary tract infections497,055 (44.6%)
       Other kidney and ureter disorders12,040 (1.1%)
       Other lower urinary tract disorders244,258 (21.9%)
      Figure thumbnail gr1
      Figure 1Excess rate (%) of emergency department visits with a principal diagnosis of kidney disease on lag days 0 to 6 following extreme heat exposure.
      Figure 2 represents the excess rate of renal ED visits associated with extreme heat exposure in the transitional months and summer. Generally, the association between extreme heat exposure and renal ED visits was stronger in the transitional months (excess rates ranged from 1.8% to 5.1%) than in summer (excess rates ranged from 1.5% to 2.7%). In May, the association of extreme heat exposure with renal ED visits lasted for a whole week following the exposure, with the strongest association occurring on lag day 2 (excess rate of 5.1% [95% CI, 3.4%-6.8%]). In September, the association was strongest on lag day 0 (excess rate of 4.2% [95% CI, 2.6%-5.8%]), but a statistically significant association was lost by lag day 4. During the summer months, the association of extreme heat exposure with renal ED admissions lasted for the whole week of observation, but the magnitude never surpassed the strength of association observed in May.
      Figure thumbnail gr2
      Figure 2Excess rate (%) of emergency department visits with a principal diagnosis of kidney disease on lag days 0 to 6 after extreme heat exposure, by month (May or September) or summer season (June through August).
      The associations between extreme heat exposure and ED visits by kidney disease subtypes are presented in Table 2. We found that extreme heat exposure was significantly associated with greater ED visits due to AKI, kidney stones, UTIs, other kidney and ureter disorders, and other lower urinary tract disorders. The association was strongest for AKI and lasted for 4 days after the extreme heat exposure (excess rates monotonically decreased from 16.5% on lag day 0 to 4.3% on lag day 4). The association between extreme heat exposure and kidney stones persisted for the whole week (excess rates ranged from 3.8% on lag day 6 to 8.2% on lag day 2). The association of extreme heat exposure with UTI was transient and lasted from lag day 0 to lag day 2. The association with other kidney and ureter disorders was observed 1 day later (lag day 1) and lasted to lag day 3 (excess rates ranged from 7.0% to 9.5%), while the association with other lower urinary tract disorders began on lag day 4 and ended on lag day 6 (excess rates ranged from 1.8% to 2.9%).
      Table 2Excess Rate of Emergency Department Visits Related to Kidney Disease on Lag Days 0 to 6 After Extreme Heat Exposure, by Subtypes
      Lag DayAKICKDLower Urinary Tract CalculiNephritis and NephrosisKidney StoneUTIOther Kidney and Ureter DisordersOther Lower Urinary Tract Disorders
      016.5 (13.9 to 19.2)1.2 (−5.8 to 8.7)0.0 (−11.4 to 12.9)−0.2 (−2.7 to 2.3)4.2 (2.5 to 5.9)2.4 (1.3 to 3.5)5.3 (−1.0 to 12.0)−3.4 (−4.9 to −2.0)
      117.8 (15.1 to 20.5)−1.1 (−8.1 to 6.3)0.2 (−11.1 to 12.9)−2.1 (−4.5 to 0.4)7.3 (5.6 to 9.1)2.4 (1.3 to 3.5)7.0 (0.6 to 13.9)−3.3 (−4.8 to −1.9)
      212.6 (10.0 to 15.2)−6.4 (−13.2 to 0.9)−4.4 (−15.5 to 8.2)−0.4 (−2.8 to 2.2)8.2 (6.5 to 10.0)1.4 (0.3 to 2.5)9.5 (2.8 to 16.5)0.9 (−0.6 to 2.4)
      38.6 (6.1 to 11.2)−0.6 (−7.7 to 7.1)−3.0 (−14.4 to 9.8)−0.1 (−2.6 to 2.4)5.5 (3.8 to 7.3)0.9 (−0.2 to 2.0)8.6 (2.0 to 15.6)1.5 (0.0 to 3.0)
      44.3 (1.9 to 6.9)0.6 (−6.7 to 8.4)−3.6 (−15.1 to 9.4)0.4 (−2.1 to 2.9)5.4 (3.7 to 7.2)0.8 (−0.3 to 1.9)4.8 (−1.7 to 11.8)1.8 (0.3 to 3.3)
      51.1 (−1.4 to 3.5)−1.9 (−9.0 to 5.8)−1.1 (−12.8 to 12.2)0.5 (−2.0 to 3.0)3.9 (2.2 to 5.7)1.6 (0.5 to 2.7)1.2 (−5.1 to 8.0)1.9 (0.4 to 3.4)
      6−0.9 (−3.2 to 1.6)−3.9 (−10.9 to 3.5)−3.7 (−15.2 to 9.4)1.5 (−1.0 to 4.1)3.8 (2.1 to 5.5)0.9 (−0.2 to 2.0)3.5 (−2.9 to 10.3)2.9 (1.3 to 4.4)
      Values expressed as percentages; values in parentheses are 95% CI. Abbreviations: AKI, acute kidney injury; CKD, chronic kidney disease; UTI, urinary tract infections.
      In Table 3, we present the modification effect of sociodemographic characteristics on the associations between extreme heat exposure and renal ED visits. Age and sex modified the association significantly. Results of stratification analyses showed that older individuals (aged >65 years) experienced more renal ED visits during the short-term period after extreme heat exposure (from lag days 0 to 3). However, for participants aged 18-65 and 5-18 years, extreme heat exposure was associated with excess risk of renal ED visits during the whole week from lag day 0 to lag day 6. Notably, we found that the association was stronger for male than female individuals. Race, ethnicity, and insurance did not significantly modify the associations. To detect a potential dose dependence in the association between extreme heat exposure and renal ED visits, we assessed excess rates according to the number of days of extreme heat exposure during the previous week (Fig 3). The association between extreme heat exposure and renal ED visits showed a dose-dependent trend from 1 to 5 days of exposure in the previous week. However, the trend did not persist beyond 5 days of exposure.
      Table 3Excess Rate of Emergency Department Visits Related to Kidney Disease on Lag Days 0 to 6 After Extreme Heat Exposure, Stratified by Sociodemographic Characteristics
      Lag Day 0Lag Day 1Lag Day 2Lag Day 3Lag Day 4Lag Day 5Lag Day 6
      Age
       <5 y1.1 (−1.2 to 3.4)1.7 (−0.7 to 4.1)−0.7 (−3.0 to 1.7)−0.6 (−2.9 to 1.8)0.8 (−1.5 to 3.2)1.0 (−1.3 to 3.4)−1.5 (−3.8 to 0.9)
       5-18 y1.1 (−0.0 to 2.3)2.5 (1.3 to 3.7)3.6 (2.4 to 4.8)2.5 (1.3 to 3.6)2.2 (1.0 to 3.4)2.1 (0.9 to 3.2)2.0 (0.8 to 3.1)
       18-65 y2.8 (1.3 to 4.2)3.0 (1.5 to 4.4)4.7 (3.3 to 6.2)3.9 (2.4 to 5.3)4.1 (2.7 to 5.6)2.9 (1.5 to 4.4)3.0 (1.6 to 4.5)
       >65 y3.9 (2.6 to 5.3)3.4 (2.0 to 4.8)2.3 (0.9 to 3.6)1.8 (0.4 to 3.1)0.4 (−0.9 to 1.8)1.0 (−0.3 to 2.3)1.1 (−0.3 to 2.4)
      P for interaction0.005
      Statistically significant value (P < 0.05).
      0.5<0.001
      Statistically significant value (P < 0.05).
      0.009
      Statistically significant value (P < 0.05).
      0.001
      Statistically significant value (P < 0.05).
      0.20.008
      Statistically significant value (P < 0.05).
      Sex
       Female1.5 (0.5 to 2.4)1.7 (0.8 to 2.7)2.3 (1.4 to 3.3)1.6 (0.7 to 2.6)1.4 (0.5 to 2.4)1.6 (0.6 to 2.5)1.6 (0.6 to 2.5)
       Male3.5 (2.4 to 4.7)4.4 (3.3 to 5.6)4.4 (3.2 to 5.6)3.5 (2.4 to 4.7)3.1 (1.9 to 4.2)2.4 (1.2 to 3.6)1.9 (0.7 to 3.1)
      P for interaction0.005
      Statistically significant value (P < 0.05).
      <0.001
      Statistically significant value (P < 0.05).
      0.004
      Statistically significant value (P < 0.05).
      0.01
      Statistically significant value (P < 0.05).
      0.02
      Statistically significant value (P < 0.05).
      0.30.8
      Race
       African American1.6 (0.1 to 3.2)1.8 (0.2 to 3.4)2.9 (1.3 to 4.5)1.7 (0.1 to 3.3)1.2 (−0.4 to 2.8)1.8 (0.2 to 3.4)0.9 (−0.6 to 2.5)
       Non–African American2.4 (1.6 to 3.3)3.0 (2.2 to 3.9)3.2 (2.3 to 4.1)2.5 (1.7 to 3.4)2.3 (1.4 to 3.1)1.9 (1.0 to 2.8)1.9 (1.0 to 2.8)
      P for interaction0.30.10.80.30.20.90.3
      Ethnicity
       Hispanic1.1 (−0.5 to 2.8)3.2 (1.5 to 4.9)3.3 (1.6 to 5.0)3.1 (1.4 to 4.8)2.3 (0.6 to 4.0)2.4 (0.7 to 4.1)3.0 (1.3 to 4.8)
       Non-Hispanic2.4 (1.5 to 3.3)2.6 (1.7 to 3.5)3.1 (2.2 to 4.0)2.1 (1.2 to 3.0)2.0 (1.1 to 2.9)1.8 (0.9 to 2.7)1.5 (0.6 to 2.4)
      P for interaction0.20.60.90.40.80.70.2
      Insurance
       Medicaid3.7 (1.6 to 5.9)4.2 (2.0 to 6.4)3.4 (1.2 to 5.6)3.3 (1.1 to 5.5)3.4 (1.2 to 5.6)2.3 (0.2 to 4.5)1.0 (−1.2 to 3.2)
       Self-pay1.6 (−0.5 to 3.6)0.8 (−1.3 to 2.9)2.1 (0.1 to 4.3)1.9 (−0.2 to 4.1)1.1 (−0.9 to 3.3)0.7 (−1.4 to 2.8)1.1 (−1.0 to 3.2)
       Other insurance2.2 (1.3 to 3.1)2.9 (2.0 to 3.8)3.2 (2.4 to 4.1)2.3 (1.4 to 3.2)2.0 (1.1 to 2.9)2.0 (1.1 to 2.9)1.9 (1.0 to 2.8)
      P for interaction0.30.070.60.60.30.50.6
      Except where indicated, values expressed as percentages; values in parentheses are 95% CI.
      a Statistically significant value (P < 0.05).
      Figure thumbnail gr3
      Figure 3Excess rate (%) of emergency department visits with a principal diagnosis of kidney disease, by number of days during the past week exposed to extreme heat.

      Discussion

      Our analyses found that extreme heat exposure was significantly associated with excess ED visits related to kidney disease (excess rates of 1.7% to 3.1%) in the warm season (May through September), with a significant dose-dependent relationship. A prior systematic review and meta-analysis revealed that high temperature was associated with a 30% increase in kidney disease morbidity; specifically, heat stress or strain increased the risk of kidney disease by 16%.
      • Lee W.S.
      • Kim W.S.
      • Lim Y.H.
      • Hong Y.C.
      High temperatures and kidney disease morbidity: a systematic review and meta-analysis.
      Similarly, in 2017 Ogbomo et al
      • Ogbomo A.S.
      • Gronlund C.J.
      • O’Neill M.S.
      • Konen T.
      • Cameron L.
      • Wahl R.
      Vulnerability to extreme-heat-associated hospitalization in three counties in Michigan, USA, 2000-2009.
      found that hospitalization rates for kidney disease in Michigan increased significantly (OR of 1.14) during extreme-heat periods (defined as daily mean temperature ≥95th percentile). A retrospective cohort study in South Korea found that temperatures above the 90th and 95th percentiles were associated with an overall cumulative relative risk of 1.18-1.23 and 1.23-1.29, respectively, for ED admissions related to kidney disease compared to minimum morbidity percentiles.
      • Kim E.
      • Kim H.
      • Kim Y.C.
      • Lee J.P.
      Association between extreme temperature and kidney disease in South Korea, 2003-2013: Stratified by sex and age groups.
      A time series analysis in Washington state found a 1.57-fold greater risk of hospital admissions related to kidney disease on a 99th percentile (36.2°C) heat day compared to days without extreme heat.
      • Isaksen T.B.
      • Yost M.G.
      • Hom E.K.
      • Ren Y.
      • Lyons H.
      • Fenske R.A.
      Increased hospital admissions associated with extreme-heat exposure in King County, Washington, 1990-2010.
      In addition, in 2017 Borg et al
      • Borg M.
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      • Williams S.
      • McDonald S.
      The impact of daily temperature on renal disease incidence: an ecological study.
      in Australia and Chen et al
      • Chen T.
      • Sarnat S.E.
      • Grundstein A.J.
      • Winquist A.
      • Chang H.H.
      Time-series analysis of heat waves and emergency department visits in Atlanta, 1993 to 2012.
      in Atlanta found that a 1°C increase in daily minimum temperature or heatwave average temperature was associated with a 1.003 to 1.009 increase in ED admissions for kidney disease overall and selected subtypes.
      In general, our study found a weaker association between extreme heat exposure and renal ED visits than in previous studies, which may be due to differences in study design, exposure definitions, and health outcome categorization. For example, we defined extreme heat exposure as when the daily temperature exceeded the 90th percentile temperature of that month over the entire study period in the county. Because there is no standard definition, researchers have previously elected to use the 90th, 95th, 97th, and 99th percentiles to define extreme heat exposure.
      • Lee W.S.
      • Kim W.S.
      • Lim Y.H.
      • Hong Y.C.
      High temperatures and kidney disease morbidity: a systematic review and meta-analysis.
      ,
      • Ogbomo A.S.
      • Gronlund C.J.
      • O’Neill M.S.
      • Konen T.
      • Cameron L.
      • Wahl R.
      Vulnerability to extreme-heat-associated hospitalization in three counties in Michigan, USA, 2000-2009.
      ,
      • Isaksen T.B.
      • Yost M.G.
      • Hom E.K.
      • Ren Y.
      • Lyons H.
      • Fenske R.A.
      Increased hospital admissions associated with extreme-heat exposure in King County, Washington, 1990-2010.
      ,
      • Kim E.
      • Kim H.
      • Kim Y.C.
      • Lee J.P.
      Association between extreme temperature and kidney disease in South Korea, 2003-2013: Stratified by sex and age groups.
      Unsurprisingly, using a higher percentile to define extreme heat exposure tends to yield stronger associations.
      • Ogbomo A.S.
      • Gronlund C.J.
      • O’Neill M.S.
      • Konen T.
      • Cameron L.
      • Wahl R.
      Vulnerability to extreme-heat-associated hospitalization in three counties in Michigan, USA, 2000-2009.
      Furthermore, we exclusively focused on ED visits, but most prior studies used hospitalizations,
      • Ogbomo A.S.
      • Gronlund C.J.
      • O’Neill M.S.
      • Konen T.
      • Cameron L.
      • Wahl R.
      Vulnerability to extreme-heat-associated hospitalization in three counties in Michigan, USA, 2000-2009.
      ,
      • Isaksen T.B.
      • Yost M.G.
      • Hom E.K.
      • Ren Y.
      • Lyons H.
      • Fenske R.A.
      Increased hospital admissions associated with extreme-heat exposure in King County, Washington, 1990-2010.
      which included more severe kidney disease cases, and thus the risks were higher. The real impacts of extreme heat exposure on kidney disease could be larger than what we found in the current study because we only included some severe cases (ED visits) and omitted more moderate cases as well as other severe cases where patients were hospitalized cases. To our knowledge, we are the first to examine dose dependence in the association of extreme heat exposure with renal ED visits. The limited number of instances in which extreme heat persisted for 6-7 days may explain why excess rates of renal ED visits were not significantly associated with having 6 or 7 extreme heat days in the week preceding a visit.
      We also found that the association of extreme heat exposure with renal ED visits was detectable by lag day 0, was greatest on lag day 2, and remained statistically significantly for the whole week. These associations were also significantly stronger in transitional months, especially in May, than in summer. Similar to our results, in 2017 Ogbomo et al
      • Ogbomo A.S.
      • Gronlund C.J.
      • O’Neill M.S.
      • Konen T.
      • Cameron L.
      • Wahl R.
      Vulnerability to extreme-heat-associated hospitalization in three counties in Michigan, USA, 2000-2009.
      reported that extreme heat exposure had the strongest association with kidney disease on the day of exposure and remained statistically significant for 1 to 5 subsequent days. Although the association of extreme heat exposure and kidney disease during transitional months and in summer has seldom been studied separately, our recent research found stronger associations between extreme heat exposure and pregnancy complications during transitional months than in summer.
      • Qu Y.
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      • Ryan I.
      • et al.
      Ambient extreme heat exposure in summer and transitional months and emergency department visits and hospital admissions due to pregnancy complications.
      Kidney disease patients may be more prone to the effect of extreme heat exposure in transitional months due to their potential lack of physical and behavioral preparation or adaptations to sudden temperature changes (eg, not using fans or air conditioners, more outdoor activities, and no early heat warning systems).
      • Lin S.
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      The impact of “unseasonably” warm spring temperatures on acute myocardial infarction hospital admissions in Melbourne, Australia: a city with a temperate climate.
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      Changes in the effects of heat on mortality among the elderly from 1998-2010: results from a multicenter time series study in Italy.
      The specific kidney disease subtypes significantly associated with extreme heat exposure in this study included AKI, kidney stones, UTIs, and other kidney and lower urinary tract disorders. AKI had the highest risks (excess rates ranged from 4.3% to 16.5%), similar to previous studies.
      • Hopp S.
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      Medical diagnoses of heat wave-related hospital admissions in older adults.
      ,
      • Isaksen T.B.
      • Yost M.G.
      • Hom E.K.
      • Ren Y.
      • Lyons H.
      • Fenske R.A.
      Increased hospital admissions associated with extreme-heat exposure in King County, Washington, 1990-2010.
      ,
      • Bobb J.F.
      • Obermeyer Z.
      • Wang Y.
      • Dominici F.
      Cause-specific risk of hospital admission related to extreme heat in older adults.
      ,
      • Chen T.
      • Sarnat S.E.
      • Grundstein A.J.
      • Winquist A.
      • Chang H.H.
      Time-series analysis of heat waves and emergency department visits in Atlanta, 1993 to 2012.
      Our results were consistent with previous studies indicating that elevated temperature was mainly associated with AKI,
      • Borg M.
      • Bi P.
      • Nitschke M.
      • Williams S.
      • McDonald S.
      The impact of daily temperature on renal disease incidence: an ecological study.
      ,
      • Hopp S.
      • Dominici F.
      • Bobb J.F.
      Medical diagnoses of heat wave-related hospital admissions in older adults.
      ,
      • Isaksen T.B.
      • Yost M.G.
      • Hom E.K.
      • Ren Y.
      • Lyons H.
      • Fenske R.A.
      Increased hospital admissions associated with extreme-heat exposure in King County, Washington, 1990-2010.
      ,
      • Bobb J.F.
      • Obermeyer Z.
      • Wang Y.
      • Dominici F.
      Cause-specific risk of hospital admission related to extreme heat in older adults.
      • Kim E.
      • Kim H.
      • Kim Y.C.
      • Lee J.P.
      Association between extreme temperature and kidney disease in South Korea, 2003-2013: Stratified by sex and age groups.
      • Kim S.E.
      • Lee H.
      • Kim J.
      • et al.
      Temperature as a risk factor of emergency department visits for acute kidney injury: a case-crossover study in Seoul, South Korea.
      ,
      • Chen T.
      • Sarnat S.E.
      • Grundstein A.J.
      • Winquist A.
      • Chang H.H.
      Time-series analysis of heat waves and emergency department visits in Atlanta, 1993 to 2012.
      ,
      • Honda T.
      • Manjourides J.
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      Daily ambient temperature is associated with biomarkers of kidney injury in older Americans.
      • Johnson R.J.
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      Climate change and the kidney.
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      kidney stones,
      • Ross M.E.
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      Assessment of the combination of temperature and relative humidity on kidney stone presentations.
      ,
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      and UTIs.
      • Borg M.
      • Bi P.
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      • Williams S.
      • McDonald S.
      The impact of daily temperature on renal disease incidence: an ecological study.
      ,
      • Bobb J.F.
      • Obermeyer Z.
      • Wang Y.
      • Dominici F.
      Cause-specific risk of hospital admission related to extreme heat in older adults.
      The exact biological mechanisms by which extreme heat exposure contributes to kidney disease are unclear. However, it is increasingly evident that, although the kidney has a major role in protecting the body from heat stress, it is also a target for heat stress–related injury.
      • Johnson R.J.
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      Heat stress can lead to increased core temperatures, dehydration, hypovolemia, blood hyperosmolality, and electrolyte imbalance, which can lead to AKI
      • Kim E.
      • Kim H.
      • Kim Y.C.
      • Lee J.P.
      Association between extreme temperature and kidney disease in South Korea, 2003-2013: Stratified by sex and age groups.
      ,
      • Knowlton K.
      • Rotkin-Ellman M.
      • King G.
      • et al.
      The 2006 California heat wave: impacts on hospitalizations and emergency department visits.
      and the formation of kidney stones and renal colic,
      • Borghi L.
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      Hot occupation and nephrolithiasis.
      and can facilitate the progression of localized infections to sepsis and shock.
      • Bobb J.F.
      • Obermeyer Z.
      • Wang Y.
      • Dominici F.
      Cause-specific risk of hospital admission related to extreme heat in older adults.
      The adverse effects of vasopressin
      • Bankir L.
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      and the fructokinase system
      • Roncal Jimenez C.A.
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      Fructokinase activity mediates dehydration-induced renal injury.
      on renal tubules from extreme heat exposure can also lead to AKI.
      • Kim E.
      • Kim H.
      • Kim Y.C.
      • Lee J.P.
      Association between extreme temperature and kidney disease in South Korea, 2003-2013: Stratified by sex and age groups.
      Meanwhile, increased circulating markers of inflammation and oxidative stress associated with elevated ambient temperatures may contribute to kidney disease exacerbation.
      • Wu S.
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      • Pan L.
      • et al.
      Ambient temperature and cardiovascular biomarkers in a repeated-measure study in healthy adults: a novel biomarker index approach.
      • Halonen J.I.
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      Associations between outdoor temperature and markers of inflammation: a cohort study.
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      • Devlin R.B.
      • et al.
      Interaction effects of temperature and ozone on lung function and markers of systemic inflammation, coagulation, and fibrinolysis: a crossover study of healthy young volunteers.
      Moreover, the medications commonly used to treat kidney disease, such as diuretics, may compromise thermoregulatory responses and aggravate syndromes related to extreme heat exposure.
      • Ogbomo A.S.
      • Gronlund C.J.
      • O’Neill M.S.
      • Konen T.
      • Cameron L.
      • Wahl R.
      Vulnerability to extreme-heat-associated hospitalization in three counties in Michigan, USA, 2000-2009.
      Our study found varied associations among different populations: older adults (aged >65 years) and male individuals had slightly higher extreme heat exposure–associated renal ED risks than other groups. Previous studies found that the groups vulnerable to extreme heat exposure varied depending on health outcomes, geographic regions, and type of diseases. However, most studies agree that older adults are most vulnerable to the renal effects of extreme heat exposure.
      • Ogbomo A.S.
      • Gronlund C.J.
      • O’Neill M.S.
      • Konen T.
      • Cameron L.
      • Wahl R.
      Vulnerability to extreme-heat-associated hospitalization in three counties in Michigan, USA, 2000-2009.
      ,
      • Gronlund C.J.
      • Zanobetti A.
      • Wellenius G.A.
      • Schwartz J.D.
      • O’Neill M.S.
      Vulnerability to renal, heat and respiratory hospitalizations during extreme heat among U.S. elderly.
      ,
      • Kravchenko J.
      • Abernethy A.P.
      • Fawzy M.
      • Lyerly H.K.
      Minimization of heatwave morbidity and mortality.
      Similar to our results, previous studies also found a higher risk of hospitalization and ED visits due to kidney disease in male than in female persons.
      • Heidari L.
      • Winquist A.
      • Klein M.
      • O’Lenick C.
      • Grundstein A.
      • Ebelt Sarnat S.
      Susceptibility to heat-related fluid and electrolyte imbalance emergency department visits in Atlanta, Georgia, USA.
      ,
      • Sun S.
      • Weinberger K.R.
      • Nori-Sarma A.
      • et al.
      Ambient heat and risks of emergency department visits among adults in the United States: time stratified case crossover study.
      Our study has several strengths. First, we included a large sample size of over 1 million ED visits related to kidney disease. Furthermore, we compared the risks for renal ED visits by different lag days and between summer and transitional months. Additionally, our detailed classification of kidney disease enabled us to assess the association of extreme heat exposure with specific subtypes as well as by sociodemographic characteristics. Finally, we used a case-crossover design, which controls most interindividual confounders.
      However, several limitations should be considered when interpreting our results. First, the sample size may be a concern when evaluating the associations of extreme heat exposure with kidney disease subtypes; however, we collected multiple years of data from one of the largest states in the United States. Second, confounders may be a concern, but we used the case-crossover design. Each case was self-compared regarding the exposure to control all individually inherited confounders. We further controlled for air pollutants, humidity, and various time-related variables (holidays). Third, we exclusively considered ED visits in our case selection, limiting our analysis to the severe cases of kidney disease. However, this may be less of a problem for the kidney diseases studied because patients with AKI, kidney stones, and UTI may be more likely to present to an ED to get urgent medical care than other patients. Fourth, we did not evaluate individual exposure to heat or access to air conditioning. Future studies with more accurate measurements of individual exposures are needed to verify our findings. Finally, cases within a county were assigned the same temperature values for a specific day due to data limits, which likely resulted in exposure misclassification. However, we assigned weather data at the level of county (n = 62 in New York), which is more granular than the data from limited monitoring sites (n =14 in New York). The temperature variance across New York state was limited in our study (SD of 4.17-9.74 °F from month to month; <10% change). Thus, we assumed that the ambient temperature was not likely to change each day significantly within a much smaller area (ie, within a county). In addition, using a case-crossover design, we compared extreme heat exposure on the case and control days; exposure was compared at different times but in the same residence. Therefore, if exposure misclassification occurs, the bias would be nondifferential between the case and control days.
      • Zhang W.
      • Lin S.
      • Hopke P.K.
      • et al.
      Triggering of cardiovascular hospital admissions by fine particle concentrations in New York state: before, during, and after implementation of multiple environmental policies and a recession.
      ,
      • Zeger S.L.
      • Thomas D.
      • Dominici F.
      • et al.
      Exposure measurement error in time-series studies of air pollution: concepts and consequences.
      In conclusion, extreme heat exposure was significantly associated with increased risk of ED visits related to multiple kidney disease types and displayed a dose-dependent relationship. This association lasted a week after exposure and was stronger during transitional months (especially May) than in summer. The kidney disease subtype showing the strongest association was AKI, although kidney stones and UTIs also showed strong associations. Age and sex may modify observed associations. The association of extreme heat exposure and kidney disease was observed not only in summer but also in transitional months.

      Article Information

      Authors’ Full Names and Academic Degrees

      Yanji Qu, MD, PhD, Wangjian Zhang, MD, PhD, Asi-Yahola M. Boutelle, MS, MDS, Ian Ryan, PhD, Xinlei Deng, PhD, Xiaoqing Liu, MD, Shao Lin, MD, PhD.

      Authors’ Contributions

      Study design/concept: YJQ, SL, AYMB; data acquisition/analysis: WJZ, YJQ, XLD, IR; mentorship and data interpretation: SL, XQL, WJZ, YJQ. Each author contributed important intellectual content during manuscript drafting or revision and agreed to be personally accountable for the individual’s contributions and to ensure that questions pertaining to the accuracy or integrity of any portion of the work, even one in which the author was not directly involved, are appropriately investigated and resolved, including with documentation in the literature if appropriate.

      Support

      This work was supported by the NYS Energy Research and Development Authority (grant no. 137487), the National Institute of Environmental Health Science (grant no. 1R15ES02800001A1), the United States Environmental Protection Agency (grant no. 83563602), and the National Institutes of Health (grant no. 1R01AG070949-01A1). The funding sources were not involved in the study design, data collection, analysis, reporting, conduction, or preparation of this article, nor the decision to submit it for publication.

      Financial Disclosure

      The authors declare that they have no relevant financial interests.

      Peer Review

      Received February 5, 2022. Evaluated by 2 external peer reviewers, with direct editorial input from a Statistics/Methods Editor, an Associate Editor, and the Editor-in-Chief. Accepted in revised form September 5, 2022.

      Supplementary Material

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