Effects of Air Conditioning-Induced Humidity Reduction on Glomerular Filtration Rate in Healthy Adults

Introduction

In the warm and humid subtropical climate of southern China, air conditioners have been widely installed in most workplaces and homes in an attempt to improve workplace productivity and thermal comfort [1]. Most people spend 80–90% of the day indoors. Insensible evaporative water loss in healthy adults can reach 0.3 L/day through the skin and approximately 0.35 L/day through the respiratory tract while at rest [2]. The lower air temperature and relative humidity in air-conditioned rooms can accelerate the rate of water loss via the respiratory tract and skin [3], causing chronic moisture loss in the body.

The rennin-angiotensin system (RAS) is one of the important humoral regulation systems in the human body [4]. When there is massive blood loss, body fluid loss, and blood pressure drop, the RAS is activated to release renin, which activates angiotensin II (Ang II) through a series of enzymatic reactions [5]. Ang II is the most active biological product of the RAS. Recently, some studies have demonstrated that inappropriate activation of the intra-renal Ang II causes changes in renal function and its structure, leading to renal injury [6,7].

Glomerular filtration rate (GFR) is widely accepted as the best overall indicator of kidney function [8,9]. Estimated GFR (eGFR), calculated from serum creatinine using validated equations, allows non-invasive evaluation of renal status in large populations. This study employed both cross-sectional and self-controlled designs to assess the short-term effect of indoor humidity reduction due to air conditioning on eGFR in a large cohort of healthy adults in southern China.

Material and Methods

ETHICAL CONSIDERATIONS:

This study was approved by the regional ethics committee of the Second Affiliated Hospital of Nanchang University (approval number (2016) 011). All procedures followed were in accordance with the ethics standards of the responsible committee on human experimentation (institutional and national) and with the Helsinki Declaration of 1975, as revised in 2008. Informed consent was obtained from all patients included in the study.

STUDY POPULATION:

The study enrolled healthy adult participants aged 20 years and older of Han ethnicity residing in southern China. The inclusion criteria were: (a) age ≥20 years; (b) normal blood pressure (systolic blood pressure <140 mmHg and diastolic blood pressure <90 mmHg); (c) fasting blood glucose <7.0 mmol/L; (d) normal results in urine microscopy, complete blood count, and routine biochemical tests; and (e) no history of clinical conditions indicative of renal impairment.

Exclusion criteria were: (a) obesity, defined as a body mass index (BMI) ≥28 kg/m²; (b) history of diabetes mellitus, hypertension, cardiovascular disease, cerebrovascular disease, liver dysfunction, infectious diseases, malignancies, kidney disease, gout, or other severe systemic conditions; and (c) individuals belonging to ethnic groups other than Han.

STUDY METHOD:

From July 2017 to September 2020, we used a multi-stage, stratified sampling method to include 20 930 healthy adults in southern China. The details of the study participants and experimental methods have been previously described [10]. Among them, when the mean daily maximum outdoor temperature was 27–33°C and the monthly mean humidity was 77–83%, we included 12 671 people. Healthy participants were defined as individuals without a history of chronic diseases (eg, hypertension, diabetes, cardiovascular disease, kidney or liver disease), and without abnormal findings on physical examination or laboratory testing (including blood pressure, serum creatinine, liver function, and urinalysis). Through the questionnaire survey, it was found that 7120 people used air conditioning and 5551 people used electric fans. All participants completed a standardized questionnaire, which collected detailed information on dietary habits (such as daily water and protein intake), smoking and alcohol consumption, physical activity level, occupation, and family history of chronic kidney disease (CKD). This information was used to evaluate potential lifestyle and genetic confounders. General demography, personal health, and family history of all participants were recorded using a questionnaire, and blood pressure was recorded using a portable blood pressure monitoring system. Temperature and relative humidity were measured using calibrated digital sensors (accuracy ±2% RH), which were placed beside the participant’s bed from 22: 00 to 06: 00 to continuously record overnight conditions. Morning blood and urine samples were collected and sent to the central laboratory of the Second Affiliated Hospital of Nanchang University for testing. Estimated glomerular filtration rate (eGFR) was calculated using both the MDRD and CKD-EPI equations. The eGFR values recorded under each condition (air conditioning or fan use) were considered the baseline value for each participant within that group or period [10].

Among those who used electric fans, we randomly selected 400 people for the self-controlled study. Thirty-eight people withdrew from the experiment due to personal reasons, and 362 participants were ultimately included. The self-controlled study design consisted of 8 home visits in June and July 2020, with 1 visit per week. Participants were instructed to drink 2000 mL of water during the day, to rest from 22: 00 to 06: 00 the next morning, to avoid drinking water within 1 hour before bedtime, and to empty their bladders before sleep. In June, participants used fans and open windows for ventilation, and the average indoor conditions were 31°C (range: 29–33°C) and 80% relative humidity (range: 77–83%). In July, participants were required to use window-mounted air conditioners set at 27°C, with windows and doors closed. This resulted in a mean room humidity of 48% (range: 44–52%), with temperature consistently maintained at 27°C. During each home visit, bedside digital sensors recorded overnight temperature and humidity continuously from 22: 00 to 06: 00. Blood pressure, blood samples, and urine samples were collected in the morning. Each participant experienced 4 weeks of fan exposure followed by 4 weeks of air conditioning exposure, and served as their own control. Each bedroom was approximately 60 m³ in volume.

METHOD OF GFR ESTIMATION:

Serum creatinine levels were determined using Jaffe’s kinetic method with a Hitachi 7600 automated analyzer (Hitachi, Tokyo, Japan). Estimated glomerular filtration rate (eGFR) was calculated using 2 equations: the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation, and a modified version of the Modification of Diet in Renal Disease (MDRD) equation, which was specifically adapted based on data from Chinese patients with chronic kidney disease. The eGFR_MDRD was calculated according to the formula:

The eGFR_CKD-EPI was calculated according to the formula:

where Scr is serum creatinine concentration (in mg/dL) and age in years.

STATISTICAL ANALYSIS:

The main objective of the analysis was to assess the change in eGFR following the air conditioning intervention. The results are expressed as mean±standard deviation. The Kolmogorov-Smirnov (K-S) test was used to evaluate the distribution of the variable. The Mann-Whitney U test and Wilcoxon signed ranks test were used to determine whether there were significant differences in eGFR values and indoor humidity between the intervention groups. The correlation between variables was measured using Pearson’s correlation test. All statistical analyses were carried out using SPSS24.0 software, and P value <0.05 was considered to be statistically significant.

Results

BASELINE CHARACTERISTICS OF PARTICIPANTS:

A total of 12 671 healthy Han adults in southern China were enrolled, including 7120 in the air conditioning group (ACG) and 5551 in the fan group (FG). The mean age of the participants was 41.8±12.3 years, and the mean BMI was 22.5±2.5 kg/m2. Table 1 summarizes the baseline characteristics of the included population.

ENVIRONMENTAL HUMIDITY DIFFERENCES BETWEEN GROUPS:

After 8 hours of rest at night, in the early morning the mean humidity of the air conditioning group was 48% (range, 44–52%) and in the fan group it was 80% (range, 77–83%), with a significant difference (P<0.001).

CROSS-SECTIONAL ANALYSIS OF EGFR AND CLINICAL PARAMETERS:

In the cross-sectional study, the eGFRMRDD was 109.1±17.0 mL/min per 1.73 m2 in the fan group. With the decrease of indoor humidity, the eGFRMRDD was 105.3±16.5 mL/min per 1.73 m2 in the air conditioning group, and the difference between the 2 groups was statistically significant (P<0.001) (Table 2). Table 3 shows the comparison of the relevant clinical data between the 2 groups after the intervention. We found that with the decrease of humidity in air-conditioned rooms, the loss of body water is accelerated, and people’s thirst increases. It can also cause body mass index (BMI) and blood pressure (BP) to drop, and serum creatinine (SCr) to increase, but it has little effect on fasting blood glucose, blood lipid, uric acid, and liver function.

SELF-CONTROLLED STUDY: EFFECT OF HUMIDITY REDUCTION ON EGFR:

We enrolled 362 healthy people for the self-control study, including 230 females (63.5%) and 132 males (36.5%). The mean age was 42.9±11.7 years. After the intervention, in the self-control study group, the fan group eGFRMDRD was 108.4±15.7 mL/min per 1.73 m2, and the eGFRMDRD in the air conditioning group was 104.6±15.8 mL/min per 1.73 m2, and the difference was statistically significant (P<0.001). Table 2 shows the eGFR calculated by the CKD-EPI formula, and the eGFR differences were statistically significant (P<0.001).

CHANGES IN CLINICAL PARAMETERS IN THE SELF-CONTROLLED STUDY:

As illustrated in Table 4, air-conditioned environments were associated with a mild decrease in BMI and blood pressure, and a significant increase in serum creatinine. However, no substantial changes were observed in fasting glucose, lipid levels, uric acid, or liver enzymes.

Discussion

LIMITATIONS:

This study explored the impact of air-conditioned environments on eGFR in healthy individuals, but it has several limitations. Further longitudinal studies are needed to evaluate the long-term effects of air-conditioned living environments on renal function. First, we hypothesize that humidity reduction may be associated with potential activation of the renin-angiotensin system (RAS), but this mechanism requires further validation. Although our findings are consistent with RAS activation, we did not measure any direct biomarkers such as plasma renin activity, angiotensin II, or aldosterone levels. This is a critical limitation. Future studies should incorporate these physiological and molecular markers to directly assess RAS involvement. Second, the mean difference in eGFR (~3.8 mL/min/1.73 m² or ~3.5%) is relatively modest and falls within the known intra-individual variation (~10%) for creatinine-based eGFR equations. While statistically significant due to the large sample size, it remains unclear whether this subtle change reflects actual renal impairment or merely normal physiological fluctuation. The clinical relevance of this finding warrants further investigation. Third, although we attempted to minimize temperature variability by fixing air conditioning temperature at 27°C and conducting both study phases during the summer, we acknowledge that temperature and humidity effects cannot be fully disentangled. The observed physiological changes are likely more attributable to humidity reduction, given the relatively modest temperature difference (31°C vs 27°C). Fourth, we relied on indirect indicators such as serum creatinine, blood pressure, and BMI to reflect hydration status. The absence of direct hydration markers (eg, urine osmolality, plasma osmolality) is a limitation. Future studies should incorporate these objective markers to more precisely characterize the role of dehydration in renal function changes. Fifth, although the self-controlled design reduces between-subject variability, it remains susceptible to confounding factors such as regression to the mean, within-subject fluctuations, and behavioral adaptation. These design issues may affect the internal validity of our findings and should be addressed in future randomized or longitudinal studies. Finally, the study did not evaluate multiple humidity or temperature gradients. Future work should involve controlled experiments under various environmental conditions to more precisely assess the independent and interactive effects of temperature and humidity on eGFR.

Conclusions

In this large cohort of healthy Han adults in southern China, exposure to reduced indoor humidity in air-conditioned environments was associated with a modest but statistically significant decrease in estimated glomerular filtration rate (eGFR). Both cross-sectional and self-controlled analyses demonstrated that lower humidity was linked to increased serum creatinine and reduced eGFR, accompanied by small declines in body mass index (BMI) and blood pressure. These findings suggest that dry indoor environments may contribute to subclinical dehydration and transient alterations in renal function, even among healthy individuals. Other metabolic parameters, including fasting glucose, lipid profile, uric acid, and liver function, remained stable. Our results highlight the potential physiological impact of indoor humidity and underscore the importance of considering environmental conditions in kidney health management.