Quantitative and Qualitative Evaluation of the Sweating Function of the Human Earlobe for Heat Stress Monitoring

Abstract

Background

This study evaluated thermoregulatory sweating at the earlobe under heat exposure as an initial step toward exploring its potential as a technically interesting candidate site for skin temperature monitoring in occupational heat stress assessment.

Methods

Two experiments were conducted in healthy male participants. Sweat rates on the medial and lateral earlobe and forearm were quantitatively measured using ventilated capsule perspiration meters at 25, 30, and 35 °C ambient temperatures with 50% relative humidity. Qualitative sweating was assessed using Minor's iodine-starch test at 40°C ambient temperature and 50% relative humidity.

Results

Earlobe sweat rates were consistently and significantly lower than forearm values, with minimal temperature-related variation. No iodine-starch reaction was observed on the earlobe, whereas sweating was detected on the auricle and face.

Conclusion

The earlobe appeared to exhibit negligible thermoregulatory sweating under the tested conditions. Given the low likelihood of sweating-related interference, the earlobe may warrant further investigation as a technically interesting candidate site for future validation studies of skin temperature monitoring in occupational heat stress assessment.

1. Introduction

Global warming is accelerating worldwide. The global average temperature in 2024 exceeded the pre-industrial baseline (1850–1900) by approximately 1.55°C, and the period from 2015 to 2024 was the warmest decade on record [1].

Japan, located in the mid-latitudes of the Northern Hemisphere, has experienced a rate of temperature increase exceeding the global average. In 2025, Japan recorded its highest daily maximum temperature on record, 41.8 °C [2].

In 2024, occupational casualties caused by heat stroke in Japan reached 1,257, the highest number since records began in 2005 [3]. The Basic Guidelines for the Prevention of Heat Stroke in the Workplace were revised in 2025 to strengthen preventive measures, encouraging the use of wearable devices to assess heat stress risk [4]. However, continuous measurement of physiological parameters such as core body temperature, sweat rate, and heart rate during work remains difficult.

Recent advances in wireless technology have enabled wearable devices for remote, real-time physiological monitoring. ISO 9886:2004 recommends core body temperature measurement to assess thermal strain [5]. Core body temperature can be estimated by measuring rectal, esophageal, abdominal, oral, tympanic membrane, external auditory canal, or urinary temperatures. Nevertheless, real-time measurements at these sites remain technically challenging during work. Consequently, most wearable devices estimate thermal strain primarily from heart rate. Yet heart rate is influenced by factors such as age, sex, ethnicity, posture, physical fitness, psychological stress, smoking habits, alcohol consumption, and obesity [6], making it less reliable than body temperature for evaluating thermal strain.

Skin temperature can be easily measured even during work; however, it is influenced by heat exchange at the skin surface through conduction, convection, radiation, and evaporation. For example, even when core temperature is rising during exercise, skin temperature may conversely decrease due to evaporative cooling from sweating. It is also affected by changes in skin blood flow and by variations in the temperature of arterial blood reaching specific body regions [5]. These factors may limit the reliability of skin temperature as an indicator of heat stress.

In addition, skin temperature is a thermal parameter that is difficult to measure accurately. When a temperature sensor is attached to the skin surface for measuring skin temperature, the lateral sides of the sensor are exposed to ambient air, which may cause the device to register the surrounding air temperature. Furthermore, sweat can enter the space between the skin and the sensor, potentially causing the measurement to reflect the temperature of the sweat.

To address these measurement challenges, we explored the earlobe as a candidate site for preliminary investigation of skin temperature monitoring. The rationale for this choice is threefold: (1) it contains abundant arteriovenous anastomoses, (2) it minimally interferes with work activities, and (3) it is unlikely to produce sweat.

An arteriovenous anastomosis (AVA) is a direct connection between an artery and a vein that bypasses capillaries. AVAs can rapidly increase blood flow to the body surface and facilitate heat dissipation during heat exposure without additional metabolic load. When a person is nude and at rest, most AVAs are open when the ambient temperature exceeds 36 °C [7]. Since AVAs respond quickly to heat and reflect arterial blood temperature close to core temperature, measuring skin temperature at AVA-rich sites may be worth exploring for future heat-stress monitoring studies.

AVAs are predominantly distributed in glabrous skin regions close to the body surface, such as the palms, fingertips, nail beds, nail folds, soles, toes, auricles, nasal tip, cheeks, and periorbital areas. Among these, the AVAs in the palms and soles are highly sensitive to temperature and serve as major sites for thermoregulation [8].

In occupational settings, attaching temperature sensors to the palms or soles is impractical, as it interferes with work tasks and walking. The face is also prone to heavy sweating, which can cause sweat to accumulate between the sensor and the skin. Given these limitations, we considered the earlobe as the most appropriate AVA-rich site, as it minimizes interference with work activities and reduces discomfort during sensor use.

Sweating can affect skin temperature measurements at the sensor site. Of the two types of sweat glands, eccrine glands are responsible for thermoregulatory sweating through evaporative cooling and are distributed throughout most of the human body, except for regions such as the lips and external auditory canal [9]. Kuno et al. measured sweat secretion at 20 body sites under high-temperature conditions and reported that although thermal sweating occurs widely, its magnitude varies significantly by region [10]. Higher sweat rates were observed on the forehead, neck, lower back, hands, and forearms, whereas lower rates were found on the palms and soles. Sweating on the head is most prominent in the region extending from the forehead and nasal tip to the lower eyelids and upper lip, while other parts exhibit markedly less secretion. Although the earlobe was not directly measured, sweat secretion in this region is likely minimal.

Minor V qualitatively evaluated whole-body sweating using the iodine-starch test and reported that, among facial regions, sweating was abundant on the forehead and around the nose and mouth, but significantly reduced on the nasal tip, cheeks, eyelids, and especially the auricle [11]. However, no description was made as to whether sweating occurs specifically on the earlobe.

We considered that earlobe skin temperature may be suitable for monitoring heat stress. Ideally, this assumption should be validated by examining the correlation between earlobe skin temperature and core temperature. As a preliminary step toward such validation, we first sought to determine whether sweating occurs at the earlobe, given that local sweating may compromise the reliability of temperature measurement. However, to our knowledge, no previous study has comprehensively assessed the sweating function of the human earlobe under thermal stress. Therefore, in the present study, to verify whether thermally induced sweating at the earlobe is minimal or absent, we quantitatively and qualitatively evaluated the sweating response at the earlobe under controlled thermal conditions.

2. Materials and methods

2.1. Measurement of sweat rate

2.1.1. Experimental protocol

We quantitatively measured sweat rate on the medial and lateral surfaces of the earlobe using ventilated capsule perspiration meters. For comparison, sweat rate on the flexor surface of the forearm was simultaneously measured under the same conditions.

The experiments were conducted between January 2023 and January 2024 in the artificial climate chambers (TBL-15FW5CPX or TBR-8E20W0P2T, Espec, Osaka, Japan) at the Shared-Use Research Center of the University of Occupational and Environmental Health, Japan.

Fifteen healthy male participants participated in this experiment. Written informed consent was obtained after they received a written explanation of the study procedure. Participants were screened by interviews to confirm the absence of current medical treatment, relevant medical history, smoking, or alcohol consumption. They were instructed to refrain from strenuous exercise and alcohol from the day before each session.

Experiments were conducted at ambient temperatures of 25, 30, and 35 °C with relative humidity maintained at 50%. All participants completed all conditions in randomized order, with at least 48 hours between sessions.

On a separate day, participants performed submaximal exercise on a bicycle ergometer (T.K.K.3070, Takei Scientific Instruments, Niigata, Japan) at an ambient temperature of 25 °C and 50% relative humidity. Expired gas was analyzed using an automatic respiratory gas analyzer (ARCO-2000, Arco System Inc., Chiba, Japan) with a breath-by-breath method to estimate maximal oxygen uptake (VO₂max). The workload corresponding to 50% of the heart rate reserve (HRR) was calculated to determine exercise intensity.

2.1.2. Experimental procedure

Fig. 1 (A) presents the experimental procedure. Participants consumed a 400-kcal meal (Calorie Mate BLOCK, Otsuka Pharmaceutical, Tokyo, Japan) and 500 ml of water (CRYSTAL GEYSER, Crystal Geyser Water Company, Calistoga, CA, USA) within one hour before the experiment. Fluid intake was restricted during the experimental session.

Fig. 1.

(A) Protocol for Experiment 1. (B) Placement of Sweat Collection Probes: Lateral and medial aspects of the earlobe, and the flexor surface of the forearm. Ta: ambient temperature, RH: relative humidity, HRR: heart rate reserve.

They were dressed to simulate the typical attire of workers in occupational settings, including a long-sleeved summer work uniform (VES2360 and VES360, Midori Anzen, Tokyo, Japan), a short-sleeved undershirt and socks made of 100% cotton, and a helmet.

In Room A at an ambient temperature of 28 °C and a relative humidity of 50%, participants were fitted with ventilated capsule perspiration probes (POS-02, SKINOS, Ueda, Japan) at the flexor surface of the left forearm and the medial and lateral earlobe. They then remained seated for 10 minutes to stabilize sweat rate.

Participants were transferred to Room B set at ambient temperatures of 25, 30, or 35 °C, with 50% relative humidity. After a 25-minute seated rest, they performed 20-minute cycling at 50% HRR with continuous sweat rate.

After exercise, they returned to Room A and remained seated for 15 minutes before probe removal.

Exercise was immediately discontinued if abnormal signs were observed by the attending physician.

2.1.3. Measurements

Sweat rate was measured every 10 seconds using ventilated capsule perspiration meters (POS-02, SKINOS, Ueda, Japan) attached to the forearm and medial/lateral earlobe. Fig. 1 (B) shows probe placement.

2.1.4. Statistical analysis

Local sweat rates (forearm, medial and lateral earlobes) were analyzed using a linear mixed-effects model. Measurement site and elapsed time, and their interaction, were treated as fixed effects, and participant ID was included as a random effect. When significant, post hoc comparisons were conducted using Tukey's Honestly Significant Difference (HSD) test. All analyses were performed using JMP Student Edition 18.0 (SAS Institute Inc., Cary, NC, USA).

2.2. Minor's iodine-starch test for qualitative evaluation of sweating

2.2.1. Experimental protocol

Based on the quantitative evaluation of sweat rates at the earlobe conducted in Experiment 1, we identified a limitation in the measurement accuracy of the ventilated capsule-type perspiration meter. The measurement accuracy of the device is ±10% with a minimum resolution of 0.01 mg/cm²/min. This measurement method simultaneously detects both the amount of water vapor in the environment and the amount of water vapor containing evaporated sweat within the enclosed space on the skin, and measures the amount of sweating from the difference. Therefore, even though the humidity in the artificial climate chamber is controlled, even slight changes in ambient humidity can cause baseline fluctuations in water vapor levels. Furthermore, it measures the water vapor that happens to be present in a 1 cm² space on the skin of the earlobe, and does not necessarily reflect the total amount of sweating in the entire earlobe. Therefore, we found that this method has inherent limitations in detecting extremely low sweat rates. We aimed to qualitatively and non-invasively determine whether the earlobe lacks functional eccrine sweating using Minor's iodine–starch test, a colorimetric method to visualize sweat activity. This technique is based on the iodine–starch reaction, in which moisture dissolves starch, and triiodide ions (I₃^-) are incorporated into the helical structure of amylose to form a charge-transfer complex. This complex exhibits a characteristic absorption spectrum that appears as a deep blue–purple coloration in the presence of water, allowing assessment of sweat gland distribution and function. It is widely applied to diagnose hyperhidrosis or anhidrosis and to evaluate sweat gland disorders and treatment efficacy [[11], [12], [13]].

Qualitative sweating was assessed using Minor's iodine-starch test in the same climate chamber as in Experiment 1, in 12 healthy men between November 2021 and March 2024. Participants were screened to exclude active medical conditions, including sweating disorders and iodine allergy.

2.2.2. Experimental procedures

The reagent was prepared by mixing iodine solution (2 g; Iodine, beads, 096-05425, FUJIFILM Wako Pure Chemical Corporation), castor oil (15 g; Castor Oil, 034-01586, FUJIFILM Wako Pure Chemical Corporation), ethanol (100 mL; Ethanol 99.5, FUJIFILM Wako Pure Chemical Corporation), and soluble starch (Starch, Soluble, FUJIFILM Wako Pure Chemical Corporation). This mixture was applied evenly to the left side of the face, auricle, and neck. Participants entered the climate chamber at an ambient temperature of 40 °C and 50% relative humidity and remained seated for 15 minutes. Sites were examined for dark purple discoloration, and photographs were taken.

3. Results

3.1. Measurement of sweat rate

Participants had a mean age of 23.2 ± 3.4 years, height of 171.6 ± 6.4 cm, weight of 67.0 ± 7.5 kg, and body mass index of 22.7 ± 2.1. All participants completed the experiments. Mean VO_2max was 39.4 ± 9.0 mL/kg/min, and mean workload at 50% HRR was 99.5 ± 16.3 W.

Local sweat rates are presented in Fig. 2. As the ambient temperature increased, sweat rate at the forearm exhibited a clear upward trend. In contrast, sweat rates on both the medial and lateral aspects of the earlobe remained consistently low, with little variation across all temperature conditions. Table 1 provides median values and interquartile range (IQR) for each site.

Fig. 2.

Sweat rate at each measurement site (N = 15). T_a: ambient temperature, RH: relative humidity. Environmental conditions: (A) T_a = 25°C, (B) T_a = 30°C, (C) T_a = 35°C; RH = 50% for all conditions. Local sweat rates at the forearm, medial earlobe, and lateral earlobe were compared using a linear mixed-effects model. Measurement site and time factor were set as fixed effects with their interaction term included, and participant ID as a random effect. Tukey's Honestly Significant Difference (HSD) test was used for post hoc comparisons when significant effects were detected.

Table 1.

Median value and interquartile range (IQR) of local sweat rates (N = 15)

Room temperature	Placement	Sweat rate(mg/cm2/s)		Measurement site	Time factor	Interaction effects
		Median	IQR		p
25 °C	medial earlobe	0.033	(0.023-0.050)	<0.0001	<0.0001	<0.0001
	lateral earlobe	0.034	(0.014-0.050)
	forearm	0.154	(0.076-0.494)
30 °C	medial earlobe	0.033	(0.023-0.050)	<0.0001	<0.0001	<0.0001
	lateral earlobe	0.034	(0.014-0.050)
	forearm	0.182	(0.069-0.708)
35 °C	medial earlobe	0.038	(0.018-0.068)	<0.0001	<0.0001	<0.0001
	lateral earlobe	0.030	(0.010-0.049)
	forearm	0.374	(0.163-0.943)

Relative humidity was 50% under all environmental conditions.

The interquartile range (IQR) is the range between the 25th and 75th percentiles, indicating the spread of the middle 50% of the data.

Local sweat rates at the forearm, medial earlobe, and lateral earlobe were compared using a linear mixed-effects model. Measurement site and time factor were set as fixed effects with their interaction term included, and participant ID as a random effect. Tukey's Honestly Significant Difference (HSD) test was used for post hoc comparisons when significant effects were detected.

Forearm sweat rates were significantly higher than earlobe rates throughout the period following the onset of exercise under all environmental conditions. No significant difference was found between the medial and lateral earlobes.

3.2. Minor's iodine-starch test for qualitative evaluation of sweating

Participants had a mean age of 24.5 ± 3.8 years (range: 20–31), and none had known medical conditions or iodine allergy.

Fig. 3 shows representative findings from Minor's iodine-starch test. Dark purplish granular discoloration corresponding to sweat gland openings was found at sites of active sweating. Such discoloration was frequent on the face and neck but was less prominent on the auricle, where it appeared on the helix and antihelix; however, none was observed on the medial or lateral earlobe. These findings were consistent across all 12 participants. Additional photographs are provided in the Supplementary Materials.

Fig. 3.

Photograph of the earlobe region subjected to Minor's iodine-starch test in one participant. (A) Medial and (B) Lateral aspects of the earlobe.

4. Discussion

To the best of our knowledge, this is the first study to quantitatively and qualitatively assess the sweating function of the human earlobe. Quantitative evaluation demonstrated that sweat responses on the medial and lateral earlobe were significantly lower than those on the forearm at ambient temperatures of 25, 30, and 35 °C. However, sweat meters have limitations in measurement accuracy, and even if the measured sweat rate at the earlobe is a very low non-zero value, it may not necessarily reflect true sweating. Qualitative assessment using Minor's iodine-starch test showed no discoloration on either side of the earlobe, indicating minimal sweat secretion.

Kuno et al. reported that facial sweating is more pronounced centrally than in other areas [10], consistent with our findings.

Minor noted that sweating is sparse on the nasal tip, cheeks, eyelids, and particularly the auricle; however, he emphasized that it is incorrect to assume that the auricle does not sweat [11]. In our study, Minor's iodine-starch test across the entire external ear revealed clear discoloration indicative of sweat secretion at the peripheral regions of the auricle. In contrast, little to no such reaction was observed in the earlobe, providing evidence that this region has particularly limited sudomotor function.

Although the earlobe is peripheral and influenced by ambient temperature, this effect may be minimized in a hot environment due to AVA opening and arterial perfusion. Compared with invasive core body temperature measurements, monitoring earlobe skin temperature is less invasive and may therefore be easier to implement as a candidate approach for future validation, especially in occupational settings.

Since sweating can occur at the auricle margins, the temperature sensor should avoid these areas and any runoff from surrounding skin. Individual differences in earlobe size should also be considered, as smaller earlobes may reduce accuracy.

The earlobe is located near established core body temperature measurement sites, such as the external auditory canal and tympanic membrane, and shares vascular supply from branches of the external carotid artery [5]. Although the earlobe is a peripheral site, these anatomical features suggest that it may warrant further investigation as a candidate site for skin-temperature sensing related to heat stress. However, whether earlobe temperature reflects changes in core body temperature or can serve as a reliable indicator of heat stress remains to be determined. To properly evaluate the potential use of earlobe temperature in assessing heat stress, further dedicated validation studies under controlled conditions are needed to examine the relationship between earlobe and core body temperature.

Monitoring earlobe temperature will require a compact, high-precision wireless temperature sensor, as mentioned above. As a protruding site, the earlobe is unlikely to interfere with work movements or personal protective equipment, such as respiratory or hearing protection. If a small and reliable sensor is developed and the earlobe temperature measured by that sensor is shown to correlate with core body temperature, practical use across various occupational settings may be feasible. Integrating the sensor into commonly worn equipment, such as sun hats or safety helmets, may enhance contact stability and reduce detachment. This approach may be particularly applicable in heat-exposed workplaces, including agriculture, construction, and manufacturing.

Once developed, field studies in occupational settings will be required to verify that earlobe temperature can be measured without interfering with workers' tasks or comfort. Future studies including more diverse populations by sex, age, and occupational backgrounds are also needed to confirm the generalizability.

5. Limitations

Since this study included only Japanese male participants with an average body build, generalizability to women, older adults, individuals with obesity, or populations with different anthropometric or physiological characteristics remains uncertain.

Sweating responses vary by sex, age, and body morphology. Although men generally exhibit greater sweat rates than women [14], thermoeffector responses may be more strongly influenced by body surface area and composition than by sex per se [15,16]. Greater body mass or fat percentage may alter heat dissipation due to increased metabolic heat production and insulation. Sweating also declines with age [15,17], likely owing to reduced sweat gland sensitivity, atrophy, attenuated vasodilation, and neural changes. Nevertheless, the observation of minimal earlobe sweating in young participants appears to be consistent with our interpretation, although further studies in more diverse populations are needed.

The influence of racial and genetic factors on sweating is not fully understood. Migrant studies suggest that heat acclimatization reflects adaptation to long-term heat exposure rather than inherent genetic factors [15]. Thus, whether similar findings apply in other climatic regions remains unclear. The observation of minimal earlobe sweating in participants living in a temperate climate does not contradict this interpretation, but this finding should be confirmed in populations from different climatic regions.

Although participants reported no known medical conditions, undiagnosed autonomic or dermatological disorders associated with hypohidrosis cannot be excluded.

Minor's iodine-starch test has procedural limitations. Variability in reagent application may cause uneven discoloration, and profuse sweating may result in dye runoff, complicating interpretation. Consequently, minimal or unclear discoloration was observed in some participants, making it difficult to exclude the possibility of slight sweating at the earlobe.

We did not perform histological examinations, such as skin biopsies, to confirm the absence of sweat glands in the earlobe. Because Minor's iodine-starch test can only indicate the absence of visible sweat activity, it cannot demonstrate the anatomical absence of eccrine glands. Therefore, we cannot rule out the possibility that the earlobe is capable of producing sweat. East Asian populations have been reported to exhibit a higher prevalence of attached than free earlobes compared with Europeans or Africans, and earlobes may also be thinner in some groups. Although ethnic differences in earlobe sweat gland distribution cannot be excluded, they were not examined in this study [18].

Overall, this study demonstrated only that thermoregulatory sweating at the earlobe is minimal. Because concomitant measurements of core body temperature, heart rate, cardiovascular strain, local earlobe temperature, and composite heat-strain indices were not included, this study cannot determine whether earlobe temperature tracks physiological heat strain. Therefore, without further validation against established heat-stress indicators, it would be premature to conclude that earlobe skin temperature is suitable for practical monitoring.

6. Conclusion

In this study, we assessed the thermoregulatory sweating of the earlobe as an initial step toward exploring its potential suitability as a skin temperature measurement site for occupational heat stress monitoring. Quantitative analysis demonstrated that sweat rates on the medial and lateral earlobe were significantly lower than those on the forearm across ambient temperatures. Minor's iodine-starch test likewise indicated minimal or absent sweat secretion.

CRediT authorship contribution statement

Chikage Nagano: Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. Kentaro Kado: Writing – review & editing, Visualization, Validation, Resources, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Kisaki Maeda: Writing – review & editing, Visualization, Validation, Resources, Methodology, Investigation, Conceptualization. Keiichi Tajima: Resources, Investigation. Kimiyo Mori: Writing – review & editing, Supervision. Seichi Horie: Writing – review & editing, Supervision, Project administration, Methodology, Funding acquisition, Conceptualization.

Ethics statement

The study was conducted in compliance with the ethical standards set forth in the Declaration of Helsinki. Ethical approval was obtained from the Ethics Committee of the University of Occupational and Environmental Health, Japan (Approval No. R4-030), and written informed consent was obtained from all participants.

Disclaimers

No additional disclaimers apply beyond the conflict-of-interest statement.

Statement on the use of AI

During the preparation of this work, the author used ChatGPT (OpenAI) to assist with the English translation of the original Japanese text. After using this tool, the author reviewed and revised the content as necessary and takes full responsibility for the final content of the published article.

Conflicts of interest

This study was funded by ENEOS Corporation and CMIC Holdings Co., Ltd. as part of a multi-institutional collaborative research project involving the University of Occupational and Environmental Health, Japan. However, these sponsors had no involvement in the study design, data collection or analysis, decision to publish, or preparation of the manuscript. S.H. also received research funding from Kamakura Seisakusho Co., Ltd. and Shigematsu Works Co., Ltd. These funding sources likewise had no role in the study design, data collection or analysis, decision to publish, or manuscript preparation. The other authors declare that they have no commercial or financial relationships that could be construed as potential conflicts of interest.

Appendix A. Supplementary data

The following is the Supplementary data to this article:

figs1.