Heat Stress Exposure and Physiological Responses among Sugarcane Workers in Thailand

Abstract

Introduction: Occupational heat stress can lead to cardiovascular and thermoregulatory changes, including elevated heart rate, increased core temperature, and altered blood pressure. Evidence remains limited regarding the physiological impact of heat stress in tropical occupational settings. Sugarcane workers in Thailand experience prolonged exposure to extreme heat during harvesting. This study assessed the impact of heat stress exposure on physiological responses among Thai sugarcane workers. Methods: Field measurements were conducted in Nakhon Sawan Province during cooler and hotter harvesting periods in 2023. Demographic, health, and work-related data were collected using a questionnaire. Heat stress exposure was assessed using Wet Bulb Globe Temperature (WBGT) across entire shifts, and a full work shift time-weighted average effective WBGT (WBGT_eff-FS-TWA), adjusted for clothing, was calculated. Resting heart rate, body temperature, and blood pressure were recorded pre- and post-shift. Associations between WBGT_eff-FS-TWA and post-shift physiological parameters were analysed using general linear regression, adjusting for pre-shift values and confounders. Results: Mean WBGT_eff-FS-TWA was 31.3 ± 2.8 °C (range: 22.9–35.4 °C). Post-shift systolic and diastolic blood pressures were significantly lower, while heart rate and body temperature were significantly higher compared to pre-shift values (p < 0.001). WBGT_eff-FS-TWA was not associated with post-shift blood pressure. However, heart rate increased by 0.61 beats/min (95% CI: 0.24–0.98) and body temperature by 0.02 °C (95% CI: 0.002–0.03) per 1 °C increase in WBGT_eff-FS-TWA. Conclusion: Heat stress exposure was linked to modest increases in heart rate and body temperature among sugarcane workers. With rising global temperatures, monitoring cardiovascular and thermoregulatory responses is critical for safeguarding workers in hot environments.

Keywords

heat exposure / wet bulb globe temperature / physiological strain / agricultural workers / harvesting period / global warming

Author Contributions

T.T.: writing—original draft; T.T., A.P.; H.A.S., M.G. and M.v.T.: conceptualization, data curation, formal analysis, investigation, methodology, project administration, resources, software; supervision, validation, visualization, and writing—review & editing. All authors have read and agreed to the published version of the manuscript.

Funding

The work was supported by Royal Thai Government Scholarship [grant numbers N/A, 2022]. The funder of the study had no role in study design, data collection, data analysis, data interpretation, writing of the report, or decision to submit the article for publication.

Institutional Review Board Statement

The study was conducted according to the guidelines of the Declaration of Helsinki and approved by the University Research Ethics Committee (UREC) of The University of Manchester (Ref No. 2022-15332-26084), and also by the Institutional Review Board of Naresuan University, Thailand (IRB No. P2-0368/2565).

Informed Consent Statement

All participants were thoroughly informed about the study’s nature beforehand and provided signed consent to participate.

Data Availability Statement

The datasets generated during and/or analysed during the current study are not publicly available but are available from the corresponding author on reasonable request.

Acknowledgments

The authors would like to acknowledge the sugarcane workers who participated in this study. The authors would like to express our sincere thanks to all our research team. The authors appreciate the financial support from Royal Thai Government Scholarship.

Conflicts of Interest

The authors declare no competing interests.

Use of AI and AI-Assisted Technologies

No AI tools were utilized for this paper.

References

[1] National Centers for Environmental Information; National Oceanic and Atmospheric Administration. Monthly Global Climate Report for Annual 2024. Available online: https://www.ncei.noaa.gov/access/monitoring/monthly—report/global/202413 (accessed on 28 July 2025).

[2] Thai Meteorological Department . Yearly Summary of Weather Conditions in Thailand, 2024. Available online: https://www.tmd.go.th/climate/summaryyearly (accessed on 25 February 2025).

[3] Office of the Cane and Sugar Board. A Situation of Sugarcane Growing in Thailand in 2023—2024 Report. Available online: https://www.ocsb.go.th/2024/reports—articles/area—yield/27524/ (accessed on 28 July 2025).

[4] Boonruksa, P.; Maturachon, T.; Kongtip, P.; et al. Heat Stress, Physiological Response, and Heat—Related Symptoms among Thai Sugarcane Workers. Int. J. Environ. Res. Public Health 2020, 17, 6363. https://doi.org/10.3390/ijerph17176363.

[5] Stem, A.D.; Gibb, M.; Roncal—Jimenez, C.A.; et al. Health Burden of Sugarcane Burning on Agricultural Workers and Nearby Communities. Inhal. Toxicol. 2024, 36, 327-342. https://doi.org/10.1080/08958378.2024.2316875.

[6] Tantipanjaporn, T.; Povey, A.; Shiels, H.A.; et al. High Levels of Heat Stress among Sugarcane Workers in Thailand. Ann. Work Expo. Health 2025, 64, 401-414. https://doi.org/10.1093/annweh/wxaf002.

[7] Tantipanjaporn, T.; Povey, A.; Shiels, H.A.; et al. 62 The Effects of Heat Stress on Heat—Related Symptoms among Sugarcane Workers in Thailand. Occup. Environ. Med. 2025, 82, A28-A29. https://doi.org/10.1136/oemed—2024—EPICOHabstracts.62.

[8] Pundee, R.; Kongtip, P.; Nankongnab, N.; et al. Cross—Shift Change of Acute Kidney Injury Biomarkers in Sugarcane Farmers and Cutters. Hum. Ecol. Risk Assess. 2021, 27, 1170-1187. https://doi.org/10.1080/10807039.2020.1812049.

[9] Wichatorn, T.; Chaiklieng, S. The Correlation between Productivity and Heat Exposure of Sugarcane Farmers in Nongbuadaeng District, Chaiyaphum Province. KKU Res. J. 2022, 22, 186-198.

[10] ISO 7243:2017; Ergonomics of the Thermal Environment—Assessment of Heat Stress Using the WBGT (Wet Bulb Globe Temperature) Index . International Organization for Standardization: Geneva, Switzerland, 2017.

[11] American Conference of Governmental Industrial Hygienists. 2025 TLVs and BEIs: Based on the Documentation of the Threshold Limit Values for Chemical Substances and Physical Agents & Biological Exposure Indices ; ACGIH: Cincinnati, OH, USA, 2025.

[12] OMRON. Upper Arm Blood Pressure Monitor HEM—7156T—A. Available online: https://omronbrandshop.ph/products/automatic—blood—pressure—monitor—hem—7156t (accessed on 24 November 2025).

[13] Braun. Braun ThermoScan ^®7 . Available online: https://www.braunhealthcare.com/uk_en/thermometer/thermoscan—7/ (accessed on 16 April 2025).

[14] Vemu, L.P.; Yang, E.; Ebinger, J. 2023 ESH Hypertension Guideline Update: Bringing Us Closer Together across the Pond. 2024. Available online: https://www.acc.org/Latest—in—Cardiology/Articles/2024/02/05/11/43/2023—ESH—Hypertension—Guideline—Update (accessed on 28 July 2025).

[15] Whelton, P.K.; Carey, R.M.; Aronow, W.S.; et al. 2017 ACC/AHA/AAPA/ABC/ACPM/AGS/APhA/ASH/ASPC/NMA/PCNA Guideline for the Prevention, Detection, Evaluation, and Management of High Blood Pressure in Adults: A Report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines. Hypertension 2018, 71, 1269-1324. https://doi.org/10.1161/HYP.0000000000000066.

[16] American Heart Association . Low Blood Pressure—When Blood Pressure Is Too Low. 2024. Available online: https://www.heart.org/en/health—topics/high—blood—pressure/the—facts—about—high—blood—pressure/low—blood—pressure—when—blood—pressure—is—too—low (accessed on 27 December 2024).

[17] Textor, J. DAGitty—Draw and Analyze Causal Diagrams. 2023. Available online: https://www.dagitty.net (accessed on 15 May 2025).

[18] IBM Corp. IBM SPSS Statistics for Windows, Version 29.0.2.0 ; IBM Corp. : Armonk, NY, USA, 2023.

[19] Cornelissen, V.A.; Smart, N.A. Exercise Training for Blood Pressure: A Systematic Review and Meta—Analysis. J. Am. Heart Assoc. 2013, 2, e004473. https://doi.org/10.1161/jaha.112.004473.

[20] de Brito, L.C.; Rezende, R.A.; da Silva Junior, N.D.; et al. Post—Exercise Hypotension and Its Mechanisms Differ after Morning and Evening Exercise: A Randomized Crossover Study. PLoS ONE 2015, 10, e0132458. https://doi.org/10.1371/journal.pone.0132458.

[21] Aly, K.; Yeung, P.K. Post—Exercise Hypotension: An Alternative Management Strategy for Hypertension and Cardiovascular Disease? J. Clin. Med. 2023, 12, 4456. https://doi.org/10.3390/jcm12134456.

[22] Halliwill, J.R. Mechanisms and Clinical Implications of Post—Exercise Hypotension in Humans. Exerc. Sport Sci. Rev. 2001, 29, 65-70.

[23] Lee, D.—H.; Ihm, S.—H.; Youn, H.—J.; et al. Age Is an Independent Risk Factor for the Early Morning Blood Pressure Surge in Patients Never—Treated for Hypertension. Korean Circ. J. 2009, 39, 322-327. https://doi.org/10.4070/kcj.2009.39.8.322.

[24] Kario, K.; Pickering, T.G.; Umeda, Y.; et al. Morning Surge in Blood Pressure as a Predictor of Silent and Clinical Cerebrovascular Disease in Elderly Hypertensives. Circulation 2003, 107, 1401-1406. https://doi.org/10.1161/01.CIR.0000056521.67546.AA.

[25] Nanchen, D. Resting Heart Rate: What Is Normal? Heart 2018, 104, 1048-1049. https://doi.org/10.1136/heartjnl—2017—312731.

[26] LeWine, E.H. What Is a Normal Heart Rate? Harvard Health Publishing, Harvard Medical School, 2023. Available online: https://www.health.harvard.edu/heart—health/what—your—heart—rate—is—telling—you (accessed on 30 December 2024).

[27] Peçanha, T.; Silva—Júnior, N.D.; Forjaz, C.L. Heart Rate Recovery: Autonomic Determinants, Methods of Assessment and Association with Mortality and Cardiovascular Diseases. Clin. Physiol. Funct. Imaging 2014, 34, 327-339. https://doi.org/10.1111/cpf.12102.

[28] Fecchio, R.Y.; Brito, L.; Leicht, A.S.; et al. Reproducibility of Post—Exercise Heart Rate Recovery Indices: A Systematic Review. Auton. Neurosci. 2019, 221, 102582. https://doi.org/10.1016/j.autneu.2019.102582.

[29] Mohd Azmi, N.A.S.; Juliana, N.; Azmani, S.; et al. Cortisol on Circadian Rhythm and Its Effect on Cardiovascular System. Int. J. Environ. Res. Public Health 2021, 18, 676. https://doi.org/10.3390/ijerph18020676.

[30] Abellán—Aynés, O.; López—Plaza, D.; Alacid, F.; et al. Recovery of Heart Rate Variability after Exercise under Hot Conditions: The Effect of Relative Humidity. Wilderness Environ. Med. 2019, 30, 260-267. https://doi.org/10.1016/j.wem.2019.04.009.

[31] Peçanha, T.; Forjaz, C.L.d.M.; Low, D.A. Passive Heating Attenuates Post—Exercise Cardiac Autonomic Recovery in Healthy Young Males. Front. Neurosci. 2017, 11, 727. https://doi.org/10.3389/fnins.2017.00727.

[32] Geneva, I. Human Body Temperature Circadian Rhythm in Health and Disease. In Heat Illness and Critical Care; Shaikh, N., Ed.; IntechOpen: London, UK, 2024.

[33] Del Bene, V.E. Temperature. In Clinical Methods: The History, Physical, and Laboratory Examinations , 3rd ed.; Walker, H.K., Hall, W.D., Hurst, J.W., Eds.; Butterworths: Woburn, MA, USA, 1990.

[34] International Organization for Standardization . ISO 9886:2004 Ergonomics—Evaluation of Thermal Strain by Physiological Measurements ; International Standard Organization: Geneva, Switzerland, 2004.

[35] Mah, A.J.; Ghazi Zadeh, L.; Khoshnam Tehrani, M.; et al. Studying the Accuracy and Function of Different Thermometry Techniques for Measuring Body Temperature. Biology 2021, 10, 1327. https://doi.org/10.3390/biology10121327.

[36] Barnett, A.G.; Sans, S.; Salomaa, V.; et al. The Effect of Temperature on Systolic Blood Pressure. Blood Press. Monit. 2007, 12, 195-203. https://doi.org/10.1097/MBP.0b013e3280b083f4.

[37] Stotz, A.; Rapp, K.; Oksa, J.; et al. Effect of a Brief Heat Exposure on Blood Pressure and Physical Performance of Older Women Living in the Community—A Pilot—Study. Int. J. Environ. Res. Public Health 2014, 11, 12623-12631. https://doi.org/10.3390/ijerph111212623.

[38] Waite, T. UK Health Security Agency: Why Some People Suffer during Heatwaves. Available online: https://ukhsa.blog.gov.uk/2018/07/23/why—some—people—suffer—during—heatwaves/ (accessed on 2 January 2025).

[39] Stevens, S.L.; Wood, S.; Koshiaris, C.; et al. Blood Pressure Variability and Cardiovascular Disease: Systematic Review and Meta—Analysis. Br. Med. J. 2016, 354, i4098. https://doi.org/10.1136/bmj.i4098.

[40] Liu, J.; Varghese, B.M.; Hansen, A.; et al. Heat Exposure and Cardiovascular Health Outcomes: A Systematic Review and Meta—Analysis. Lancet Planet. Health 2022, 6, e484-e495. https://doi.org/10.1016/S2542—5196(22)00117—6.

[41] Halonen, J.I.; Zanobetti, A.; Sparrow, D.; et al. Relationship between Outdoor Temperature and Blood Pressure. Occup. Environ. Med. 2011, 68, 296-301. https://doi.org/10.1136/oem.2010.056507.

[42] Meshi, E.B.; Kishinhi, S.S.; Mamuya, S.H.; et al. Thermal Exposure and Heat Illness Symptoms among Workers in Mara Gold Mine, Tanzania. Ann. Glob. Health 2018, 84, 360-368. https://doi.org/10.29024/aogh.2318.

[43] Teymori, G.; Jafari, M.J.; Asilian Mahabadi, H.; et al. The Relationship between WBGT Index with Physiological Responses of Open—Pit Mine Workers in Hot and Dry Weather. J. Sabzevar Univ. Med. Sci. 2016, 23, 360-369.

[44] Bruce—Low, S.S.; Cotterrell, D.; Jones, G.E. Heart Rate Variability during High Ambient Heat Exposure. Aviat. Space Environ. Med. 2006, 77, 915-920.

[45] Siquier—Coll, J.; Bartolomé, I.; Pérez—Quintero, M.; et al. Heart Rate and Body Temperature Evolution in an Interval Program of Passive Heat Acclimation at High Temperatures (100 ± 2 °C) in a Sauna. Int. J. Environ. Res. Public Health 2023, 20, 2082. https://doi.org/10.3390/ijerph20032082.

[46] Crandall, C.G.; Wilson, T.E. Human Cardiovascular Responses to Passive Heat Stress. Compr. Physiol. 2015, 5, 17-43. https://doi.org/10.1002/cphy.c140015.

[47] ISO 8996:2021; Ergonomics of the Thermal Environment—Determination of Metabolic Rate ; International Organization for Standardization: Geneva, Switzerland, 2021.