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Figure.  Flow of Participants Through the Study
Flow of Participants Through the Study
Table.  Outcome Variables at Baseline and During the Last Hour of Simulated Heat Wave Exposure and Comparisons Between the Interventions
Outcome Variables at Baseline and During the Last Hour of Simulated Heat Wave Exposure and Comparisons Between the Interventions
1.
Friedrich  MJ.  Tracking progress on mitigating health effects of climate change.  JAMA. 2019;321(3):238. doi:10.1001/jama.2018.21621PubMedGoogle Scholar
2.
Ravanelli  NM, Hodder  SG, Havenith  G, Jay  O.  Heart rate and body temperature responses to extreme heat and humidity with and without electric fans.  JAMA. 2015;313(7):724-725. doi:10.1001/jama.2015.153PubMedGoogle ScholarCrossref
3.
Anderson  GB, Bell  ML.  Lights out: impact of the August 2003 power outage on mortality in New York, NY.  Epidemiology. 2012;23(2):189-193. doi:10.1097/EDE.0b013e318245c61cPubMedGoogle ScholarCrossref
4.
Wellenius  GA, Eliot  MN, Bush  KF,  et al.  Heat-related morbidity and mortality in New England: evidence for local policy.  Environ Res. 2017;156:845-853. doi:10.1016/j.envres.2017.02.005PubMedGoogle ScholarCrossref
5.
Fouillet  A, Rey  G, Laurent  F,  et al.  Excess mortality related to the August 2003 heat wave in France.  Int Arch Occup Environ Health. 2006;80(1):16-24. doi:10.1007/s00420-006-0089-4PubMedGoogle ScholarCrossref
6.
Semenza  JC, McCullough  JE, Flanders  WD, McGeehin  MA, Lumpkin  JR.  Excess hospital admissions during the July 1995 heat wave in Chicago.  Am J Prev Med. 1999;16(4):269-277. doi:10.1016/S0749-3797(99)00025-2PubMedGoogle ScholarCrossref
Research Letter
October 8, 2019

A Preliminary Study of the Effect of Dousing and Foot Immersion on Cardiovascular and Thermal Responses to Extreme Heat

Author Affiliations
  • 1Faculty of Health Sciences, University of Sydney, Sydney, Australia
  • 2School of Public Health, University of Sydney, Sydney, Australia
JAMA. 2019;322(14):1411-1413. doi:10.1001/jama.2019.13051

Air conditioning1 and electric fans2 are used to mitigate physiological strain during extreme heat exposure but may not always be available.3 Drinking water is commonly recommended4; however, when applied externally, water also can reduce body heat via evaporation from the skin or conduction when body parts with high surface-area-to-mass ratio are submerged. We assessed whether a self-dousing or a foot immersion intervention mitigated increases in cardiovascular (heart rate) and thermal (core temperature) strain and dehydration during conditions simulating hot, humid and very hot, dry heat waves.

Methods

After University of Sydney ethics approval, written consent was obtained from student volunteers. Three interventions were tested. For the control intervention, participants consumed 0.25 L of 22°C (72°F) water every 30 minutes. Additional interventions were (1) foot immersion: water consumption plus submersion of lower legs to mid-calf into 40 L of water maintained at 22°C (72°F) for 20 minutes, followed by 10 minutes out, then repeated or (2) self-dousing: external application of water maintained at 22°C (72°F) across the chest, arms, back, legs, and face with a sponge following the instructions “wet your skin whenever you start feeling warm.” Each intervention lasted 120 minutes and was conducted in a climate chamber under either hot, humid (40°C [104°F], 50% relative humidity) or very hot, dry (47°C [117°F], 10% relative humidity) conditions. Interventions were separated by more than 48 hours. Within each environmental condition, the order of the interventions was randomized.

Outcomes were core (rectal) temperature and heart rate (3-lead electrocardiography) measured continuously, whole-body sweat rate (determined using preintervention to postintervention changes in body mass measured on a platform scale), and whole-body thermal discomfort (assessed every 15 minutes using a visual analog scale with anchors at 0 mm [not uncomfortable], 40 mm [slightly uncomfortable], 80 mm [uncomfortable], and 120 mm [very uncomfortable]). The mean of each outcome variable during the last hour of exposure was analyzed within environmental conditions using 1-way repeated analysis of variance with Sidak post hoc testing. The significance threshold used was P = .05. Version 26 of SPSS software (IBM) was used for all analyses. Clinical significance was defined using a difference in heart rate of 5/min, which can detect variations in heart failure outcomes, and a difference of 125 g/h in whole-body sweat rate, which is used to determine fluid requirements for soldiers.

Results

Of 22 volunteers, 8 participated in both environmental exposure conditions and 14 in 1 environmental condition. There were 15 participants for each environmental condition (mean age, 28 [SD, 5] years; exposure to hot, humid conditions: 40% female and mean body mass index [BMI; calculated as weight in kilograms divided by height in meters squared], 24.0 [SD, 2.5]; exposure to very hot, dry conditions: 20% female and mean BMI, 25.5 [SD, 3.1]; Figure). No significant differences in outcome variables were observed at rest.

In the comparison of the foot immersion vs control interventions during exposure to hot, humid conditions, the whole-body sweat rate was significantly lower (114 g/h vs 179 g/h, respectively; difference, −66 g/h [95% CI, −109 to −23 g/h]; P = .003) and the thermal discomfort level was lower (45 mm vs 66 mm; difference, −21 mm [95% CI, −39 to −3 mm]; P = .02); however, heart rate was not (75/min vs 80/min; difference, −5/min [95% CI, −11/min to 1/min]; P = .14). The findings for whole-body sweat rate and heart rate were similar under very hot, dry conditions, but thermal discomfort was not significant (Table).

In the comparison of the self-dousing vs control interventions during exposure to hot, humid conditions, heart rate was significantly lower (75/min vs 80/min, respectively; difference, −5/min [95% CI, −10/min to 0/min]; P = .04) as well as the whole-body sweat rate (93 g/h vs 179 g/h; difference, −87 g/h [95% CI, −144 to −29 g/h]; P = .004) and thermal discomfort level (38 mm vs 66 mm; difference, −27 mm [95% CI, −49 to −6 mm]; P = .01). Results were similar under very hot, dry conditions.

In the comparison of the self-dousing vs foot immersion intervention during exposure to very hot, dry conditions, the whole-body sweat rate and thermal discomfort level were lower with self-dousing; however, no differences between interventions were observed under hot, humid conditions. No differences in core temperature or effects of intervention order for any outcome were found.

The reductions in heart rate with the foot immersion and self-dousing interventions attained clinical significance in hot, humid conditions as did the reductions in whole-body sweat rate with self-dousing in very hot, dry conditions.

Discussion

In this preliminary study, foot immersion lowered sweating during exposure to both environmental conditions and lowered thermal discomfort only in hot, humid conditions. The self-dousing intervention lowered heart rate, sweating, and thermal discomfort under both environmental conditions and was more effective than the foot immersion intervention at reducing sweating and thermal discomfort in very hot, dry conditions. Core temperature was unaltered. However, morbidity and mortality from dehydration and cardiovascular failure during heat waves typically surpass those from hyperthermia alone.5 Limitations include testing of only a small number of healthy volunteers. Responses must be evaluated among older adults and among those who have health conditions, are taking medications, or both, which could potentially disrupt thermoregulation.6

Section Editor: Jody W. Zylke, MD, Deputy Editor.
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Article Information

Accepted for Publication: August 8, 2019.

Corresponding Author: Ollie Jay, PhD, Thermal Ergonomics Laboratory, Faculty of Health Sciences, University of Sydney, Sydney, NSW 2141, Australia (ollie.jay@sydney.edu.au).

Author Contributions: Drs Morris and Jay had full access to all of the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.

Concept and design: Morris, Capon, Jay.

Acquisition, analysis, or interpretation of data: Morris, Gruss, Lempert, English, Hospers, Jay.

Drafting of the manuscript: Morris, Gruss, Lempert, English.

Critical revision of the manuscript for important intellectual content: Morris, Hospers, Capon, Jay.

Statistical analysis: Morris, Gruss, Hospers.

Obtained funding: Capon, Jay.

Administrative, technical, or material support: Lempert, English, Jay.

Supervision: Jay.

Conflict of Interest Disclosures: Dr Jay reported receiving grants from the Australian National Health and Medical Research Council, MS Research Australia, Tennis Australia, and the National Rugby League; receiving grants and personal fees from Cricket Australia and the US Department of Defense; and receiving personal fees from the Gatorade Sport Science Institute. No other disclosures were reported.

Funding/Support: This study was supported by grants from the AdaptNSW Human Health and Social Impacts Node, which is hosted by theUniversity of Sydney with funding from the New South Wales Department of Planning, Industry and Environment and Ministry of Health.

Role of the Funder/Sponsor: The funder had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.

Additional Contributions: We thank Georgia Chaseling, PhD (University of Sydney), and Mohammad Fauzan Bin Maideen, MPthy (University of Sydney), for their uncompensated assistance during data collection.

References
1.
Friedrich  MJ.  Tracking progress on mitigating health effects of climate change.  JAMA. 2019;321(3):238. doi:10.1001/jama.2018.21621PubMedGoogle Scholar
2.
Ravanelli  NM, Hodder  SG, Havenith  G, Jay  O.  Heart rate and body temperature responses to extreme heat and humidity with and without electric fans.  JAMA. 2015;313(7):724-725. doi:10.1001/jama.2015.153PubMedGoogle ScholarCrossref
3.
Anderson  GB, Bell  ML.  Lights out: impact of the August 2003 power outage on mortality in New York, NY.  Epidemiology. 2012;23(2):189-193. doi:10.1097/EDE.0b013e318245c61cPubMedGoogle ScholarCrossref
4.
Wellenius  GA, Eliot  MN, Bush  KF,  et al.  Heat-related morbidity and mortality in New England: evidence for local policy.  Environ Res. 2017;156:845-853. doi:10.1016/j.envres.2017.02.005PubMedGoogle ScholarCrossref
5.
Fouillet  A, Rey  G, Laurent  F,  et al.  Excess mortality related to the August 2003 heat wave in France.  Int Arch Occup Environ Health. 2006;80(1):16-24. doi:10.1007/s00420-006-0089-4PubMedGoogle ScholarCrossref
6.
Semenza  JC, McCullough  JE, Flanders  WD, McGeehin  MA, Lumpkin  JR.  Excess hospital admissions during the July 1995 heat wave in Chicago.  Am J Prev Med. 1999;16(4):269-277. doi:10.1016/S0749-3797(99)00025-2PubMedGoogle ScholarCrossref
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