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Figure 1.
Critical Humidity Inflection Points for Heart Rate and Core Temperature
Critical Humidity Inflection Points for Heart Rate and Core Temperature

A, The critical humidity at which rapid elevations in heart rate and core temperature occurred was evaluated in elderly adults exposed to stepwise increases in relative humidity while resting at 42°C. The protocol was performed without and with fan use. B, Error bars indicate 95% confidence intervals. The horizontal line indicates the mean. The inflection points for heart rate (P = .68) and core temperature (P = .11) did not differ between conditions.

Figure 2.
Heart Rate and Core Temperature During the Protocol
Heart Rate and Core Temperature During the Protocol

Elderly adults were exposed to stepwise increases in relative humidity while resting at 42°C. The protocol was performed without and with fan use (randomized). A significant condition × relative humidity interaction was observed for heart rate and core temperature (both P < .001). Fan use resulted in greater heart rate and core temperature, but the magnitude of difference between conditions generally became less as relative humidity increased. Error bars indicate 95% confidence intervals. Data markers indicate mean values.

1.
Patz  JA, Frumkin  H, Holloway  T, Vimont  DJ, Haines  A.  Climate change: challenges and opportunities for global health.  JAMA. 2014;312(15):1565-1580.PubMedGoogle ScholarCrossref
3.
Gupta  S, Carmichael  C, Simpson  C,  et al.  Electric fans for reducing adverse health impacts in heatwaves.  Cochrane Database Syst Rev. 2012;7(7):CD009888.PubMedGoogle Scholar
4.
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.PubMedGoogle ScholarCrossref
5.
Kenney  WL, Craighead  DH, Alexander  LM.  Heat waves, aging, and human cardiovascular health.  Med Sci Sports Exerc. 2014;46(10):1891-1899.PubMedGoogle ScholarCrossref
6.
Semenza  JC, Rubin  CH, Falter  KH,  et al.  Heat-related deaths during the July 1995 heat wave in Chicago.  N Engl J Med. 1996;335(2):84-90.PubMedGoogle ScholarCrossref
Research Letter
September 6, 2016

Cardiac and Thermal Strain of Elderly Adults Exposed to Extreme Heat and Humidity With and Without Electric Fan Use

Author Affiliations
  • 1Institute for Exercise and Environmental Medicine, University of Texas Southwestern Medical Center, Dallas
  • 2Faculty of Health Sciences, University of Sydney, Lidcombe, Australia
JAMA. 2016;316(9):989-991. doi:10.1001/jama.2016.10550

Heat-related morbidity and mortality are important health challenges posed by global climate change.1 Electric fans provide a low-cost and accessible cooling intervention, although their effectiveness remains debatable. Due to theoretical risks of accelerated heat gain and dehydration, fan use is discouraged above ambient temperatures of approximately 35°C.2 However, empirical data to support or refute their use during heat wave conditions are sparse.3

Fan use delays elevations in heart rate and core temperature of young adults exposed to 42°C.4 However, it remains unknown if fans are effective in vulnerable populations, such as the elderly who display altered cardiovascular and thermoregulatory responses during heat exposure.5 We hypothesized that fan use would delay elevations in heart rate and core temperature of elderly adults exposed to extreme heat and humidity.

Methods

Following written informed consent, elderly adults (aged 60-80 years) participated between June 2015 and January 2016 in the study approved by the institutional review boards at the University of Texas Southwestern Medical Center and Texas Health Presbyterian Hospital Dallas. All participants were volunteers recruited from the Dallas and Fort Worth areas.

Wearing shorts (men) or shorts and a sports bra (women), participants sat in a chamber maintained at 42°C. After 30 minutes at a relative humidity of 30%, relative humidity was increased 2% every 5 minutes to 70% (100 minutes total). On separate randomized days, participants performed the protocol with or without a 16-in fan (Dayton) facing them from 1 m (air velocity, 4 m/s). No fluid intake was allowed during the protocol. Measurements included heart rate from an electrocardiogram (Solar 8000i, GE Healthcare), core temperature from an esophageal thermocouple (Tab thermocouple 9 Fr, Covidien), and sweat loss from nude body weight measurements.

Segmented regression identified the critical relative humidity at which rapid elevations in heart rate and core temperature occurred.4 A representative analysis of core temperature is presented in Figure 1. Heart rate and core temperature were also analyzed by 2-way repeated measures analysis of variance (Prism 6, GraphPad). Critical relative humidity and sweat loss were analyzed with paired-sample t tests. Statistical significance was set at a P value less than .05 (2-tailed).

Results

Study participants included 3 men and 6 women (mean age, 68 years [SD, 4 years]; height, 166 cm [SD, 10 cm]; weight, 72 kg [SD, 13 kg]). No heart rate inflection was observed in 1 participant during both conditions; these data were therefore excluded from critical relative humidity analyses. Similar critical relative humidity values for heart rate (53% for fan vs 56% for no fan; difference, 3% [95% CI, −16% to 11%], P = .68) and core temperature (65% for fan vs 63% for no fan; difference, 2% [95% CI, −0.7% to 5.8%], P = .11) were observed (Figure 1). However, significant condition × relative humidity interactions for heart rate (P < .001) and core temperature (P < .001) were observed (Figure 2). Fan use resulted in greater heart rate and core temperature, but the magnitude of differences generally became less as relative humidity increased. Sweat loss was similar between conditions (0.8 L for fan vs 0.8 L for no fan; difference, 0 L [95% CI, −0.2 to 0.2], P > .99).

Discussion

In contrast to young adults,4 fan use did not modify the critical relative humidity for rapid elevations in heart rate and core temperature of elderly adults exposed to extreme heat and humidity. Rather, fan use resulted in greater heart rate and core temperature. Although differences were small, their cumulative effect may become clinically important with fan use during more prolonged heat exposure. Fan use elevates sweat loss in young adults.4 This was not observed in elderly adults, suggesting that age-related impairments in sweating capacity possibly limit the effectiveness of electric fans. Overall, this preliminary study indicates that electric fans may be detrimental for attenuating cardiovascular and thermal strain of elderly adults during heat waves.

Study limitations include using an ambient temperature at the upper limit encountered during heat waves. In young adults, fan use was most effective at more common heat wave temperatures (36°C).4 It remains unclear if the same is true for elderly adults. Studies are also needed to examine fan use in other vulnerable populations during heat waves, such as those with disease and comorbidities.6

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

Corresponding Author: Craig G. Crandall, PhD, Institute for Exercise and Environmental Medicine, University of Texas Southwestern Medical Center, 7232 Greenville Ave, Dallas, TX 75231 (craigcrandall@texashealth.org).

Author Contributions: Drs Gagnon and Crandall 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: Gagnon, Romero, Jay, Crandall.

Acquisition, analysis, or interpretation of data: All Authors.

Drafting of the manuscript: Gagnon, Crandall.

Critical revision of the manuscript for important intellectual content: All Authors.

Statistical analysis: Gagnon, Crandall.

Obtaining funding: Crandall.

Administrative, technical, or material support: Gagnon, Crandall.

Study supervision: Gagnon, Crandall.

No additional contributions: Cramer, Jay.

Conflict of Interest Disclosures: All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest and none were reported.

Funding/Support: This article was supported by grant GM-068865 from the National Institutes of Health (Dr Crandall) and grant W81XWH-12-1-0152 from the US Department of Defense (Dr Crandall); and a postdoctoral fellowship from the Natural Sciences and Engineering Research Council of Canada (Dr Gagnon).

Role of the Funder/Sponsor: The funders/sponsors had no role in 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 Hai Ngo, BSc; Paula Y. S. Poh, PhD; and Ken Kouda, MD (all from the Institute for Exercise and Environmental Medicine), for their assistance with data collection, as well as Beverley Adams-Huet, MS (University of Texas Southwestern Medical Center), for statistical assistance. No compensation was provided for their contributions.

References
1.
Patz  JA, Frumkin  H, Holloway  T, Vimont  DJ, Haines  A.  Climate change: challenges and opportunities for global health.  JAMA. 2014;312(15):1565-1580.PubMedGoogle ScholarCrossref
3.
Gupta  S, Carmichael  C, Simpson  C,  et al.  Electric fans for reducing adverse health impacts in heatwaves.  Cochrane Database Syst Rev. 2012;7(7):CD009888.PubMedGoogle Scholar
4.
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.PubMedGoogle ScholarCrossref
5.
Kenney  WL, Craighead  DH, Alexander  LM.  Heat waves, aging, and human cardiovascular health.  Med Sci Sports Exerc. 2014;46(10):1891-1899.PubMedGoogle ScholarCrossref
6.
Semenza  JC, Rubin  CH, Falter  KH,  et al.  Heat-related deaths during the July 1995 heat wave in Chicago.  N Engl J Med. 1996;335(2):84-90.PubMedGoogle ScholarCrossref
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