Heat-related Illness Prevention Research Articles

US Farmworkers’ Barriers to Preventing Heat‐Related Illness: An Integrative Review

ABSTRACTBackgroundOccupational heat‐related illness (HRI) is problematic in the United States. Farmworkers are disproportionately affected by HRI compared to other workers. Uncovering barriers that farmworkers face to the prevention of HRI is crucial to reducing HRI‐associated morbidity and mortality. This integrative review aimed to understand US farmworkers’ barriers to preventing HRI.MethodsAn integrative review occurred following Whittemore and Knafl guidelines. Literature searches occurred on PubMed, Medline, and Agricola. After applying inclusion and exclusion criteria and removing duplicates, nine articles remained for review.ResultsThere were nine articles in the review. The majority of articles used a convenience sample. In all studies, the populations were farmworkers or agricultural workers. The study designs included cross‐sectional, mixed methods, qualitative focus groups, and a longitudinal study. The articles revealed several themes related to prevention barriers: access to prevention, education and training, work culture, and compensation. Farmworkers often lack access to proper prevention measures, education, and training. Work culture and compensation were obstacles to preventing HRI as some workers feel powerless to speak up for themselves, whereas others are tempted to forgo breaks because of the way they are compensated.ConclusionsThis review indicates the need for more research to understand the barriers farmworkers face to HRI prevention. Providing prevention measures without considering obstacles to their use is ineffective in reducing HRI. Because many farmworkers lack oversight and regulation of prevention measures, focusing on barriers and areas over which farmworkers have more control could have a significant impact.

Strategy for Improving the Application of Occupational Health and Safety Management System in Heat Related Illness Prevention in XYZ Industrial Area

Strategy for Improving the Application of Occupational Health and Safety Management System in Heat Related Illness Prevention in XYZ Industrial Area

Wearable Sensor Technology to Predict Core Body Temperature: A Systematic Review.

Heat-related illnesses, which range from heat exhaustion to heatstroke, affect thousands of individuals worldwide every year and are characterized by extreme hyperthermia with the core body temperature (CBT) usually > 40 °C, decline in physical and athletic performance, CNS dysfunction, and, eventually, multiorgan failure. The measurement of CBT has been shown to predict heat-related illness and its severity, but the current measurement methods are not practical for use in high acuity and high motion settings due to their invasive and obstructive nature or excessive costs. Noninvasive predictions of CBT using wearable technology and predictive algorithms offer the potential for continuous CBT monitoring and early intervention to prevent HRI in athletic, military, and intense work environments. Thus far, there has been a lack of peer-reviewed literature assessing the efficacy of wearable devices and predictive analytics to predict CBT to mitigate heat-related illness. This systematic review identified 20 studies representing a total of 25 distinct algorithms to predict the core body temperature using wearable technology. While a high accuracy in prediction was noted, with 17 out of 18 algorithms meeting the clinical validity standards. few algorithms incorporated individual and environmental data into their core body temperature prediction algorithms, despite the known impact of individual health and situational and environmental factors on CBT. Robust machine learning methods offer the ability to develop more accurate, reliable, and personalized CBT prediction algorithms using wearable devices by including additional data on user characteristics, workout intensity, and the surrounding environment. The integration and interoperability of CBT prediction algorithms with existing heat-related illness prevention and treatment tools, including heat indices such as the WBGT, athlete management systems, and electronic medical records, will further prevent HRI and increase the availability and speed of data access during critical heat events, improving the clinical decision-making process for athletic trainers and physicians, sports scientists, employers, and military officers.

The effect of the participatory heat education and awareness tools (HEAT) intervention on agricultural worker physiological heat strain: results from a parallel, comparison, group randomized study

BackgroundFarmworkers are at risk of heat-related illness (HRI). We sought to: 1) evaluate the effectiveness of farmworker Spanish/English participatory heat education and a supervisor decision-support mobile application (HEAT intervention) on physiological heat strain; and 2) describe factors associated with HRI symptoms reporting.MethodsWe conducted a parallel, comparison group intervention study from May–September of 2019 in Central/Eastern Washington State, USA. We used convenience sampling to recruit adult outdoor farmworkers and allocated participating crews to intervention (n = 37 participants) and alternative-training comparison (n = 38 participants) groups. We measured heat strain monthly using heart rate and estimated core body temperature to compute the maximum work-shift physiological strain index (PSImax) and assessed self-reported HRI symptoms using a weekly survey. Multivariable linear mixed effects models were used to assess associations of the HEAT intervention with PSImax, and bivariate mixed models were used to describe factors associated with HRI symptoms reported (0, 1, 2+ symptoms), with random effects for workers.ResultsWe observed larger decreases in PSImax in the intervention versus comparison group for higher work exertion levels (categorized as low, low/medium-low, and high effort), after adjustment for maximum work-shift ambient Heat Index (HImax), but this was not statistically significant (interaction − 0.91 for high versus low/medium-low effort, t = − 1.60, p = 0.11). We observed a higher PSImax with high versus low/medium-low effort (main effect 1.96, t = 3.81, p < 0.001) and a lower PSImax with older age (− 0.03, t = − 2.95, p = 0.004), after covariate adjustment. There was no clear relationship between PSImax and the number of HRI symptoms reported. Reporting more symptoms was associated with older age, higher HImax, 10+ years agricultural work, not being an H-2A guest worker, and walking > 3 min to get to the toilet at work.ConclusionsEffort level should be addressed in heat management plans, for example through work/rest cycles, rotation, and pacing, in addition to education and other factors that influence heat stress. Both symptoms and indicators of physiological heat strain should be monitored, if possible, during periods of high heat stress to increase the sensitivity of early HRI detection and prevention. Structural barriers to HRI prevention must also be addressed.Trial registrationClinicalTrials.gov Registration Number: NCT04234802, date first posted 21/01/2020.

Effect of heat exposure on dehydration and kidney function among sea salt workers in Thailand.

Excessive heat exposure and dehydration among agricultural workers have been reported to reduce kidney function and lead to chronic kidney disease of unknown etiology (CKDu). This cross-sectional study aimed to assess heat exposure, factors related to dehydration and the relationship between dehydration and biomarkers of kidney function among sea salt workers in Thailand. Wet bulb globe temperature (WBGT) was used at the time workers started work outdoors on salt farms. Urine-specific gravity, urine osmolarity, and serum creatinine were collected from 50 workers after work. The results showed that more than 50% of the participants were dehydrated after work. The maximum hours spent working per day was 10. The average water intake was 1.51 L. Urine specific gravity was highly significant correlated with urine osmolality (rs = 0.400, p<0.01), and urine osmolality was significantly correlated with the estimated glomerular filtration rate (eGFR) (rs = 0.349, p<0.05). In bivariate analysis adjusted for age, sex, and current alcohol consumption, we found that a WBGTTWA ≥ 30°C (OR = 0.08, 95% CI = 0.01-0.44, p = 0.003) and hours spent working (OR=2.22, 95% CI = 1.42-3.47, p <0.001) were independently associated with dehydration. This suggests that workers should increase their time spent on breaks and increase water consumption. Educational program on heat exposure and heat-related illness prevention strategies should be provided.

Apparent temperature and heat-related illnesses during international athletic championships: A prospective cohort study.

International outdoor athletics championships are typically hosted during the summer season, frequently in hot and humid climatic conditions. Therefore, we analyzed the association between apparent temperature and heat-related illnesses occurrence during international outdoor athletics championships and compared its incidence rates between athletics disciplines. Heat-related illnesses were selected from illness data prospectively collected at seven international outdoor athletics championships between 2009 and 2018 using a standardized methodology. The Universal Thermal Climate Index (UTCI) was calculated as a measure of the apparent temperature based on weather data for each day of the championships. Heat-related illness numbers and (daily) incidence rates were calculated and analyzed in relation to the daily maximum UTCI temperature and between disciplines. During 50 championships days with UTCI temperatures between 15℃ and 37℃, 132heat-related illnesses were recorded. Average incidence rate of heat-related illnesses was 11.7 (95%CI 9.7 to 13.7) per 1000 registered athletes. The expected daily incidence rate of heat-related illnesses increased significantly with UTCI temperature (0.12more illnesses per 1000 registered athletes/°C; 95%CI 0.08-0.16) and was found to double from 25 to 35°C UTCI. Race walkers (RR=45.5, 95%CI 21.6-96.0) and marathon runners (RR=47.7, 95%CI 23.0-98.8) had higher heat-related illness rates than athletes competing in short-duration disciplines. Higher UTCI temperatures were associated with more heat-related illnesses, with marathon and race walking athletes having higher risk than athletes competing in short-duration disciplines. Heat-related illness prevention strategies should predominantly focus on marathon and race walking events of outdoor athletics championships when high temperatures are forecast.

Looking ahead of 2021 Tokyo Summer Olympic Games: How Does Humid Heat Affect Endurance Performance? Insight into physiological mechanism and heat-related illness prevention strategies.

The combination of high humidity and ambient temperature of the 2021 Tokyo Summer Olympic Game will undoubtfully result in greater physiological strains and thereby downregulates the endurance performance of athletes. Although many research studies have highlighted that the thermoregulatory strain is greater when the environment is hot and humid, no review articles have addressed the thermoregulatory and performance differences between dry and humid heat and such lack of consensuses in this area will lead to increase the risk of heat-related injuries as well as suboptimal preparation. Furthermore, specific strategies to counteract this stressful environment has not been outlined in the current literature. Therefore, the purposes of this review are: 1) to provide a clear evidence that humid heat is more stressful than dry heat for both male and female athletes and therefore the preparation for the Tokyo Summer Olympic should be environmental specific instead of a one size fits all approach; 2) to highlight why female athletes may be facing a disadvantage when performing a prolonged endurance event under high humidity environment and 3) to highlight the potential interventional strategies to reduce thermal strain in hot-humid environment. The summaries of this review are: both male and female should be aware of the environmental condition in Tokyo as humid heat is more stressful than dry heat; Short-term heat acclimation may not elicit proper thermoregulatory adaptations in hot-humid environment; cold water immersion with proper hydration and some potential per-cooling modalities may be beneficial for both male and female athletes in hot-humid environment.

Are Cal/OSHA Regulations Protecting Farmworkers in California From Heat-Related Illness?

Objective:Determine compliance with and effectiveness of California regulations in reducing farmworkers’ heat-related illness (HRI) risk and identify main factors contributing to HRI.Methods:In a cross-sectional study of Latino farmworkers, core body temperature (CBT), work rate, and environmental temperature (WBGT) were monitored over a work shift by individual ingestible thermistors, accelerometers, and weather stations, respectively. Multiple logistic modeling was used to identify risk factors for elevated CBT.Results:Although farms complied with Cal/OSHA regulations, worker training of HRI prevention and hydration replacement rates were insufficient. In modeling (AOR [95% CI]) male sex (3.74 [1.22 –11.54]), WBGT (1.22 [1.08 – 1.38]), work rate (1.004 [1.002 – 1.006]), and increased BMI (1.11 [1.10 – 1.29]) were all independently associated with elevated CBT.Conclusion:Risk of HRI was exacerbated by work rate and environmental temperature despite farms following Cal/OSHA regulations.

Risk Factors for Reaching Core Body Temperature Thresholds in Florida Agricultural Workers.

There is a compelling need to identify agricultural workers at risk for heat related illness (HRI). Data from Florida agricultural workers (N = 221) were collected over 3 summer workdays (2015 to 2017) to examine risk factors for exceeding NIOSH-recommended core temperature (Tc) thresholds (38 °C [Tc38] and 38.5 °C [Tc38.5]) using generalized linear mixed models. On an average workday, 49% of participants exceeded Tc38 and 10% exceeded Tc38.5. On average, participants first exceeded both thresholds early in the day; the Tc38 threshold mid-morning (10:38 AM), and Tc38.5 about a half hour later (11:10 AM). Risk factors associated with exceeding Tc38 included years working in US agriculture, body mass index, time performing moderate-to-vigorous physical activity, increasing heat index, and field crop work. The high prevalence of core temperatures exceeding recommended limits emphasizes the serious need for mandated HRI prevention programs for outdoor workers.

“We Force Ourselves”: Productivity, Workplace Culture, and HRI Prevention in Florida’s Citrus Groves

Efforts to disseminate heat-related illness (HRI) prevention practices among Latino farmworkers represent a critical occupational safety strategy in Florida. Targeted initiatives, however, require understanding the workplace dynamics that guide agricultural safety behaviors. This article reports focus group data collected in 2018 from citrus harvesters in central Florida and provides an in-depth perspective on the workplace culture that shapes their implementation of heat safety measures. Results indicate that citrus harvesters regularly suffered HRI symptoms yet rarely reported or sought treatment for their injuries. In some cases, the risks of developing HRI were accepted as a facet of agricultural work and harvesters blamed themselves for their illnesses. Implementation of safety practices hinged less on knowledge than on the availability of water and rest breaks and the quality of employer-employee relations and exchanges. Thus, trust was a determinant of workers’ attitudes toward management that contributed to a harvesting operation’s safety climate. Results highlight the difficulties of putting into practice measures that are not rewarded by the workplace culture and suggest that the extent to which intervention strategies promote not only individual safety behaviors but organizational accountability may predict their effectiveness.

A Multi-Level Approach to Heat-Related Illness Prevention in Agricultural Workers

A Multi-Level Approach to Heat-Related Illness Prevention in Agricultural Workers

Knowledge and Practices to Avoid Heat-Related Illness among Hispanic Farmworkers along the Florida-Georgia Line

ABSTRACTObjectives: Farmworkers who harvest and weed field crops are at increased risk for heat exposure and heat-related illness (HRI). The study objectives were to: (1) train crew leaders to use the Occupational Safety and Health Administration (OSHA) heat safety tool app and evaluate the utility of the app from a crew leader perspective; and (2) characterize heat safety knowledge, preventive practices, and perceptions of HRI risk among Hispanic farmworkers.Methods: Before harvest season, six crew leaders completed a 2-hour OSHA heat illness prevention training, including evaluation of a heat safety mobile app. Between August and October 2018, 101 Hispanic farmworkers participated in cross-sectional surveys about heat safety. Survey participants responded to questions about HRI prevention, HRI knowledge, and sociodemographics.Results: Crew leaders using the heat safety app rated the app very highly on relevance, functionality, value and privacy. Farmworkers did not report being overly concerned about HRI based on their survey responses. Nevertheless, 19% of farmworkers had experienced nonspecific symptoms from working in the heat, such as headache, dizziness, and nausea. In the multivariate linear regression model, farmworkers had lower heat safety knowledge scores if they were H-2A visa holders, female, and only “a little bit concerned,” compared to others who were “very concerned” about working in the heat.Conclusion: The results of this study indicate the need for continued heat safety training for both crew leaders and farmworkers to reduce the risk of HRI, especially among less experienced farmworkers.

Outcomes of a Heat Stress Awareness Program on Heat-Related Illness in Municipal Outdoor Workers

Heat stress is an occupational hazard. Exposed workers may suffer heat-related illness, disease exacerbation, increased injuries, and reduced productivity. Response strategies include mitigation policies and preparedness. Frequency of heat-related illness and workers' compensation costs before and after implementation of a voluntary Heat Stress Awareness Program were evaluated retrospectively in outdoor workers from 2009 to 2017. The program consisted of training, acclimatization, and medical monitoring as outlined in NIOSH's Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments. Of the 604 workers assessed, those with two or more risk factors reported a heat-related illness at greater frequency, which decreased after program implementation. Median workers' compensation costs decreased by 50%. Heat-related illness prevention programs can be effective in reducing the frequency and severity of these occupational injuries as well as associated costs.

Evaluation of Occupational Exposure Limits for Heat Stress in Outdoor Workers - United States, 2011-2016.

Heat stress, an environmental and occupational hazard, is associated with a spectrum of heat-related illnesses, including heat stroke, which can lead to death. CDC's National Institute for Occupational Safety and Health (NIOSH) publishes recommended occupational exposure limits for heat stress (1). These limits, which are consistent with those of the American Conference of Governmental Industrial Hygienists (ACGIH) (2), specify the maximum combination of environmental heat (measured as wet bulb globe temperature [WBGT]) and metabolic heat (i.e., workload) to which workers should be exposed. Exposure limits are lower for workers who are unacclimatized to heat, who wear work clothing that inhibits heat dissipation, and who have predisposing personal risk factors (1,2). These limits have been validated in experimental settings but not at outdoor worksites. To determine whether the NIOSH and ACGIH exposure limits are protective of workers, CDC retrospectively reviewed 25 outdoor occupational heat-related illnesses (14 fatal and 11 nonfatal) investigated by the Occupational Safety and Health Administration (OSHA) from 2011 to 2016. For each incident, OSHA assessed personal risk factors and estimated WBGT, workload, and acclimatization status. Heat stress exceeded exposure limits in all 14 fatalities and in eight of 11 nonfatal illnesses. An analysis of Heat Index data for the same 25 cases suggests that when WBGT is unavailable, a Heat Index screening threshold of 85°F (29.4°C) could identify potentially hazardous levels of workplace environmental heat. Protective measures should be implemented whenever the exposure limits are exceeded. The comprehensive heat-related illness prevention program should include an acclimatization schedule for newly hired workers and unacclimatized long-term workers (e.g., during early-season heat waves), training for workers and supervisors about symptom recognition and first aid (e.g., aggressive cooling of presumed heat stroke victims before medical professionals arrive), engineering and administrative controls to reduce heat stress, medical surveillance, and provision of fluids and shady areas for rest breaks.

Evaluation of a Novel Syndromic Surveillance Query for Heat-Related Illness Using Hospital Data From Maricopa County, Arizona, 2015.

We evaluated a novel syndromic surveillance query, developed by the Council of State and Territorial Epidemiologists (CSTE) Heat Syndrome Workgroup, for identifying heat-related illness cases in near real time, using emergency department and inpatient hospital data from Maricopa County, Arizona, in 2015. The Maricopa County Department of Public Health applied 2 queries for heat-related illness to area hospital data transmitted to the National Syndromic Surveillance Program BioSense Platform: the BioSense "heat, excessive" query and the novel CSTE query. We reviewed the line lists generated by each query and used the diagnosis code and chief complaint text fields to find probable cases of heat-related illness. For each query, we calculated positive predictive values (PPVs) for heat-related illness. The CSTE query identified 674 records, of which 591 were categorized as probable heat-related illness, demonstrating a PPV of 88% for heat-related illness. The BioSense query identified 791 patient records, of which 589 were probable heat-related illness, demonstrating a PPV of 74% for heat-related illness. The PPV was substantially higher for the CSTE novel and BioSense queries during the heat season (May 1 to September 30; 92% and 85%, respectively) than during the cooler seasons (55% and 29%, respectively). A novel query for heat-related illness that combined diagnosis codes, chief complaint text terms, and exclusion criteria had a high PPV for heat-related illness, particularly during the heat season. Public health departments can use this query to meet local needs; however, use of this novel query to substantially improve public health heat-related illness prevention remains to be seen.

Cost of preventing workplace heat-related illness through worker breaks and the benefit of climate-change mitigation

The exposure of workers to hot environments is expected to increase as a result of climate change. In order to prevent heat-related illness, it is recommended that workers take breaks during working hours. However, this would lead to reductions in worktime and labor productivity. In this study, we estimate the economic cost of heat-related illness prevention through worker breaks associated with climate change under a wide range of climatic and socioeconomic conditions. We calculate the worktime reduction based on the recommendation of work/rest ratio and the estimated future wet bulb glove temperature, which is an index of heat stresses. Corresponding GDP losses (cost of heat-related illness prevention through worker breaks) are estimated using a computable general equilibrium model throughout this century. Under the highest emission scenario, GDP losses in 2100 will range from 2.6 to 4.0% compared to the current climate conditions. On the other hand, GDP losses will be less than 0.5% if the 2.0 °C goal is achieved. The benefit of climate-change mitigation for avoiding worktime loss is comparable to the cost of mitigation (cost of the greenhouse gas emission reduction) under the 2.0 °C goal. The relationship between the cost of heat-related illness prevention through worker breaks and global average temperature rise is approximately linear, and the difference in economic loss between the 1.5 °C goal and the 2.0 °C goal is expected to be approximately 0.3% of global GDP in 2100. Although climate mitigation and socioeconomic development can limit the vulnerable regions and sectors, particularly in developing countries, outdoor work is still expected to be affected. The effectiveness of some adaptation measures such as additional installation of air conditioning devices or shifting the time of day for working are also suggested. In order to reduce the economic impacts, adaptation measures should also be implemented as well as pursing ambitious climate change mitigation targets.

Risk Factors for Heat-Related Illness in Washington Crop Workers

ABSTRACTCrop workers are at high risk of heat-related illness (HRI) from internal heat generated by heavy physical work, particularly when laboring in hot and humid conditions. The aim of this study was to identify risk factors for HRI symptoms in Washington crop workers using an audio computer-assisted self-interview (A-CASI) instrument that has undergone reliability and validity evaluation. A cross-sectional A-CASI survey of 97 crop workers in Washington State was conducted during the summer of 2013. Potential HRI risk factors in demographic, training, work, hydration, clothing, health, and environmental domains were selected a priori for evaluation. Mixed-effects logistic regression was used to identify risk factors for self-reported symptoms associated with heat strain and HRI (dizziness/light-headedness or heavy sweating) experienced at work in hot conditions. An increase in age was associated with a lower odds of HRI symptoms (odds ratio [OR] = 0.92; 95% confidence interval [CI] = 0.87–0.98). Piece rate compared with hourly payment (OR = 6.20; 95% CI = 1.11–34.54) and needing to walk for more than 3 minutes to get to the toilet, compared with less than 3 minutes (OR = 4.86; 95% CI = 1.18–20.06), were associated with a higher odds of HRI symptoms. In this descriptive study of risk factors for HRI symptoms in Washington crop workers, decreased age (and less work experience), piece rate pay, and longer distance to the toilet were associated with self-reported HRI symptoms. Modifiable workplace factors should be considered in HRI prevention efforts that are evaluated using objective measures in representative working populations.

Exertional Heat-Related Illnesses at the Grand Canyon National Park, 2004–2009

The Grand Canyon National Park has approximately 4 million visitors between April and September each year. During this period, outdoor activity such as hiking is potentially hazardous owing to extreme heat, limited shade, and steep, long ascents. Given the high visitation and the public health interest in the effects of extreme heat, this study calculated morbidity rates and described heat-related illness (HRI) among visitors. We conducted a retrospective cross-sectional study from April 1 through September 30, during 2004-2009. From a review of Ranger Emergency Medical Services (EMS) incident report files, we extracted information on those that met the case definition of greater than 1 hour of outdoor heat exposure with an HRI assessment or diagnosis, HRI self-report, or signs or symptoms of HRI without another etiology noted. Visitor and temperature data were obtained from respective official sources. Grand Canyon EMS responded to 474 nonfatal and 6 fatal HRI cases, with the majority (84%) being US residents, 29% from Western states. Of the nonfatal cases, 51% were women, the median age was 43 years (range, 11-83 years), and 18% reported a cardiovascular condition. Clinical HRI assessments included dehydration (25%), heat exhaustion (23%), and suspected hyponatremia (19%). Almost all (90%) were hiking; 40% required helicopter evacuation. The highest HRI rates were seen in May. HRI remains a public health concern at the Grand Canyon. High-risk evacuations and life-threatening conditions were found. Majority were hikers, middle-aged adults, and US residents. These findings support the park's hiker HRI prevention efforts and use of park EMS data to measure HRI.

Public Health Impact of Heat-Related Illness Among Migrant Farmworkers

Migrant farmworkers are at risk for heat-related illness (HRI) at work. The purpose of this study was to determine which risk factors could potentially reduce the prevalence of HRI symptoms among migrant farmworkers in Georgia. Trained interviewers conducted in-person interviews of adults who attended the South Georgia Farmworker Health Project clinics in June 2011. The analysis was conducted in 2011-2012. Population intervention models were used to assess where the greatest potential impact could be made to reduce the prevalence of HRI symptoms. In total, 405 farmworkers participated. One third of participants had experienced three or more HRI symptoms in the preceding week. Migrant farmworkers faced barriers to preventing HRI at work, including lack of prevention training (77%) and no access to regular breaks (34%); shade (27%); or medical attention (26%). The models showed that the prevalence of three or more HRI symptoms (n=361, 34.3%) potentially could be reduced by increasing breaks in the shade (-9.2%); increasing access to medical attention (-7.3%); reducing soda intake (-6.7%); or increasing access to regular breaks (-6.0%). Migrant farmworkers experienced high levels of HRI symptoms and faced substantial barriers to preventing these symptoms. Although data are cross-sectional, results suggest that heat-related illness may be reduced through appropriate training of workers on HRI prevention, as well as regular breaks in shaded areas.

A challenging adventure

The Grand Canyon National Park has approximately 4 million visitors between April and September each year. During this period, outdoor activity such as hiking is potentially hazardous owing to extreme heat, limited shade, and steep, long ascents. Given the high visitation and the public health interest in the effects of extreme heat, this study calculated morbidity rates and described heat-related illness (HRI) among visitors.We conducted a retrospective cross-sectional study from April 1 through September 30, during 2004–2009. From a review of Ranger Emergency Medical Services (EMS) incident report files, we extracted information on those that met the case definition of greater than 1 hour of outdoor heat exposure with an HRI assessment or diagnosis, HRI self-report, or signs or symptoms of HRI without another etiology noted. Visitor and temperature data were obtained from respective official sources.Grand Canyon EMS responded to 474 nonfatal and 6 fatal HRI cases, with the majority (84%) being US residents, 29% from Western states. Of the nonfatal cases, 51% were women, the median age was 43 years (range, 11–83 years), and 18% reported a cardiovascular condition. Clinical HRI assessments included dehydration (25%), heat exhaustion (23%), and suspected hyponatremia (19%). Almost all (90%) were hiking; 40% required helicopter evacuation. The highest HRI rates were seen in May.HRI remains a public health concern at the Grand Canyon. High-risk evacuations and life-threatening conditions were found. Majority were hikers, middle-aged adults, and US residents. These findings support the park’s hiker HRI prevention efforts and use of park EMS data to measure HRI.