The National Institute for Occupational Safety and Health (NIOSH) received a health hazard evaluation (HHE) request from the management of the Grand Canyon National Park in Arizona. The request indicated that many employees, especially park rangers assigned to the park's inner canyon and wilderness areas, are at high risk for heat stress illness because duties requiring moderate to extreme physical exertion are conducted in outdoor temperatures that usually exceed 100 F during spring and summer. The requesters asked NIOSH to evaluate heat stress in the work environment and make recommendations to prevent heat illness among the employees. The first of two investigations was conducted September 5-12, 1999. Core body temperatures (CBTs), physiological hydration measurements, and self-reported heat strain indicators, were collected from patrolling corridor rangers and trail crew personnel rebuilding an inner-canyon section of the North Kaibab trail. The second evaluation took place June 26 - July 5, 2000; CBT, heart rate, blood electrolytes, and pre- and post-activity body weights were measured on wilderness and corridor rangers, preventative search and rescue rangers (PSARs), and maintenance rangers. Individual and task-specific metabolic rates were estimated, and wet bulb globe temperatures (WBGTs) were measured during both evaluations. Monitored activities included hiking into and out of the canyon between the South Rim and Phantom Ranch, patrolling the inner canyon, and patrolling the South Rim. Most activities included trail-dependent elevation changes of up to 4,900 feet (ft). The results were compared to the NIOSH recommended action limits and recommended exposure limits (RALs/RELs) and the American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Values (TLVs). NIOSH and ACGIH assess heat stress using sliding scale limits based on environmental and metabolic heat loads. In addition, ACGIH provides physiological heat strain limits in situations of excess heat stress. For individuals with normal cardiac performance, sustained (over several minutes) heart rate should remain below 180 beats per minute (bpm) minus age (in years), maximum CBT should remain below 100.4 F for unselected, unacclimatized personnel (101.3 F for medically selected, acclimatized personnel), recovery heart rate at one minute after a peak work effort should be below 110 bpm, and there should be no symptoms of sudden and severe fatigue, nausea, dizziness, or lightheadedness. Results of both surveys indicated that most trail crew members and rangers were exposed to heat stress in excess of the screening criteria. During the 2000 survey, daily high WBGTs at Phantom Ranch averaged 92 F with a one-day peak of 98.8 F, and none of the participants could be considered fully acclimatized (used to working in the extreme heat.) All participants experienced heat strain to some degree, i.e., some or all of their measured physiologic parameters exceeded at least one of the evaluation criteria during each activity. Five of six participants hiking out of the canyon exceeded a CBT of 100.4 F an average of 43% of their activities, and two participants exceeded 101.3 F about 20% of the time. One ranger exceeded a CBT of 102.2 F for 14% of the activity, a level that calls for immediate termination of exposure even when the individual is being monitored. All of those hiking out also exceeded the heart rate criterion. All monitored participants hiking into the canyon exceeded the heart rate and CBT criteria, as did five of six PSAR participants patrolling the south rim, and all three employees conducting inner-canyon patrol. All but one of the measured participants developed mild dehydration (body weight loss of 1.5% or less) during their activities in the heat, but their electrolytes remained within normal limits. One case of hypernatremia (attributed to dehydration) occurred. The baseline osmolality (a measure of serum solute concentration) was elevated in approximately 50% of the participants, which indicates some degree of dehydration prior to activities. During both surveys, the majority of employees reported that they had a history of heat-related symptoms or illnesses while working in the park but had never reported them. Health hazards exist from excessive heat and overwork for employees assigned to the corridor trails and inner canyon of Grand Canyon National Park. Every participant was exposed to combinations of environmental and metabolic heat in excess of occupational criteria, and every participant with heart rate and core body temperature measurements exceeded one or both of the criteria for at least part of the sampling period. The majority of participants reported a history of heat-related symptoms or illnesses while working at the park, however, the park does not have a formal heat stress program for its employees or a system for reporting heat-related illnesses. Recommendations are made regarding reductions in workload and the establishment of work monitoring, heat stress training, and medical surveillance programs.
Division of Surveillance, Hazard Evaluations, and Field Studies, Hazard Evaluations and Technical Assistance Branch, National Institute for Occupational Safety and Health, 4676 Columbia Parkway, Cincinnati, OH 45226, USA