Occupational exposure to heat can result in injuries, disease, reduced productivity, and death. To address this hazard, the National Institute for Occupational Safety and Health (NIOSH) has evaluated the scientific data on heat stress and hot environments and has updated the Criteria for a Recommended Standard: Occupational Exposure to Hot Environments [NIOSH 1986a]. This document was last updated in 1986, and in recent years, including during the Deepwater Horizon oil spill response of 2010, questions were raised regarding the need for revision to reflect recent research and findings. In addition, there is evidence that heat stress is an increasing problem for many workers, particularly those located in densely populated areas closer to the equator where temperatures are expected to rise in relation to the changing climate [Lucas et al. 2014]. This revision includes additional information about the physiological changes that result from heat stress; updated information from relevant studies, such as those on caffeine use; evidence to redefine heat stroke and associated symptoms; and updated information on physiological monitoring and personal protective equipment and clothing that can be used to control heat stress. Workers who are exposed to extreme heat or work in hot environments indoors or outdoors, or even those engaged in strenuous physical activities may be at risk for heat stress. Exposure to extreme heat can result in occupational illnesses caused by heat stress, including heat stroke, heat exhaustion, heat syncope, heat cramps, heat rashes, or death. Heat can also increase workers' risk of injuries, as it may result in sweaty palms, fogged-up safety glasses, dizziness, and may reduce brain function responsible for reasoning ability, creating additional hazards. Other heat injuries, such as burns, may occur as a result of contact with hot surfaces, steam, or fire. Those at risk of heat stress include outdoor workers and workers in hot environments, such as fire fighters, bakery workers, farmers, construction workers, miners (particularly surface miners), boiler room workers, and factory workers. In 2011, NIOSH published with the Occupational Safety and Health Administration (OSHA) a co-branded infosheet on heat illness. Through this combined effort, many recommendations were updated, including those on water consumption. In addition, factors that increase risk and symptoms of heat-related illnesses were more thoroughly defined. In 2013, NIOSH published <a href="https://www.cdc.gov/niosh/docs/wp-solutions/2013-143/"target="_blank">"Preventing Heat-related Illness or Death of Outdoor Workers"</a>. Outdoor workers are exposed to a great deal of exertional and environmental heat stress. Chapters on basic knowledge of heat balance and heat exchange largely remain unchanged, although clothing insulation factors have been updated to reflect current International Organization for Standardization (ISO) recommendations. Additional information on the biological effects of heat has become available in recent studies, specifically increasing the understanding of the central nervous system, circulatory regulation, the sweating mechanism, water and electrolyte balance, and dietary factors. New knowledge has been established about risk factors that can increase a worker's risk of heat-related illness. Those over the age of 60 are at additional risk for suffering from heat disorders [Kenny et al. 2010]. Additional studies have examined sex-related differences regarding sweat-induced electrolyte loss and whole-body sweat response, as well as how pregnancy affects heat stress tolerance [Meyer et al. 1992; Navy Environmental Health Center 2007; Gagnon and Kenny 2011]. As obesity and the increasingly overweight percentage of the population in the United States continue to increase, this is now a major health concern in workers. Heat disorders among the obese and overweight occur more frequently than in lean individuals [Henschel 1967; Chung and Pin 1996; Kenny et al. 2010]. Another factor affecting heat-related illness is use of drugs, including cocaine, alcohol, prescription drugs, and caffeine. Caffeine use has long been argued against, as it has a diuretic effect and may reduce fluid volume, leading to cardiovascular strain during heat exposure [Serafin 1996]. However, more recent studies have found that the effect of caffeine on heat tolerance may be much less than previously suspected [Roti et al. 2006; Armstrong et al. 2007a; Ely et al. 2011]. The definition of heat stroke has also changed in recent years. Heat stroke is now classified as either classic heat stroke or exertional heat stroke which is more common in workplace settings. Characteristics of the individual (e.g., age and health status), type of activity (e.g., sedentary versus strenuous exertion), and symptoms (e.g., sweating versus dry skin) vary between these two classifications [DOD 2003]. Re-education is needed in the workplace especially about symptoms. Many workers have incorrectly been taught that as long as they were still sweating they were not in danger of heat stroke. Measurements of heat stress are largely unchanged since the last revision, although additional information has been added about bimetallic thermometers and the psychrometric chart. The latter is a useful graphic representation of the relationships among dry bulb temperature, wet bulb temperature, relative humidity, vapor pressure, and dew point temperature. Such charts are especially valuable for assessing the indoor thermal environment. In addition, many modern computers and mathematical models can be used to calculate heat stress indices, based on weather station data. Heat stress can be reduced by modifying metabolic heat production or heat exchange by convection, radiation, or evaporation. In a controlled environment, these last three can be modified through engineering controls, including increasing ventilation, bringing in cooler outside air, reducing the hot temperature of a radiant heat source, shielding the worker, and using air conditioning equipment. Heat stress can also be administratively controlled through limiting the exposure time or temperature (e.g., work/rest schedules), reducing metabolic heat load, and enhancing heat tolerance (e.g., acclimatization). Although most healthy workers will be able to acclimatize over a period of time, some workers may be heat intolerant. Heat intolerance may be related to many factors; however, a heat tolerance test can be used to evaluate an individual's tolerance, especially after an episode of heat exhaustion or exertional heat stroke [Moran et al. 2007]. Additional preventive strategies against heat stress include establishing a heat alert program and providing auxiliary body cooling and protective clothing (e.g., water-cooled garments, air-cooled garments, cooling vests, and wetted overgarments). Employers should establish a medical monitoring program to prevent adverse outcomes and for early identification of signs that may be related to heat-related illness. This program should include preplacement and periodic medical evaluations, as well as a plan for monitoring workers on the job. Health and safety training is important for employers to provide to workers and their supervisors before they begin working in a hot environment. This training should include information about recognizing symptoms of heat-related illness; proper hydration (e.g., drinking 1 cup [8 oz.] of water or other fluids every 15-20 minutes); care and use of heat-protective clothing and equipment; effects of various factors (e.g., drugs, alcohol, obesity, etc.) on heat tolerance; and importance of acclimatization, reporting symptoms, and giving or receiving appropriate first aid. Supervisors also should be provided with appropriate training about how to monitor weather reports and weather advisories. The NIOSH Recommended Alert Limits (RALs) and Recommended Exposure Limits (RELs) were evaluated. It was determined that the current RALs for unacclimatized workers and RELs for acclimatized workers are still protective for most workers. No new data were identified to use as the basis for updated RALs and RELs. Most healthy workers exposed to environmental and metabolic heat below the appropriate NIOSH RALs or RELs will be protected from developing adverse health effects. The Wet Bulb Globe Temperature-based limits for acclimatized workers are similar to those of OSHA, the American Conference of Governmental Industrial Hygienists, the American Industrial Hygiene Association, and the ISO. In addition, the Universal Thermal Climate Index (UTCI), originally developed in 2009, is gaining acceptance as a means of determining environmental heat stress on workers [Blazejczyk et al. 2013]. During the 2014 peer review of the draft criteria document, concerns were expressed about the sufficiency of the scientific data to support the NIOSH ceiling limits for acclimatized and unacclimatized workers. In fact, many acclimatized workers live and work in temperatures above the ceiling limits without adverse health effects. Further consideration of the scientific data led to the decision to remove the ceiling limit recommendations from the document. Although research has produced substantial new information since the previous revision of this document, the need for additional research continues. Two newer areas of research that will likely continue to grow are the effects of climate change on workers and how heat stress affects the toxic response to chemicals. It is likely but unclear to what extent global climate change will impact known heat-exposure hazards for workers, especially with regard to severity, prevalence, and distribution [Schulte and Chun 2009; Schulte et al. 2015]. Toxicological research has shown that heat exposure can affect the absorption of chemicals into the body. Most of what is known on this subject comes from animal studies, so a better understanding of the mechanisms and role of ambient environment with regard to human health is still needed [Gordon 2003; Gordon and Leon 2005]. With changes in the climate, the need for a better understanding will become increasingly important [Leon 2008]. In addition to the updated research, this criteria document includes more resources for worker and employer training. Information about the use of urine color charts, including a chart and additional information, is in Appendix B. The National Weather Service Heat Index is in Appendix C, along with the OSHA-modified corresponding worksite protective measures and associated risk levels. NIOSH recommends that employers implement measures to protect the health of workers exposed to heat and hot environments. Employers need to ensure that unacclimatized and acclimatized workers are not exposed to combinations of metabolic and environmental heat greater than the applicable RALs/RELs. Employers need to monitor environmental heat and determine the metabolic heat produced by workers (e.g., light, moderate, or heavy work). Additional modifications (e.g., worker health interventions, clothing, and personal protective equipment) may be necessary to protect workers from heat stress, on the basis of increases in risk. In hot conditions, medical screening and physiological monitoring are recommended. Employers, supervisors, and workers need to be trained on recognizing symptoms of heat-related illness; proper hydration; care and use of heat-protective clothing and equipment; effects of various risk factors affecting heat tolerance (e.g., drugs, alcohol, obesity, etc.); importance of acclimatization; importance of reporting symptoms; and appropriate first aid. Employers should have an acclimatization plan for new and returning workers, because lack of acclimatization has been shown to be a major factor associated with worker heat-related illness and death. NIOSH recommends that employers provide the means for appropriate hydration and encourage their workers to hydrate themselves with potable water <15 degrees-C (59 degrees-F) made accessible near the work area. Workers in heat <2 hours and involved in moderate work activities should drink 1 cup (8 oz.) of water every 15-20 minutes, but during prolonged sweating lasting several hours, they should drink sports drinks containing balanced electrolytes. In addition, employers should implement a work/rest schedule and provide a cool area (e.g., air-conditioned or shaded) for workers to rest and recover. These elements are intended to protect the health of workers from heat stress in a variety of hot environments.