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Heat illness is a spectrum of disorders due to increased body temperature. It can be caused by either environmental conditions or by exertion. It includes minor conditions such as heat cramps, heat syncope, and heat exhaustion as well as the more severe condition known as heat stroke. [1] It can affect any or all anatomical systems. [2]
Heat stress can result in heat-related illnesses, such as heat stroke, hyperthermia, heat exhaustion, heat cramps, heat rashes, and chronic kidney disease (CKD). [2] [3] Although heat exhaustion is less severe, heat stroke is a medical emergency and requires emergency treatment, which if not provided, can lead to death. [4]
In urban areas people are at an increased susceptibility to hyperthermia. This is due to a phenomenon called the urban heat island effect. [36] Since the 20th century in the United States, the north-central region (Ohio, Indiana, Illinois, Missouri, Iowa, and Nebraska) was the region with the highest morbidity resulting from hyperthermia.
Heat syncope, also referred to as exercise-associated collapse, is a moderate form of heat-related illness characterized by a temporary loss of consciousness. Unlike heat exhaustion, heat cramps and heat syncope do not have systemic effects. Heat exhaustion is a precursor to heat stroke, a severe form of heat-related illness.
People have different metabolic rates that can fluctuate due to activity level and environmental conditions. [ 23 ] [ 24 ] [ 25 ] ASHRAE 55-2017 defines metabolic rate as the rate of transformation of chemical energy into heat and mechanical work by metabolic activities of an individual, per unit of skin surface area.
With humid heat, the moisture in the air can prevent the evaporation of sweat. [21] Regardless of acclimatization, humid heat poses a far greater threat than dry heat; humans cannot carry out physical outdoor activities at any temperature above 32 °C (90 °F) when the ambient humidity is greater than 95%.
The start of heat stroke can be sudden or gradual. [3] Heatstroke is a life-threatening condition due to the potential for multi-organ dysfunction, [6] with typical complications including seizures, rhabdomyolysis, or kidney failure. [3] Heat stroke occurs because of high external temperatures and/or physical exertion.
An excessive heat warning is a notice issued by the National Weather Service of the United States within 12 hours of the heat index reaching one of two criteria levels. In most areas, a warning will be issued if there is a heat index of at least 105 °F (41 °C) for more than three hours per day for two consecutive days, or if the heat index is greater than 115 °F (46 °C) for any period of time.