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Heat-related illness during extreme weather emergencies

Note: An abbreviated version of this report has been published: Blum LN, Bresolin LB, Williams MA, for the Council on Scientific Affairs: Heat-related illness during extreme weather emergencies. JAMA. 1998;279:1514.

The following is the full text of the CSA Report that was presented at the 1997 AMA Annual Meeting; it represents the scientific and medical literature on this subject as of June 1997.

Full text

In July 1995, a four-day period of extreme heat resulted in or contributed to the death of more than over 700 residents of Chicago.1,2 This latest example of the insidiously deleterious effect of heatwaves prompted a renewed interest by the public health community in creating and publicizing plans for addressing extreme weather emergencies. In June 1996, public health professionals convened in Chicago at a forum sponsored by Medic Alert to consider heat preparedness policies. This forum was held in association with the 1996 Annual Meeting of the American Medical Association (AMA) House of Delegates. Based on that forum, the AMA has agreed to collaborate with Medic Alert in a program to educate physicians and the public about the prevention and treatment of weather-related illness. 

Thermoregulation 

Maintaining a consistent internal temperature is essential to normal body functioning. A series of biological mechanisms work in concert to maintain core body temperature of humans in a narrow range around 37°C.3 Variations of as little as 10% can result in life-threatening illness.3 The complex thermoregulatory functions of humans are governed and coordinated by the hypothalamus.3 

Body heat is generated predominantly through two mechanisms. The first is through release of energy as a product of metabolism. The second is through muscular activities, which can elevate basal heat production from its resting point of approximately 294 kJ/h to as high as 4200 kJ/h during periods of extreme exertion.4 As heated blood is pumped to the anterior hypothalamus, it triggers a series of biological mechanisms to dissipate heat and cool the body.4 

There are four major mechanisms by which heat is lost from the body.3,  5 Conduction of body heat occurs when heat is dissipated passively from the body into surrounding air, liquid, or solids. Convection occurs when the heat is transferred to moving currents of air across the body surface, which then carries the heat away from the body.3 In radiation, the body transfers energy electromagnetically (infrared radiation) into its environment. Finally, heat is lost through the evaporation of perspiration. The body facilitates these processes when triggered by the hypothalamus to divert blood away from its core to surface areas (vasodilatation) or to initiate perspiration. Typically, the hypothalamus also cues the body to replace fluid lost through perspiration by triggering thirst.3,6 

Conduction, convection, and radiation are only effective when ambient temperature is lower than body temperature.5 As environmental temperature reaches or passes body temperature, the body becomes reliant on perspiration to dissipate heat.3 When ambient humidity surpasses about 60%, this mechanism also becomes less effective, since heat loss requires evaporation.5 Evaporation is also less effective when there is no air movement or convection to carry saturated air away from the skin, or when clothing is worn. 

Heat-related illnesses

Heat-related illnesses range along a continuum of severity. Minor syndromes, if untreated, can progress to more serious syndromes. Minor illnesses include heat cramps, heat syncope, and heat edema.7 

Heat cramps are related to hyponatremia and depletion of other electrolytes. They typically occur following periods of exertion, during which profuse sweating occurs. It is not unusual for onset to occur during the cool-down period. The syndrome is characterized by intense, painful cramping of lower extremities and abdominal muscles. Treatment consists of discontinuing exertion (if this has not already happened) and replenishing fluid volume with salted drinks. In most cases, this can be done orally. Muscle massage is generally not helpful.8,  9,  10 

Heat edema is a secondary effect of prolonged peripheral vasodilatation, followed by orthostatic pooling of blood in the extremities. It is usually responsive to elevating the legs, punctuated by periods of gentle movement to increase venous return.4,8,  9,  10 

Heat syncope is marked by a sudden loss of consciousness. Its cause is similar to heat edema, in that it generally results from peripheral pooling of blood with consequent orthostatic hypotension. Loss of consciousness can often be avoided by sitting or lying down when light-headedness or weakness is experienced.4,8,  9,  10 

Of greater severity is heat exhaustion, which arises from significant depletion of blood plasma volume, often accompanied by hyponatremia and/or peripheral blood pooling. Victims of heat exhaustion present with a cluster of vague symptoms, including mild disorientation, generalized malaise, weakness, nausea, vomiting, headache, tachycardia, and hypotension. Body temperature may be slightly elevated, and profuse sweating is common. While readily treatable, heat exhaustion can progress to the more serious syndrome of heat stroke if undiagnosed. The most common misdiagnosis for heat exhaustion is symptoms of viral illness. Persons experiencing heat exhaustion should be removed to a cool environment and receive fluid and electrolyte replacement. In more severe cases, active cooling measures should be undertaken. These include loosening clothing, increasing airflow across the skin with a fan, misting the person with tepid water, or applying ice to extremities. In cases where very cold water or ice packs are used, massage can counteract vasoconstriction.4,8,  9,  10 

The most serious form of heat-related illness is heat stroke. Heat stroke constitutes an emergency and, left untreated, can rapidly lead to death.5 Heat stroke occurs when the body's thermoregulatory abilities are compromised, resulting in core body temperatures in excess of 40°C (104°F).5 There are two forms of heat stroke. Classic heat stroke occurs without exertion, generally among people at risk--the elderly, infants, persons with chronic illnesses, etc.6-8 Exertional heat stroke, in contrast, is most common among young, otherwise healthy adults who are engaging in rigorous exercise in the absence of heat acclimatization (e.g., athletes, construction workers, soldiers).8 Morbidity related to heat stroke stems from the cell destruction arising from heating, as well as the effects of disseminated intravascular coagulation. Death can occur rapidly, through cardiac failure or hypoxia. It can also result from complications arising in subsequent days, for example through renal failure from dehydration, rhabdomyolysis, or both.4,7,  8,  9,  10 

Neurologic effects of heat stroke include warning symptoms of headache, dizziness, and weakness. Confusion or euphoria often precede an abrupt loss of consciousness. During this acute phase, patients can vary widely in their neurologic presentation, becoming combative, experiencing vivid hallucinations, or quietly slipping away into a comatose state. While unconscious, patients can evidence seizure activity and may develop status epilepticus. Consciousness usually returns within a few hours, once cooling treatment begins. However, transient neurologic symptoms--difficulty speaking, impaired concentration, irritability, etc.--may persist for several weeks. In some cases, permanent neurologic damage occurs, the most common of which is a cerebellar syndrome affecting gait, articulation, hyperreflexia, and nystagmus.6 

Treatment protocols for heat stroke are aimed at active cooling measures to bring core body temperatures down to, but not lower than 39°C.11 Medical interventions to replace blood volume, electrolytes, oxygen, and to prevent complications have been detailed in several protocols.4,6,  8,  12 Dramatic cooling mechanisms once recommended (e.g., immersion in ice water or swaddling in cold, wet sheets) are now not recommended, because they are less effective in promoting heat dissipation than modern methods and can make medical monitoring difficult. In general, the more severe and prolonged the acute symptoms, the worse the prognosis.11 

Risk factors for heat-related illness 

A number of factors lead to increased risk for heat-related illness. Meteorologic factors are those that increase exposure and inhibit heat dissipation; these include prolonged periods of ambient temperatures at or above body temperature, high humidity indices, increased barometric pressure, and reduced wind.3 

Geographic factors contribute to increased risk in cities. This risk is related to the 'heat island' effect of urban areas. Large expanses of concrete and other building materials absorb heat during the day and re-radiate heat after sundown (when the temperature would otherwise drop). Clusters of large buildings can impede breezes, and reduced numbers of trees are available to provide shade for buildings and their inhabitants.1,5,  7,  10 

Socioeconomic factors also lead to increased risk.1,7 Perhaps the single most important is lack of access to air conditioning. People who do not own air conditioners, who have no access to air conditioned common areas, or who do not have access to transportation are at much greater risk.1,5,7 Research indicates that use of electric fans does not significantly reduce risk, because convection requires that the moving air currents be cooler than body temperature.1,7,13 Studies have also indicated that people living in apartment buildings, living in upper floors of buildings, or living in flat-roofed buildings are at increased risk. Individuals also may not activate air conditioners or fans out of concern about their utility bills.1,  7 Psychosocial factors also affect risk. Fear of crime, for example, can lead people to leave windows and doors locked or to resist leaving their homes to visit a cooling center.1,10 Social isolation plays a role, in that heat stroke victims are much less likely to have a concerned support network (friends, family, neighbors, church members) to monitor them during heat emergencies.1,  10 

Finally, physiological and medical factors can also greatly increase risk. Normal aging processes, for example, can render older adults at increased risk.4,5 Research indicates that the elderly are less likely to perceive excess heat, have a longer latency before the onset of sweating, and are less likely to experience thirst in the initial stages of dehydration.4,7 Infants are also at increased risk. Children have a greater surface area-mass ratio than adults, which induces a greater heat transfer between the environment and the body. Further they are governed by adults' decisions about how warmly to be dressed or bundled, and they are unable to communicate discomfort from heat or thirst. Chronic medical problems can increase vulnerability to cardiovascular insufficiency or renal problems. The medically and physically disabled are more dependent on others to help them regulate their fluid intake or to remove clothing, travel to cooler environments, etc. Alcohol impairs the body's ability to thermoregulate.13 Several medications also impede thermoregulation.1,10,13,14 Most notable among these are neuroleptic medications and other medications with anticholinergic mechanisms of action. Amphetamines and lithium can increase metabolic heat production.10 Finally, individuals who have a prior history of significant heat-related illness are at increased risk of experiencing trouble again in subsequent heat emergencies.5 

Consequently, it is those individuals who are vulnerable in many ways who are also most vulnerable to heat stroke--the elderly, children, individuals with physical or mental disabilities, alcoholics, the chronically ill, and the socially isolated. 

Prevention strategies 

In the 1995 Chicago experience over 700 people were judged to have died, either directly from heat stroke or from heat-related exacerbations of underlying medical problems.1 The ensuing investigation underscored the need for all communities to have an emergency plan in place to prevent heat-related morbidity and mortality. These plans hinge on successful identification of individuals at risk, as well as of the weather conditions that constitute a medical emergency. 

A variety of indices have been designed15 that attempt to integrate the effects of heat, humidity, air pressure, and other factors into a single value. In addition to identifying thresholds of concern, it is important to monitor the cumulative effects over time of these factors. Most heat-related deaths do not occur on the first day of a heat wave. Rather, deaths tend to increase over several days time.1,7 

Municipal heat emergency plans also need to establish safe havens--air conditioned spaces to which people can retreat and receive fluids and medical assessment, if necessary. Mechanisms must be established to check on people identified as being at risk and to provide transportation to cooling centers as necessary. Networks of city employees, social service volunteers, and community support groups can be organized to provide this function.1,7 

A communication plan must also be activated, engaging the news media to inform the public about environmental risks, educate them about the signs of heat-related illness, encourage them to check on each other, and remind them of steps they can take to reduce their risk. Individual strategies include regular fluid intake (at least 8 oz./hr) regardless of thirst; avoidance of exertion during the day; maintaining salt intake; traveling to cool areas; wearing light, loose clothing; and vigilance to early physiologic warning signs (e.g., light-headedness, mild nausea or confusion, sleepiness, profuse sweating).1,7,14 Individual responsibility is crucial to coping with oppressive heat conditions, through taking commonsense precautions and looking out for neighbors and friends who may be particularly vulnerable. 

Targeted communications are also useful for athletic coaches, supervisors of road crews, and leaders of other groups that may be at risk by virtue of their activities. Education for these leaders on how to prevent, identify, and react to signs of heat illness may be particularly effective in reducing exertional heat stress. 

Community strategies 

Communities can develop plans and procedures for weather-related emergencies. A good model is the plan developed by Chicago in the wake of the 1995 crisis.16 In that plan, Chicago lays out resources available and strategies for involvement of city, county, and state governmental agencies; private and nonprofit entities; and members of the news media. The plan calls for governmental bodies to: 

  • disseminate information about at-risk populations to citizens and community organizations
  • develop and coordinate with the National Weather Service, private weather forecasters, and community organizations a consistent message regarding the dangers of heat, precautions, treatment, and emergency management
  • establish a Heat Awareness Week each year to explain the effects of heat, how to stay cool, and the public and private resources available to individuals in need
  • coordinate available public and private resources into a network--with defined contacts, commitments, and responsibilities--that can be activated in the event of an extreme weather emergency. 

The plan calls on private businesses and nonprofit agencies to provide education and outreach opportunities through distribution of materials, organization of workshops, and identification of individual volunteers. The private sector is also called upon for donation of low-cost cooling devices, supplies for cooling centers, refrigerator magnets with emergency information, transportation, and printing services. 

Finally, print and broadcast media are encouraged to distribute standardized packets and audiovisual messages about preventive strategies and to notify the public when a heat-related emergency has been declared. 

Roles for physicians 

Physicians are an essential component of a multi-pronged effort to identify and educate individuals at risk of heat-related illness.1,7 Patients who are at increased risk by virtue of their medical conditions or psychosocial circumstances can be identified by their physicians. Physicians can educate and counsel patients about their risk, including prevention strategies, early signs and symptoms, and strategies for seeking help when they are in trouble.1,7 Physicians can also counsel parents of young children and caretakers of the elderly or disabled about how to ensure the safety of their charges. Patients living alone can, with their physician's assistance, identify a support person or agency who will be responsible for assisting the patient when help is needed. Where appropriate, physicians can register at-risk patients with local health departments, to ensure that they are checked during heat crises. 

Medic alert 

In 1996, Medic Alert, in association with the AMA, held a heat policy preparedness forum in Chicago, as well as providing consumer information to other warm-weather locales deemed to be at risk for heat-related disasters. Medic Alert has also prepared educational and press materials that detail ways to protect the public during heat and cold weather crises. Medic Alert will be collaborating with the AMA and other medical organizations to educate physicians about emergency treatment for heat-affected patients. 

Recommendations 

The following statements, recommended by the Council on Scientific Affairs, were adopted by the AMA House of Delegates as AMA policy at the 1997 AMA Annual Meeting.

The AMA recognizes the significant public health threat imposed by heat-related emergencies, and provides the following policy:

  1. Physicians should identify patients at risk for extreme heat-related illness such as the elderly, children, individuals with physical or mental disabilities, alcoholics, the chronically ill, and the socially isolated. Patients, family members, friends, and caretakers should be counseled about prevention strategies to avoid such illness. Physicians should provide patients at risk with information about cooling centers and encourage their use during heat emergencies.
  2. The AMA encourages patients at risk for heat-related illness to consider wearing appropriate medical identification.

References

  1. Semenza JC, Rubin CH, Falter K, Selaniki J, Flanders D, Howe, HL, Wilhelm JL. Heat-related deaths during the July 1995 heat wave in Chicago. N Engl J Med. 1996;335;84-90.
  2. Voelker R. Probe of heat wave deaths under way. JAMA. 1995; 274:595-596.
  3. Caroline NL. Heat exposure. Emergency Care in the Streets. Boston: Little, Brown, and Company; 1995:809-816.
  4. Brody GM. Hyperthermia and hypothermia in the elderly. Clin Geriatr Med. 1994;10:213-229.
  5. Kilbourne EM. Illness due to thermal extremes. Public Health Prev Med. 1992;13:491-501.
  6. Goetz CG, Klawans HL. Hyperthermic states: heat stroke and malignant hyperthermia. In: Vinken PJ, Bruyn GW, eds. Neurological Manifestations of Systemic Disease. Amsterdam, Holland: North Holland Publishing Company; 1979:543-561.
  7. Kilbourne EM. Heat waves. In: Gregg MB, ed. The Public Health Consequences of Disasters: The Center for Disease Control Monograph. Atlanta: Department of Health and Human Services; 1989:51-61.
  8. Lee-Chiong TL, Stitt JT. Heatstroke and other heat-related illnesses. Heatstroke. 1995;98:26-36.
  9. Bross MH, Binford TN, Carlton FB. Heat emergencies. Am Fam Physician. 1994;50:389-397.
  10. Kilbourne M, Choi K, Jones S, Thacker SB. Risk factors for heatstroke. JAMA. 1982;47;3332-3336.
  11. Auerbach PS, Fleisher GR, Knochel JP. Heatstroke: be ready for summer. Patient Care. 1993;May 15:52.
  12. Iseke RJ. Heat-related illnesses. In Noble J, ed. Textbook of Primary Care Medicine. 2nd Ed. New York: Mosby, 1996: 63-66.
  13. Heat-related illnesses and deaths--United States, 1994-1995. Morbid Mortal Weekly Rep. 1995;44:465-469.
  14. Heat-wave-related mortality--Milwaukee, Wisconsin, July 1995. From the CDC. JAMA. 1996; 276:275.
  15. Dukes-Dobos FN. Heat stress. In Brooks SM, Gochfeld M, Herzstein J, Jackson RJ, Schenker MB, eds. Environmental Medicine. New York: Mosby, 1995: 563-572.
  16. City of Chicago. Mayor's Commission on Extreme Weather Conditions. Chicago: City of Chicago, 1995.

Also visit the AMA Center for Disaster Preparedness and Emergency Response.

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Last updated: Sep 24, 2007
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