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California Environmental Health Tracking Program

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Last Edited: 9/16/2012

Heat and How It Affects Our Health

The public health risks associated with extreme heat events vary across California's communities and among the individuals within the communities. 

Back to Extreme Heat and Climate Change


What is heat illness and mortality?

Heat-related illness is a broad spectrum of disease.  The spectrum goes from mild heat cramps to life-threatening heat stroke.

  • Heat stress: Any individual- regardless of age, sex, or health status- can develop heat stress if engaged in intense physical activity and/or exposed to environmental heat. The body aims to maintain core temperature of 37C (98.6F). When the core temperature rises, the body’s response is to sweat and to increase circulation of blood to the skin’s surface to increase cooling. When a body’s capacity to cool isn’t enough to combat heat exposure, a range of heat-related symptoms and conditions can occur.
  • Heat cramps: Severe painful cramping of muscles in the legs or abdomen. Exertion, with profuse sweating, is a common cause of heat cramps. Heat cramps often occur during cool-down after the activity has stopped. If heat cramps occur, activity should be stopped and salted drinks (e.g. some sports drinks) should be consumed to replenish fluid volume.
  • Heat syncope: The main characteristic of heat syncope is fainting, preceded by warning signs such as light-headedness or weakness. Sitting or lying down at the first warning signs can avoid loss of consciousness.
  • Heat edema: Swelling in legs due to accumulation of fluids in the tissues. This is a result of prolonged dilation of small arteries in the legs. It can usually be treated by alternating between elevating the legs and gentle movement of the legs to increase circulation.
  • Heat exhaustion: Heat exhaustion the consequence of extreme depletion of blood plasma volume. Symptoms include mild disorientation, weakness, nausea, vomiting, headache, rapid heartbeats, and hypotension. Because of these symptoms, it can often be misdiagnosed as acute viral illness. An individual with these symptoms should be moved to a cool environment, provided salted fluids (e.g. some sports drinks), and cooled. An individual can be cooled by loosening clothing, fanning while misting or wiping down with cool water, or placing ice packs on their extremities alternating with massaging of the extremities. If untreated, heat exhaustion could progress to heat stroke, the most serious form of heat-related illness.
  • Heat stroke: Heat stroke is an extreme medical emergency. If untreated, it can result in death or permanent neurological damage. Heat stroke occurs when a person’s core body temperature rises about 104F (40C). The first signs of heat stroke include headache, dizziness, and weakness, which can be followed by confusion or euphoria and a sudden loss of consciousness. A person suffering heat stroke may have hallucinations, be combative, or become comatose. Unconscious patients may suffer seizures. An individual suffering from heat stroke should receive medical attention immediately. The individual should be cooled as quickly as possible to bring his core body temperature to 102F (39C) or below. Blood volume and electrolytes should also be replaced.

Heat-related illness and death is preventable if appropriate prevention strategies are adopted and implemented by communities and individuals.

Find out more in our collaborative report on Heat-Related Illness and Mortality  (PDF, 4.3 MB)

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How has temperature changed in California?

  • California appears to be slowly warming. Temperatures are increasing most rapidly in Southern California and in urban areas.
  • Minimum temperatures at night have been increasing at a higher rate than daytime temperature increases. This results in fewer opportunities for the body to cool overnight during heat waves.
  • "Heat islands" play a large role in the elevated nighttime temperatures. Heat islands are geographic areas that are warmer than surrounding areas. They form as cities replace natural land cover. Pavement, buildings, and other infrastructure retain heat more than natural surfaces.
  • The number and duration of extreme heat events are expected to increase. This will lead to increases in heat-related illnesses and deaths if precautionary measures are not put in place.

See maps of temperature in California

Find out more in our collaborative report on Heat-Related Illness and Mortality  (PDF, 4.3 MB)

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Which individuals and counties are at highest risk?

Individuals who are at highest risk for developing heat-illness include:

  • Elderly
  • Children 
  • Participants in athletic events
  • Outdoor workers
  • Medically compromised and socially isolated

California counties have varying and unique indicators of risk, including high proportions of elderly, socially isolated populations, children, outdoor workers, the poor, the chronically ill, and the medically under served.  County and local health officials are urged to identify vulnerable communities within their jurisdiction. Simple methods can be used such as targeting all ZIP codes where more than 20% of the residents live under the U.S. poverty level, for example.


The urban areas of California such as Los Angeles, San Diego, and the San Francisco Bay Area, have the largest absolute numbers of people vulnerable to heat such as children and the elderly. Yet, other areas of the State, which are away from the major urban centers, especially counties with lower socioeconomic status, have high proportions of populations of socially isolated elderly and elderly living in nursing homes.


Recent and future changes in locations where there are higher average temperatures and where heat waves are most strongly expressed may change geographic risk in California.  The change in risk is due to lack of historic experience with higher temperatures and limited physiologic acclimatization of the residents, and overall adaptive capacity of the community.  Among the factors that influence adaptive capacity to deal with higher temperatures are characteristics of the built environment, such as the amount of impervious surfaces (e.g., paved areas, sparse tree cover), housing types, presence of air conditioning which can influence the ability to cool off at night, and the level of community preparedness (e.g., heat emergency contingency plans, or availability of cooling centers and transportation to those centers).


It is recommended that local health officials focus efforts to prepare areas of the state not historically accustomed to heat.  Foothill and mountainous communities throughout the State may be particularly subject to respiratory and heat stress due to lack of historic adaptability, higher ozone levels, higher elevations, and increasing temperatures. Further degradation of air quality due to climate change will amplify occurrence and severity of known air pollution-related adverse health effects.


See maps of vulnerable populations in California

Find out more in our collaborative report on Heat-Related Illness and Mortality  (PDF, 4.3 MB)

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Deaths due to the July 2006 heat wave

 In the two weeks of the July 2006 heat wave, seven counties accounted for 80% of the reported deaths. 

map of deaths due to heat in July 2006

Find out more in our collaborative report on Heat-Related Illness and Mortality  (PDF, 4.3 MB)

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How can I prevent heat illness?

  • Get plenty to drink: Sweating removes needed salt and minerals from the body. When it is hot, drink more water, juice, and sports drinks. Avoid drinks with caffeine (tea, coffee, and cola) and alcohol. Be sure to eat regularly.
  • Wear light clothing and sunscreen: Wear as little clothing as possible when you are at home. Choose lightweight, light-colored, loose-fitting clothing. In the sun, wear a wide-brimmed hat and use a sunscreen with a sun protection factor (SPF) of 15 or higher and follow package directions.
  • Stay cool indoors: Stay in an air conditioned area. If you don't have an air conditioning, go to a public building, shopping mall, or other location with air conditioning. A cool shower or bath is also a good way to cool off.
  • Schedule outdoor activities carefully: Try to be less active during the hottest part of the day, the late afternoon. If you must be out in the heat, plan your activities so that you are outdoors either before noon or in the evening. While outdoors, rest often in a shady area. Never leave kids or pets in a parked car.
  • Pace yourself: If you are not used to working or exercising in hot weather, start slowly and pick up the pace gradually. Take frequent, regularly scheduled breaks. Stop activity and rest if you become lightheaded, confused, weak, or feel faint.
  • Use a buddy system: During a heat wave, check on your friends and family and have someone do the same for you. If you know someone who is elderly or has a health condition, check on them twice a day during a heat wave. Watch for signs of heat exhaustion or heat stroke. 
  • The use of fans may be dangerous in certain situations. When temperatures are above 100F or when there is a combination of high humidity (greater than 33%) and high temperatures (90F or higher), fans may contribute to illness. However, using a fan along with wetting-down of the skin of an affected person may help the cooling process.

Find out more in our collaborative report on Heat-Related Illness and Mortality  (PDF, 4.3 MB)

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How should I treat heat illness?


Heat cramps

Heat cramps are muscle pains and spasms, which usually involve the stomach muscles or the legs, and often occur during cool-down after the activity has stopped.  If you or another individual experiences these symptoms:

  • Stop the activity and sit quietly in a cool place
  • Drink clear juice or a sports beverage
  • Rest for a few hours to avoid heat exhaustion or heat stroke
  • Get medical help if heat cramps do not stop after one hour
  • If you have heart problems or are on a low-sodium diet, get medicinal attention for heat cramps

Heat exhaustion

Heat exhaustion happens when the body has lost too much water and salt in sweat.  Warning signs include:

  • Heavy sweating
  • Cramps
  • Headache
  • Nausea or vomiting
  • Tiredness
  • Weakness
  • Dizziness
  • Fainting

If heat exhaustion is not treated, it can turn into heat stroke.  If you or another individual experiences these symptoms:

  • Drink cool, nonalcoholic beverages
  • Rest, lying down
  • Take a cool shower, bath, or sponge bath
  • Turn on air-conditioning
  • Change into lightweight clothing

Heat stroke

Heat stroke is the most serious illness in the heat-related illness spectrum.  Heat stroke happens when the body can no longer control its temperature.  The body cannot sweat and is unable to cool itself.  Warning signs include:

  • Red, hot, dry skin
  • Very high body temperature
  • Dizziness
  • Nausea
  • Confusion
  • Strange behavior
  • Unconsciousness
  • Rapid pulse
  • Throbbing headache

Heat stroke can cause death or disability if treatement is not given.  If you or another individual experiences these symptoms:

  • Get medical help quickly
  • Get the victim to a shady area
  • Cool the person off with a cool shower, garden hose, etc
  • Do not give the victim fluids to drink
  • If emergency medical personnel are delayed, call the hospital for further instructions


Find out more in our collaborative report on Heat-Related Illness and Mortality  (PDF, 4.3 MB)

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Recommendations for local and statewide prevention strategies

Recommendations for short-term strategies

  • County and local health officials should identify vulnerable communities within their jurisdiction. This includes children, the elderly, the chronically ill, outdoor workers, and the poor and medically underserved.
  • Health officials should assess strategies for communication and services, particularly to the populations at risk.
  • Transportation to cooling centers should be established and available in each county.
  • Social contacts and caretakers of at-risk populations should be educated about preventative actions. For example, social contacts should know to move vulnerable individuals without air conditioning to cooling centers as soon as a heat wave emergency is declared, regardless of symptoms.
  • Outreach and education about personal cooling strategies and first aid for heat illness should be implemented. A combination of stakeholders should participate in a State facilitated process to continue developing recommendations. These groups should contain physicians experienced in heat-related illnesses, health educators, public health officials, and representatives of vulnerable communities and/or their caretakers and social contacts.

Recommendations for long-term strategies

  • In addition to currently affected areas, attention should be paid to areas of the state not historically exposed to heat. These areas are less prepared for extreme heat events. Future heat waves may increase risk in these areas due to lack of historic physiologic adaptability and capacity to cool.
  • Policies should be developed requiring city and land-use planners and public health officials to work together to minimize the heat island effect. Heat islands are geographic areas that are warmer than surrounding areas. Increased development of impervious areas (e.g. pavement) in urban areas contribute to the heat island effect.
  • Strategic planning and resources are needed to build public health response and surveillance infrastructure. This includes early and accurate heat warning systems and heat-related disease surveillance systems.
  • Coordinated planning and action is needed to prepare for climate change-related increases in temperature. With strategic planning, sparse resources can be adequate to begin the urgent tasks required to reduce heat-related illness and mortality.

Find out more in our collaborative report on Heat-Related Illness and Mortality  (PDF, 4.3 MB)

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