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Climate Change

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California Environmental Health Tracking Program

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Last Edited: 4/15/2014

Extreme Heat and Climate Change

The effects of extreme heat are already being felt throughout California and the entire country.  During the 2006 California heat wave, there were 16,166 more visits to the Emergency Department and 1,182 more hospitalizations than usual in the state.1  Similar extreme heat events are expected to increase over time.  The health impacts of extreme heat events range from mild heat cramps to death.  Certain populations, such as elderly people living alone, face much greater health risks during heat waves.


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1.  Knowlton K, Rotkin-Ellman M, King G, et al. The 2006 California Heat Wave: Impacts on Hospitalizations and Emergency Department Visits. Environ Health Perspect. January 2009. 117(1):61-7.

How does climate change affect extreme heat?

Temperatures will rise more dramatically as the amount of man-made greenhouse gases increase and shift the earth's temperature balance.

  • The “greenhouse effect
    • Greenhouse gases in the atmosphere, such as carbon dioxide and water vapor, absorb heat radiated from the earth’s surface.  This heat is then radiated back to earth’s surface.  This phenomenon occurs naturally, but human-induced emissions can strengthen its effect.  
  • California appears to be slowly warming, with temperatures increasing most rapidly in Southern California and in urban areas. 

More detailed information regarding temperature change in California is available here.


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How does extreme heat impact health?

People suffer from heat-related illness when the body's natural temperature controls become overloaded.  As body temperature rises uncontrollably, damage can be done to an individual's brain or other vital organs.  Climate change and our built environment both contribute to extreme heat.

  • Nighttime temperatures rising
    • Nighttime is usually the time when our bodies can cool off and recover from the heat.  However, nighttime temperatures have been increasing at a higher rate than daytime temperatures. This results in fewer opportunities for the body to cool overnight during heat waves, increasing the risk for heat-related illness.
  • Heat islands
    • The existence of "heat islands" also contributes to this problem.  Heat islands are geographic areas that are warmer than surrounding areas.  They form as cities, with their vast areas of concrete and metal, replace natural land covers such as vegetation and soil.  Pavement, buildings, and other infrastructure retain more heat than natural surfaces, so temperatures in cities subsequently rise.

Without precautionary measures, as the number and duration of extreme heat events increase, there will be an increase in heat-related illnesses and deaths.  Learn more about identifying and preventing heat-related illness in Heat and How it Affects Our Health. More information can also be found in our collaborative report on Heat-Related Illness and Morbidity (PDF, 4.3 MB).


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Who will be most affected?

CEHTP has worked to document what populations in California may be more vulnerable to the effects of extreme heat.  Maps showing the location of particularly vulnerable populations are available here.  Populations vulnerable to heat include:

  • The elderly
  • Children
  • Socially isolated individuals
  • Individuals with existing health issues
  • Individuals living in poor or urban areas
  • Individuals who are active outdoors (workers, athletes, etc.)

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What can you do?

We know that climate change will bring an increase in extreme heat for many communities.  There are ways to make sure you and your community are prepared and stay safe during heat spells.

  • Reduce sun exposure from 10 a.m. to 4 p.m. when UV rays are the strongest, and minimize physical activities during that time.
  • When working outside, drink plenty of water or juice, even if not thirsty.  Take frequent rest breaks in the shade.
  • Wear a wide-brimmed hat that covers you face and neck, and wear cool, loose fitting clothing.  Apply sunscreen frequently.
  • NEVER leave infants, children, the elderly, or the frail unattended in a car.  The same applies to family pets.  The temperature inside the car can climb to lethal levels in just 10 minutes.
  • Use cool compresses, mists, and cool showers or baths to prevent overheating.  If you or someone else experiences a rapid pulse, deliriousness, or has a temperature above 102 degrees, call 911 immediately.
  • Encourage greening in your community.  Tree planting provides much needed shade while also removing greenhouse gases from the atmosphere.

Other extreme heat tips and warning signs are available here:

Resouces on climate change and extreme heat

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