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5.9 Extreme Heat The Bay Area, especially away from the coast and bay, can experience extreme heat days, where the Heat Index, a function of heat and relative humidity, is high. Extreme heat days pose a public health threat, causing symptoms such as exhaustion, heat cramps, and sunstroke if the Heat Index is over 90F. The National Weather Service has developed a Heat Index Program Alert which gets triggered when high temperatures are expected to exceed 105 to 110 for at least two consecutive days. Heat emergencies occur when residents are subject to heat exhaustion and heatstroke, and are more likely to occur in areas not adapted to heat and without air conditioning, cooling centers, or vegetation to mediate heat impacts in exposed areas. Certain populations are typically the most at risk during extreme heat emergencies, including people with disabilities, chronic diseases, the elderly, and children.45 Extreme heat emergencies typically build over time with cumulative effects. Because of this, and the fact that they do not cause substantial physical damage to the built environment, they do not elicit the same immediate response that other hazards do. However, they claim many lives in comparison to other disasters. The California Climate Adaptation Strategy, citing a California Energy Commission Study, states that heat waves have claimed more lives in California than all other disaster events combined.46 5.9.1 Historic Extreme Heat No heat emergencies in California have been declared a disaster at the state or federal level between 1960 and 2008.47 The Spatial Hazard Events and Loss Data for the United States estimates approximately 47 heat events in California during this time. In 2006 a notable heat wave spread throughout most of the United States and Canada, causing 140 fatalities in California.48 5.9.2 Probability of Future Extreme Heat Climate change is expected to generate an increase in ambient average air temperature, particularly in the summer. The outer Bay Area will likely experience greater temperature increases than coastal or bayside jurisdictions, though likely not as great as in the eastern- 45 State of California Multi-Hazard Mitigation Plan, California Governor’s Office of Emergency Services 46 Messner, S. et al. (2009) 47 State of California Multi-Hazard Mitigation Plan, California Governor’s Office of Emergency Services 48 Ibid