As temperatures warm, emergency department visits due to heat-related illnesses are rising in California. Hospitalizations and deaths spike in years with especially high summertime temperatures. Because heat-related cases are unrecognized and underreported, the actual numbers are likely considerably higher.

Heat causes more reported deaths per year on average in the United States than any other weather hazard. Yet, heat-related illnesses and deaths are generally preventable. Heat-related illnesses include a broad spectrum of diseases, ranging from mild heat cramps to life-threatening heat stroke. Heat exposures have recently been linked to mental health illnesses and adverse birth outcomes, such as preterm births and low birth weights. For more information, download the Heat-related Deaths and Illnesses chapter.

• More heat-related deaths occurred in 2006, the year of a prolonged heatwave, than in any other year.

• Emergency department visits and hospitalizations were highest in 2017, when summertime temperatures were unusually high, and in 2006.

• Emergency department visits show an increasing trend, as expected with the warming temperatures associated with climate change.

• Heat-related illnesses and deaths, while severely underreported, represent a widely recognized burden of climate change on human health.

• Tracking heat-related illnesses and deaths provides critical information for policies and actions to protect communities from heat exposure. These include providing access to cooling centers, issuing heat alerts, and strengthening community resilience, such as by planting trees and installing cool pavements.

• Warmer temperatures increase the threat of heat-related illnesses and deaths; temperatures do not have to be extremely hot to be harmful. Warm nights and high humidity are particularly dangerous:  without night-time cooling, the body cannot recover after the day’s heat, and high humidity prevents sweat from cooling down the body.

• People who are not acclimated to heat are at greater risk when hot days occur earlier in the season, or when temperatures are much higher than normal in cooler regions, where homes are less likely to have air conditioning.

• The elderly, the poor, non-Hispanic Blacks, and those who are socially isolated or with underlying health conditions are at greater risk of heat-related health effects. Workers in firefighting, agriculture, and construction are at risk because their occupations involve greater exposures to heat. Rates of occupational heat-related illnesses are increasing.

During hot weather, people are exposed to both heat and higher levels of air pollutants. Heat accelerates the formation of certain air pollutants and traps them closer to the ground. This combination increases the risk of respiratory, cardiovascular, and other health effects.

Credit: iStock/Daniel Stein

• California Department of Public Health (CDPH), Be Informed: Extreme Heat
• Centers for Disease Control and Prevention (CDC), National Environmental Public Health Tracking: Heat and Heat-related Illness
• National Integrated Heat Health Information System (NIHHS), HEAT.gov  
• Tracking California, Heat-related Illness