LOS ANGELES — Almost a third of California’s 58 counties are expected to experience at least five extremely hot days this month, according to a U.S. Department of Health and Human Services Climate and Health Outlook released Friday.

The outlook defines an extremely hot day as when the daily maximum temperature exceeds the 95th percentile of historic temperature distribution in that county.

What You Need To Know

  • Ventura and San Bernardino are among 18 counties expected to have at least five extremely hot days in May

  • Extreme heat is when the daily maximum temperature exceeds the 95th percentile of historic temperatures for an area

  • Extreme heat can lead to illness and death

  • Seniors, people with existing health conditions and those with poor access to health care are most at risk

In Southern California, San Diego and Tulare Counties are expected to meet that criteria for five to nine days this May, while Ventura, Kern, Kings, Fresno, Inyo, San Bernardino, Riverside and Imperial Counties are expected to have at least ten extremely hot days.

“We’ve seen what exposure to extreme heat can do,” Health and Human Services Secretary Xavier Becerra said in a statement. “It can lead to illness and death and makes it much harder to do a day’s work outdoors.”

People who are elderly and live alone, who have existing health conditions or who have poor access to health care are most at risk from extreme heat days, the outlook said, as are those who live in rural areas, work outdoors, make a low income, face difficulty paying utility bills, live in poor housing or live in urban areas without adequate tree cover.

A Biden administration initiative to protect people’s health by giving advance notice of climate-related risks, Friday’s Climate and Health Outlook on Extreme Heat combined long-term temperature forecasts from the National Oceanic Atmospheric Association with data from the National Integrated Heat Health Information System to pinpoint areas where residents are at greatest risk this spring and to identify vulnerable populations.

California is one of 14 states with counties that are projected to have at least five or more extremely hot days this month. Texas, New Mexico, Arizona, Utah, Oklahoma, Kansas, Idaho, Nevada, Montana, Colorado, Georgia, Florida and Alabama are also on the list. The outlook estimates that the total at-risk population for extreme heat this month is almost 32 million people.

Of the areas on the list, 66% have a high number of people without health insurance, and 36% have a high number of people living in poverty. Over two-thirds have a high number of people living in areas without adequate tree cover, and one quarter have a high number of people aged 65 or older living alone.

During last summer’s heat dome over the west coast, heat-related deaths from June 26 to July 10 increased from 12 to 25 in California year over year, according to the state’s Department of Public Health. Heat-related emergency department visits during that period also spiked, according to the U.S. Centers for Disease Control.

The outlook reported that warmer temperatures increase the risk of hospitalization for heart disease, as well as heat exhaustion leading to heat stroke. It can also worsen asthma and chronic obstructive pulmonary disease and contribute to dehydration that can lead to kidney injury. 

People who work in construction, farming, landscaping and other strenuous outdoor occupations are at higher risk of heat-related illness.

Intended for use by public health officials, employers with outdoor workers and schools, the outlook includes guidelines from the Occupational Safety and Health Administration on how to prevent heat illness. It recommends drinking cool water even if you are not thirsty, resting for long enough to recover from the heat, taking breaks in a shady or cool area, wearing a hat and otherwise dressing for the heat.

The outlook also includes best practices for emergency managers and health officials to protect vulnerable populations, noting that energy demand spikes should be expected during the summer with increased air conditioning usage.

“Our communities across the country will soon be facing heatwaves that will be an additional strain on our health systems,” Assistant Secretary for Health Admiral Rachel Levine said in a statement. “This information will save lives.”