Seniors and children have to be particularly careful.
And for many seniors, air conditioning is either not available or too expensive to use.
Ice water refills is how 80-year-old Holly Reed gets through the day. He finds refuge at the Pasadena Senior Center because he doesn't want to turn on the AC at home.
"I don't use it because I can't afford it," Reed said. "You worry about running out of money at the end of the month because you can't pay anything."
Like many seniors, Reed lives by himself. An LA County climate vulnerability map shows the places where most seniors live are also the areas getting hit with the most extreme-heat days.
ER specialist Dr. Roger Yang at Huntington Health, an affiliate of Cedars Sinai, says during sizzling temperatures he usually sees one heat-exhaustion patient a day.
"Maybe during these times they should get family involved to watch over them. Drinking water is very important and I know a lot of our elderly don't do that," Yang said.
But elderly patients are the ones more likely to suffer more serious forms of heat illness.
"By the time you have heat stroke, most of these people are confused and not acting normal. Patients will start going into multi organ failure so their livers can fail," Yang said.
For elderly members in Pasadena who can't leave their homes, the Pasadena Senior Center provides in-person check-ins during meal deliveries and through regular phone calls.
"That telephone call to a number of seniors who have asked to be called goes on during the entire week to be sure everyone is fine," said Pamela Kaye with the Pasadena Senior Center.
Seniors don't usually have to be active to suffer from heat illnesses, but during hot weather, exertion is how many children and young athletes get into trouble.
And children are more susceptible because they don't cool off as efficiently as adults.
LA County's climate vulnerability map shows high concentrations of children living in extreme heat areas.
Dr. Yang says it's up to adults to watch for signs.
"They'll be acting dizzy," Yang said. "They'll be nauseous. If they are extremely sweaty, then that's probably signs that they're heading towards heat exhaustion."
And those warning signs are true for adults as well.
Experts say if you know a senior or older adult who doesn't have air conditioning, check on them, and make sure they stay hydrated and know not to wait until they're thirsty to drink.
Here's a list of cooling centers in LA County, and a way to find an LA County pool nearest you.
And check out LA 's "Beat the Heat" site for more information on the signs and symptoms of heat-related illness.