Workers at Santa Clara Valley Medical Center say it takes about three days of excessive heat before they see an increase in heat related illnesses. However, some people are more vulnerable than others.
Being homeless is tough, but being homeless when temperatures are in the triple digits, can be unbearable.
"It's sticky, it's muggy, you can pass out," said James Mitchell, a homeless person.
Emergency Housing Consortium Life Builders usually closes its homeless shelter doors on Little Orchard Street in San Jose from 9:30 a.m. to 3:30 p.m. However, with temperatures in the triple digits, it's staying open 24 hours a day through Thursday.
"I think extreme heat can be just as dangerous as extreme cold. We get people dehydrated, suffering from heat exhaustion and heat stroke. So they need a place to get out. If you don't get off the streets you can be in a lot of trouble and even die," said Ray Bramson, from EHC Lifebuilders.
Another group that's vulnerable to excessive heat is pregnant women. Dehydration can irritate the uterus and cause contractions, possibly leading to pre-term labor.
"Someone contracting dehydration may actually be in pre-term labor so they should, rest get hydrated," said Dr. Jennifer Domingo, MD, an Obstetrician and Gynecologist.
Dr. Domingo says if a pregnant woman continues to have four or more contractions an hour or the contractions become more painful, she should see her doctor.
The triple digit heat didn't stop the 12 year olds from playing in their all star game between Los Gatos and Willow Glen. League officials put up shade for the crowd, put cool, wet towels in the dugouts and made sure the players had water.
"We have the Gatorade jugs that we put in the dugout. We fill them with ice and water, and we put cups in there so they have the water jug the whole entire time," said Jenna Chalmers, from San Jose American.
"Parents have a sixth sense when they're kids are struggling with something. And you can tell just by looking at them," and Ron Ricci, a father.
Ricci's son Jake plays catcher for the Los Gatos All-Star team. He knew it was going to be hot at game time, so he limited Jake's exposure to the sun.
"He stays inside for most of the day. He doesn't swim and stays out of the sun for the day. And make sure he hydrates and drinks a lot of water," said Ron Ricci.
If kids are playing outdoor activities during these hot temperatures, the American Academy of Pediatrics recommends five ounces of water every 20 minutes for children weighing 90 pounds and nine ounces for kids weighing 130 pounds.
The Almaden Community Center is just one of 13 cooling centers in San Jose and a full list may be viewed at http://www.sccgov.org.
Cooling centers are available in cities throughout the region. A list of cooling centers statewide opened by local authorities and tips to prevent heat illness are available at http://www.oes.ca.gov.