The heat is on in Utah.

The National Weather Service has issued a heat advisory for the state, with daytime temperatures expected to reach the upper 90s and low 100s through Tuesday.

By 3:30 p.m. Monday, the weather service reported on its Twitter feed, the temperature in Salt Lake City had reached 103 degrees. The temperature hovered around 103 for a couple of hours but didn’t reach the record of 104, the weather service said.

Residents should feel some relief when the sun goes down with overnight temperatures dipping into upper 60s and mid 70s.

Grace Harris, a physician who treats patients at the InstaCare at Intermountain Health Care’s Memorial clinic in Sugar House, said Monday that people should watch for the symptoms of dehydration: headache, nausea and weakness.

“That’s when you have a chance,” Harris said.

More severe symptoms — confusion, vomiting, weak pulse — indicate heatstroke. If someone with those symptoms shows up at Harris’ InstaCare clinic, she said, “We’d start IVs, call 911 and get them to the [emergency room].”

Infants under a year old, the elderly and people with existing health problems are more susceptible to heat-related illnesses, Harris said.

But anyone can be at risk. Take the case of Mitch Petrus, the 32-year-old former New York Giants offensive lineman, who died from heatstroke in Arkansas last week during a heat wave that struck most of the central and eastern United States.

People are advised to stay out of the sun, particularly in the afternoon, when temperatures are hottest. But if someone is more likely to suffer in the heat, Harris said, “it’s wiser to stay indoors and seek shade,” no matter what time of day.

People should drink plenty of fluids, Harris said, but avoid caffeine, a diuretic that will make a person urinate more. People should also avoid alcohol in the heat, because of its high sugar content (which the body will try to flush out by urinating) and because it can impair judgment and cause a person to fall asleep outside in the heat.

Also, Harris said, “don’t be shy with the salt.” Sweat is water and salt, and drinking fluids will replace one but not the other.

Salt Lake County’s Aging and Adult Services has set up several indoor, air-conditioned spaces — called “cool zones” — for those who may need them. The list of senior centers, libraries and recreational facilities is available at https://slco.org/aging-adult-services/cool-zone/.

The agency also is seeking new fans for vulnerable and homebound adults. Donations can be taken to senior centers or the County Government Center, South Building, 2001 S. State St., Salt Lake City.

To combat heat-related illness, the American Red Cross urges people to follow these tips:

• Wear lightweight fabrics and light colors; dark colors absorb more heat.

• Eat small meals, and eat more often.

• Avoid exercise and strenuous activity when it’s hot outside.

• If one must work outside, take frequent breaks when temperatures are at their peak.

• Never leave children or pets alone in enclosed vehicles.

• Check on family, friends or neighbors who do not have air conditioning, or may spend more time alone, or may be more susceptible to problems (such as the elderly).