(CNN) -- The scorching temperatures affecting almost half of the U.S. population isn't just causing heatstrokes -- it's also causing people to feel drained and more susceptible to other health problems. The humidity can wreak havoc and feel suffocating to people who have breathing or heart-related problems.
The National Weather Service issued heat watches, warnings and advisories Wednesday morning in more than 30 states, stretching from most of the Midwest to the mid-Atlantic and northeastern states, warning that temperatures will feel like 100 to 110 degrees or higher during the afternoon.
In some places, the heat index values -- which measure how hot it feels -- have been as high as 131 degrees, according to the weather service.
Across the United States, about 141 million people are under heat advisories and warnings.
It has been hot enough for an iReporter to fry an egg on his dashboard.
It's unclear how many deaths in the United States can be directly attributed to the recent heat wave.
Most of the heat-related deaths reported in the news media occurred in older adults, who are at highest risk of heatstroke.
In south-central Kansas, a 65-year-old man collapsed while working on his tractor on a hot Monday afternoon. His temperature was at 107 degrees, and it was unclear whether pre-existing medical conditions could have contributed to his death, according to KSN, a CNN affiliate in Kansas.
A 69-year-old man in Legion Park, Oklahoma, died with his core body temperature recorded at 108 degrees. A normal body temperature is around 98.6 degrees Fahrenheit.
The medics' equipment only reaches 108, which means that the man's temperature could have been higher, according to KTUL, CNN's affiliate in Tulsa.
The Kansas City, Missouri, Health Department was notified Tuesday of that city's 13th suspected heat-related death this year.
Jeff Hershberger, public information officer for the department, said the deaths have been scattered throughout the summer. Four were reported to the Health Department within the last two days.
The deaths will be examined by the medical examiner's office to determine whether heat was the cause. Last year, the Kansas City Health Department saw four confirmed heat-related deaths.
The Centers for Disease Control and Prevention estimates about 300 heat-related deaths a year.
Between 1979 and 2003, excessive heat exposure killed 8,015 Americans, according to the CDC. That's more people than the number who died from hurricanes, lightning, tornadoes and floods combined.
Major cities like Chicago; Raleigh, North Carolina; and Washington, D.C., can expect to reach 100 degrees this week, forecasters said.
In metropolitan Oklahoma City, emergency services, which serves about 1.1 million Oklahomans, has received about five heat-related calls a day since June 17. On busier days, it receives up to 12 calls a day.
"There's no relief. The air you breathe is hot. You can't get away from the heat," said Lara O'Leary, public information officer for Emergency Medical Services Authority in Oklahoma City.
Many people calling for emergency help had breathing problems because of the humidity -- the air felt too heavy for people who have respiratory or cardiac problems, O'Leary said.
"It's a medical nightmare out there," she said. "We know the heat is creating adverse breathing problems. They're causing falls from people who are passing out and falling."
Residents who live in heat-stricken areas are being asked to check on their older neighbors, especially the ones who live in isolation.
Heat illnesses can start with cramps and progress to heat exhaustion, with symptoms like severe headache, nausea, vomiting and severe weakness.
The most severe heat-related illness is a heatstroke, in which a person can lose consciousness and the muscles can break down to the point of kidney failure.
Here are some tips from the CDC on ways to deal with extreme heat:
• Drink plenty of fluids
• Schedule outdoor activities carefully
• Stay cool indoors
• Do not leave children in cars
• Wear appropriate clothing and sunscreen