For most people, high heat and humidity is uncomfortable.
For those with certain medical conditions, hot temperatures can be dangerous, even deadly.
A hospital room at Texas Health Fort Worth is far from how Barbie Stanislawski envisioned spending her summer.
"Everyone thinks fun in the sun outside in the beach," said Stanislawski. "It's not that way for us."
As a heart disease patient, her heart does not pump perfectly under the best of circumstances.
Cardiologists say heat makes it even worse.
"When the body sweats, blood volume decreases so the heart has to work harder to pump blood with oxygen and various nutrients to the body's tissues," explained Dr. Sreenivas Gudimetla, a cardiologist at Texas Health Fort Worth. "When the heart can't keep up, the body gets dangerously overheated."
"It can cause heart attacks," said Gudimetla. "It can cause sudden death. It can cause a whole variety of problems."
Hot humid air can also trigger asthma.
Pregnant women, whose body temperature is already higher than normal, are also at much higher risk of heat exhaustion.
For Stanislawski, the symptom of heat stress on her heart was extreme coughing.
"The heat is like a death warrant during the summer," Stanislawski said. "It's hard on me."
Some medications can also cause problems.
Diuretics, for example, cause people to lose water weight, putting them at high risk of quick dehydration.
Anyone with a medical condition should consult their doctor before spending much time outside in this extreme heat.
The average, healthy person can adapt to high temperatures in seven to 10 days.
People with heart disease and other common medical conditions may never be able to adapt to the heat.