Besides being hot and sticky, Lisa McDavid of Drexel, N.C., said, "I get really tired, can't breathe out of my nose and my chest starts hurting."
McDavid suffers from seasonal allergies, a deviated septum and mitral valve prolapse, a condition that causes one of the heart valves to not close properly.
"When it's hot but not that humid, I can stand being outside a little longer, but when it's humid, I feel like I'm suffocating."
McDavid certainly isn't the only one suffering from the adverse effects of heat on her health. With about half the country baking in a heat wave, hospitals in some of the worst-hit areas are reporting cases of people coming into emergency rooms with heat-related illnesses. Many expect more as the heat wave continues.
And many cases don't necessarily involve heat stroke or heat exhaustion.
"We do not see a lot of hot people but rather people with diseases, alcohol, drugs, old age and disability whose conditions are worsened by the heat," said Dr. James Adams, professor and chair of the Department of Emergency Medicine at Northwestern Memorial Hospital.
Very high heat and humidity can affect everyone, but experts say in addition to children and the elderly, people with the medical conditions that follow are especially susceptible to heat-related illness.
Allergy and asthma specialists say they are seeing more patients whose illnesses have been triggered by the heat and humidity as well as by increased levels of pollutants in the air.
"[We] have seen many new patients for the first time with a diagnosis of asthma made worse by heavy pollens and extreme temperature and humidity levels," said Dr. Clifford Bassett, medical director of Asthma & Allergy Care of New York.
Bassett also said that in addition to pollen, mold levels increase when it's very humid.
The heat wave is also causing more serious breathing problems, including very severe asthma attacks and a worsening of chronic obstructive pulmonary disease (COPD). At Temple University Hospital in Philadelphia, several patients needed emergency treatment for both these conditions. One of them even needed a breathing tube.
"Both of these patients had air conditioning -- room air conditioning, not the whole house -- and were using it, yet it wasn't enough to prevent the heat from exacerbating their symptoms," said Dr. Alvin Wang, an emergency room physician at Temple.
During a heat wave, experts say room air conditioners may not make the environment cool enough.
Bassett advises anyone with allergies or asthma to stay where it's air conditioned, and to change and clean the filters frequently. If you need to go outside, check the pollen counts and pay special attention to ozone alerts.
"Just about any underlying medical condition can be exacerbated by the stress of heat and dehydration, but the people most at risk are those with serious underlying heart and lung disease," said Dr. Corey Slovis, professor and chair of the Department of Emergency Medicine at Vanderbilt University School of Medicine in Nashville, Tenn.
"During times of extreme heat, people are prone to dehydration," said Dr. Phil Ragno, director of cardiovascular health and wellness at Winthrop University Hospital in Mineola, N.Y. "The more activities we perform, we're losing fluid through perspiration, and that decreases the volume of blood in our system. Blood vessels also dilate when it's hot, and as a result, the heart has to pump harder to circulate a smaller amount of blood."
Ragno also says people with heart conditions should drink a lot of fluids before they leave the house when it's hot and should keep hydrated throughout the day.
"People with heart conditions should weigh themselves each morning. If their weight is down a bit, it might not be body weight, but body fluid they're losing, which is a sign of impending troubles," Ragno said.