Summer doesn't officially start until Wednesday evening, but in some parts of the country the sweltering summer heat came early as the temperature in Denver and Phoenix eclipsed 100 on Monday and over the next few days, parts of the East Coast will bake in the 90s.
Thousands of people end up in hospitals because of heat-related illnesses every year and according to the most recent data from the Centers for Disease Control and Prevention, nearly 3,500 people died after exposure to excessive heat between 1999 and 2003.
Despite the dangerously high heat and humidity, medical experts say there are simple but important ways people can stay cool on oppressively hot days and avoid problems like heat exhaustion and heat stroke.
Dr. Jeffrey Rabrich, director of EMS and disaster preparedness at St. Luke's & Roosevelt Hospitals in New York, expects to see quite a number of people come to the emergency room with heat-related symptoms over the next few days as temperatures in New York City climb into the 90s.
"Generally, whenever we have a heat wave or high humidity, we get a lot of patients in with symptoms that include dehydration, lightheadedness and passing out," Rabrich said. "Most people are not too severe and we can treat them by cooling them off and giving them fluids. But every once in a while, we get a couple of cases of heat stroke."
Heat stroke is a life-threatening condition that requires immediate medical attention. Doctors warn it can be caused by being out in high temperatures for too long or by being overly active in very hot weather.
People should monitor themselves and others for the classic signs of heat stroke.
"Watch out for symptoms like fatigue, dizziness, lightheadedness, nausea, or if the skin becomes hot or sweating stops," Rabrich said.
Heat exhaustion is less serious, but if not treated, can progress to heat stroke. According to the American Red Cross, symptoms of heat exhaustion include cool, moist skin, headache, dizziness and nausea.
Extremely high heat and humidity can be especially hazardous to the very old and the very young.
"It's hard to tell when young children are getting hot or dehydrated. They can't tell you they're thirsty, and with the very old, their thirst sense decreases with age," said Dr. Corey Slovis, chair of emergency medicine at Vanderbilt University Medical Center in Nashville. "Others who are at high risk for heat-related illness are those with heart and lung disease. People with heart disease may take diuretics, which make them urinate a lot, and they can become dehydrated."
"People who are bed-ridden and ill are also at a much higher risk," said Jeffrey Pellegrino, who serves on the National Scientific Advisory Council for the American Red Cross. "People who live on upper floors, in urban environments or in temperate climates who are not used to the heat are also at higher risk."
Pellegrino said it normally takes about a week for the body to acclimate.
Those who work outside in the heat should also take special precautions to prevent heat-related illnesses by periodically getting out of the heat and staying hydrated, especially when they start to feel hot.
If someone is experiencing heat stroke, seek medical attention, but also keep them cool and moist.
"Immediate first aid for anyone with heat stroke symptoms is to keep them wet and windy," Slovis said. "Keep them wet and keep a breeze on them."
But there are simple steps people can take to avoid experiencing any hot weather symptoms.