THURSDAY, July 28 (HealthDay News) -- Summer is a time to be out and about, but it's also a time when high temperatures can take a big toll on health.

Experts at the U.S. Centers for Disease Control and Prevention (CDC) say that from 2001 to 2009, almost 6,000 people annually were seen in emergency rooms for heat-related illness caused while playing sports or engaged in other outdoor activities.

"Because heat-related illness is a preventable illness, any number is too high," said lead researcher Ellen E. Yard, an Epidemic Intelligence Service Officer at the CDC. Her team published a study on the problem in the July 29 issue of the agency's journal Morbidity and Mortality Weekly Report.

Their concerns are especially timely as many areas in the nation are suffering through record heat waves, with probably more blistering temperatures to come before fall arrives.

"You need to be proactive in protecting yourself from heat illness," Yard said. "If heat illness does occur, recognize it right away and treat it appropriately."

Yard said there are things that can be done to prevent heat-related illness, particularly for active people. First, drink lots of water when you are outside. In addition, try to limit activity to the coolest parts of the day -- early morning and late evening, she said.

"It's really important to pace yourself. Especially when it first starts getting warm, your body is not going to be used to that," she said. "You really want to slowly increase the intensity and frequency of your activity over one to two weeks."

In the study, the most common activities linked to heat illnesses were football and exercise. Most of the illnesses -- about 73 percent -- occurred among males. One-third of the illnesses were in teenagers.

Among women, the most common activities causing heat illness were baseball and softball for those aged 14 and younger, track and field for those 15 to 19 years, and exercise for women 20 and older, the researchers found.

Heat illness includes a range of symptoms, usually starting with mild dehydration and quickly progressing to heat exhaustion and possibly to heat stroke, Yard said. Heat stroke is rarer, but can be potentially fatal.

Signs of possible heat illness include heavy sweating, feeling weak, headache, nausea or vomiting. These are signs of heat exhaustion, Yard said.

"That's a sign that you need to get these people immediately out of the heat and into air conditioning or an ice bath," she said. "That's why it is important to have an exercise partner or teammates or coaches who can recognize the signs of heat illness and get you the prompt attention that it needs," she added.

If a person starts to show signs of mental confusion it could be heat stroke, Yard noted. "That's a medical emergency that needs immediate emergency attention," she said.

According to Dr. Lisandro Irizarry, chair of the emergency department at The Brooklyn Hospital Center in New York City, the most common heat-related illness sending people to the emergency room is heat exhaustion.

"It's rare to see someone in heat stroke," Irizarry said. "Usually the person you see who has heat stroke is an elderly person on multiple medications that restrict the person's ability to sweat," he explained.

However, there are times when heat illness should be treated in an emergency room, he added. "If you are significantly confused you should go to the emergency department," he said.

"In addition, if you have symptoms of another illness you have, like heart disease with chest pain or shortness of breath, that's another reason to go to the hospital," Irizarry continued. Also, feeling faint is another reason to go to the hospital, he said.

For most people who are active outside during heat waves, Irizarry recommends limiting the time spent exercising and keep drinking water. "Know your limitations," he said.

Irizarry also suggests exercising indoors where it is air conditioned. "If you're a runner, get to the gym and run on a treadmill," he said.

More information

For more information on heat-related illness, visit the U.S. Centers for Disease Control and Prevention.