The Allegheny County Health Department has issued a heat alert, warning that the high temperatures and humidity can pose serious health problems for the elderly, children up to age 4, the overweight, people who work or exercise outdoors, and people with heart or respiratory problems.

The risk is for heat exhaustion or the more serious, potentially life-threatening heat stroke, the health department said.

Heat exhaustion is a physical response to an excessive loss of water and salt due to sweating. Warning signs include heavy sweating, paleness, muscle cramps, fatigue, dizziness, headache, nausea or vomiting, and fainting. Medical help should be sought immediately if symptoms are severe or if the person has heart problems or high blood pressure. Otherwise, help cool the person off. Seek medical help if the symptoms worsen or last longer than an hour.

Heat stroke occurs when the body cannot control its temperature and the sweating mechanism fails. Warning signs may include a body temperature above 103 degrees; red, hot and dry skin; a rapid, strong pulse; throbbing headache; dizziness; nausea; confusion; and unconsciousness.

Death or permanent disability may result without emergency treatment for heat stroke. Cool the victim as fast as possible by any available means until paramedics arrive.

Never leave a child or a pet alone in a vehicle on a hot day. A child may become disoriented in just five minutes, unconscious in 10 minutes, and brain-damaged in 20 minutes.

Use the buddy system and check on the elderly or the infirm who do not have air conditioning and are less able to take care of themselves.

Pittsburgh's senior centers in Homewood and the South Side will be open until 7 p.m. -- three hours later than usual -- today, Wednesday and Thursday to make sure older residents have someplace to escape the heat.

Refreshments and activities will be provided.

The Homewood center is at 7321 Frankstown Ave., and the South Side center is at 12th and Bingham streets.

Allegheny County also announced that senior centers in Sharpsburg and Tarentum will remain open until 7 p.m. the same days.

The heat prompted Carnegie Library of Pittsburgh to close four branches located in buildings that don't have air conditioning.

Closing at 1 p.m. today are branches in Lawrenceville, Mount Washington, South Side and the West End.

They are expected to reopen Wednesday.

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