UPDATED 07/20/11 – 4:41 p.m.

CHICAGO (CBS) — The entire Chicago area and beyond is under an excessive heat warning, and temperatures hit 100 degrees in some areas.

The National Weather Service reported that the air temperature reached 100 degrees at Northerly Island in Chicago, with a heat index of 112 at 4 p.m. The same temperature was also reported in south suburban Lansing.

“If the heat index reaches 110 degrees at O’Hare Airport, which is what we are forecasting, then today will be the highest, essentially the most oppressive heat since the July 1995 heat wave,” NWS Meteorologist Dino Izzi said. In 1995, 750 people were killed has temperatures soared in the high 90s to above 100 degrees over several days.

The temperature had hit 99 degrees at O’Hare by just after 3 p.m. The heat index at O’Hare as of 3 p.m.: 109 degrees.

As CBS 2’s Suzanne Le Mignot reports, doctors were warning local residents to take the heat seriously, especially after the Fire Department received 87 heat-related calls between 7 a.m. and 1 p.m. on Wednesday In temperatures like this, doctors say no one is invincible.

At Swedish Covenant Hospital, emergency room Dr. Matthew Ajluni said, “We’re fully staffed and we’re prepared for any influx of patients that might be affected by heat stroke.”

Ajluni said that people can be overcome by the heat in a matter of minutes.

“It can be very serious and it’s important that people take measures to help prevent heat stroke, which is seeking a cool shaded area, frequently drinking plenty of water, wearing a brimmed hat to prevent too much sun exposure,” he said.

At Children’s Memorial Hospital, child safety experts demonstrated just how quickly the interior of a car heats up and how dangerous that can be for a child.

During the demonstration, when it was 99 degrees outside, a thermometer inside a parked car showed the temperature at 109 degrees.

“It’s actually gone up; since the first interview, it’s gone up to to 112 or 114 now. So it keeps rising and it’s impossible for anyone, especially a child, to sustain that type of temperature,” Jessica Choi with Safe Kids Illinois said.

Doctors also said parents should remember to dump the water out of wading pools when they’re not being used under supervision.

“We see a lot of kids drowning in wading pools here at Children’s during the summer,” Dr. Karen Sheehan said. “These little toddler pools, with just a few inches of water, are very dangerous,” she said.

Kids at day camp spent the afternoon at Gompers Park cooling off at a Chicago Park District pool.

Heidi Olenoski visits regularly to help her son beat the heat.

“What I do is I bring him to the Chicago Park District so that he can swim every single day. It is so hot out here, that this is the most awesome thing for the kids to be able to do is to cool off in a great pool,” she said.

Don’t expect the nighttime hours to bring any relief from the heat. The forecast temperature at midnight is 85 degrees, with a heat index of 92.

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And the heat index is what the temperature feels like in the shade when humidity is factored in. In the sun, it could feel more like 125.

If the temperature hits 101 at O’Hare International Airport, it will tie a record. If it goes higher, it will top a record set back in 1980.

The temperature topped 100 degrees in Rockford on Tuesday.

An excessive heat warning is in effect for all Chicago area counties through 4 a.m. Friday, since highs both Wednesday and Thursday are expected in the upper 90s, with heat indices between 105 and 115 degrees.

As of noon Wednesday, the Cook County Medical Examiner’s office said there had been no heat-related deaths.

The Department of Family and Support Services said that, as of 3 p.m. Wednesday, its staff had received 209 requests for well-being checks since Monday, but no one was seriously ill or taken to a hospital.

There were 87 heat-related calls to the Fire Department just between 7 a.m. and 1 p.m. on Wednesday and, as of midday, 41 calls to the Anti-Cruelty Society, where they were expecting that number to double, with calls about pets left outside with no water.

There were also 312 calls about open fire hydrants, which creates a dangerous situation by sapping the water pressure available for emergencies and also because children who play in the spray of an open hydrant might not be visible to passing cars.

Meantime, the AAA Chicago Motor Club said that, on Tuesday, it received 1,176 heat-related calls for road service. As of 3 p.m. Wednesday, they had received another 752 calls.

As CBS 2’s Susanna Song reports, while it is often cooler by the lake, the heat and high humidity will be unavoidable no matter where you go Wednesday. Those who need relief from the heat can visit any of the city’s six cooling centers, of which a list is available here.

LISTEN: Newsradio 780’s Bernie Tafoya reports

Those in need of a cool space may also go to Park District field houses, libraries and police stations.

WBBM Newsradio 780’s Bernie Tafoya reports city Human Services crews have been gearing up for yet another sweltering day, but so far, the city has not been inundated with requests for hot weather-related help.

Since Monday, the city says there have been about 209 requests for well-being checks to be made by the Department of Family and Support Services.

“That number is fairly moderate,” said Deputy Commissioner Joel Mitchell. “We believe that our messaging has been working. We’ve been proactive. We’ve been getting the word out for neighbors to check on neighbors and family members to check on their families.”

The temperatures were not as hot Tuesday, but those who had to be outside still found themselves in a state of great discomfort.

“Sometimes it will get hot so fast I get a headache, but thank God I’ve been able to survive and press on through,” said Anthony Mitchel of the South Side.

Those who work outside are in the greatest danger. John Smith of Oak Lawn was operating a steamroller Tuesday.

“You get dizzy. You lose your breath. You get lightheaded,” he said.

Over in the emergency room at Advocate Christ Medical Center in Oak Lawn, the hospital staff is prepared for an influx of patients with heat-related illnesses. The ER has a large supply of saline IVs refrigerated to help cool down overheated bodies, as well as plenty of fans and cooling places.

“The body is still stressed and strained from that first day of exposure, and then each subsequent day drains the body a little bit more,” said Dr. Brian Sayger of Advocate Christ.

Signs of heat exhaustion include nausea, vomiting, headaches and body cramps.

Doctors warn the elderly, the very young and the disabled are most at risk.

But also at risk are those who are healthy but insist on going out running or engage in other strenuous activity. Stroger Hospital of Cook County ER Dr. Moses Lee pointed to the 2007 Chicago Marathon, when 250 people were hospitalized because of heat-related illnesses in the 88-degree temperature, and the race had to be stopped early.

The Anti-Cruelty Society reported during the heat wave, calls to its Human Investigation unit had more than doubled, mostly for pets without water or shelter.

The agency points out any dog that has to be outdoors should be in a cool area with access to water. Jogging with a dog is a bad idea, and keeping a dog in a hot car – windows open or not – could be extremely dangerous.

Pets can also suffer heat stroke. They exhibit difficult, rapid breathing, a reddened tongue and gums, a blank stare or a sudden collapse. Elderly and overweight pets, and pets with flat feces such as pugs or Persian cats are particularly susceptible to heat stroke, the agency said.