It was unlike any disaster in the city's history -- silent, invisible and only slowly revealing just how deadly it was.

Not until doors of apartments and houses were pushed open one after another, when victims were found inside rooms where the air itself seemed to be on fire, did its strength become clear.

Ten years ago, a heat wave in Chicago killed more than 700 people in the span of four days -- more than twice as many deaths as in the Great Chicago Fire of 1871.

In the decade since, the city has honed a heat response system that has become a model for the nation. Today, any time the mercury climbs above 90 degrees, city workers start calling and visiting the frail and elderly. A heat index of 105 degrees prompts an emergency plan that turns air-conditioned city buildings into 24-hour cooling centers.

More than 17,000 people who have contact with the elderly -- from police officers to garbage collectors to cable company employees -- have been trained to report those at risk to the city.

And just last month, Mayor Richard M. Daley called on electricity supplier Commonwealth Edison to wire its system to the city's emergency response center so officials are immediately notified of outages during the summer's hottest months.

"We've learned a lot, not only in Chicago but throughout the country and throughout the world, how dangerous heat is," the mayor said recently.

That was not the case 10 years ago, when Daley and others in a city more prepared for extreme cold than extreme heat initially brushed off concerns about temperatures rising above 100 degrees.

"They plainly learned something," said New York University sociologist Eric Klinenberg, who was highly critical of Daley and city officials in his book, "Heat Wave: A Social Autopsy of Disaster in Chicago." "They refused to take the dangers of heat seriously until it was too late, [and] now Chicago is one of the leaders nationally in taking these heat waves seriously."

The city's Department on Aging commissioner, Joyce Gallagher, said officials now realize that they must start checking on residents before the heat gets so bad that they may be in trouble.

"We treat any day over 90 as a potential emergency now, to make sure everybody is okay," she said.

It was a different story in 1995.

July 14 was shaping up to be a scorcher when Edmund Donoghue, Cook County's medical examiner, arrived at work. The day before, the temperature hit 106 degrees with high humidity that made it feel as though it were 125 degrees.

In the afternoon, Donoghue got word that two toddlers had been found dead inside a van. Though he knew the heat was to blame, Donoghue did not think it signaled anything other than someone had made a terrible mistake.

Nor was Daley worried. "It's very hot, but let's not blow it out of proportion," the mayor said that day.

That seemed to be the attitude of most residents. Streets hummed with the sound of air conditioners and the voices of people spending the night on their porches or yards.

That night Donoghue's phone at home rang with news that when he came to work the next day there would be 40 bodies waiting for him.

That was more than twice as many as the office receives on an average day and a few more than it had ever received in a single day. "I had an inkling something was up," he said.

The next morning, he showed up to find the number of bodies had jumped to 87. Eventually, officials would have to call in refrigerated trucks to store the overwhelming number of corpses.

Tim Walsh could also see something was going wrong. Walsh's job as a paramedic routinely took him into homes of people who do not get much attention: the elderly poor. He knew that they often did not have air conditioning, and he knew something else.

"A lot of these older people keep their houses shut for fear of crime," said Walsh, who is now a fire department lieutenant. "A lot of them didn't even open windows or . . . call their families because they didn't want to be a pest."

Taken together, all those factors added up to a killer that was, in effect, stalking a very vulnerable population. "Heat waves are special disasters, because they pinpoint the poor," Klinenberg said.

Their deaths, like their lives, often went unnoticed. One elderly woman's body was discovered in her house only after her husband regained consciousness in a hospital and asked about her a couple days after he was found comatose in his car.

"There wasn't this concept of everybody looking in on everybody else, people taking care of each other," said Jerry Noble, an emergency room doctor at the hospital.

Today there is evidence of change.

Helen Navarro still regrets not checking on her 81-year-old neighbor during the heat wave until it was too late.

"I keep thinking I should have checked yesterday," she said. "That's why I don't wait a day to check on somebody. That's never going to happen to me again."

Other heat waves -- such as one that killed more than 19,000 people in Europe two years ago -- continue to remind Chicagoans of the danger of sweltering weather.

"That showed it wasn't a fluke, that it could happen all over again," Gallagher said.

Cook County medical examiners load bodies into a refrigerated truck during Chicago's 1995 heat wave, which killed more than 700 people.