On this day in 1995, a heat advisory is issued in Chicago, Illinois, warning of an impending record-breaking heat wave. By the time the heat breaks a week later, nearly 1,000 people are dead in Illinois and Wisconsin.

On July 13, the temperature in the city hit 106 degrees Fahrenheit and the heat index, which combines temperature with humidity for an estimate of how hot it feels, was above 120 F. During the week, the daytime temperature never went below the mid-90s and even the nighttime temperature stayed in the mid-80s. The use of air conditioning by those who had it caused records to be set for energy use and, subsequently, some power failures. People opened so many hydrants to cool themselves off in the streets that water pressure in several communities was lost. Attempts to close the hydrants were often met with violent resistance. When the heat warped train rails and made them unusable, commuting delays became common.

The young and the old were the most vulnerable to the heat. Hundreds of children were hospitalized from heat ailments. By July 14, paramedics and area hospitals were overwhelmed and unable to keep up with the continuing emergency. Soon, the corpses began to pile up. At the Chicago morgue, 17 bodies were usually handled per day. But just midway through the heat wave, there was a backlog of hundreds of bodies. Refrigerated trucks had to be brought in to hold the excess.

Most of those who died were older men who lived alone, despite the fact that senior women outnumbered senior men in the area. Researchers believe that strength of social connections to the community, which can be greater with women, was the most important factor in determining who became a victim of the heat wave.

Without any explicit criteria about how to identify a death due to heat, it was difficult to accurately count the deaths from the disaster. However, about 740 more people died in Chicago during the heat wave than in a normal week. Cook County’s chief medical examiner, Edmund Donoghue, estimated that there were 465 heat-related deaths in the city. Mayor Richard Daley questioned this number, but may have been trying to downplay the toll to avoid criticism of the city’s poor preparations for the heat. In fact, it was several days into the heat wave before the city implemented its heat-emergency plan.

Four years later, when another heat wave hit the city, better preparation and a more rapid response limited the deaths to just over 100.