Why Chicago’s 1995 heat was so dangerous
The deadly Chicago heat wave did not need to cross a universal “survivability” temperature to overwhelm people. The danger came from prolonged heat, humid air, warm nights and unequal access to ways of cooling down. Hundreds of people died, many in homes where indoor temperatures stayed dangerously high.
Sweat cools only when it can evaporate
Sweat itself does not remove much heat. As it evaporates from the skin, it carries heat away. When the air is humid, it already contains more water vapor, so sweat evaporates more slowly. The body may respond by producing more sweat, but that does not guarantee effective cooling—and it can accelerate dehydration.
The wet-bulb temperature is a measure that combines heat and humidity, based on how much cooling evaporation can provide. It helps describe heat stress, but it is not a single cutoff at which every person suddenly dies. Risk depends on exposure time, health, activity, air movement and access to water and cooling.
What happens when cooling falls behind
As the body struggles to shed heat, blood vessels near the skin widen to release warmth. The heart must work harder to circulate blood, while sweating reduces the body’s water supply. If heat continues to build, the body’s temperature-control systems can fail, leading to heat illness, organ damage and potentially death. This is a progressive emergency, not an instant cardiovascular collapse triggered by one reading.
Why the danger continued indoors
Pavement and buildings absorb heat during the day and release it slowly, keeping city neighborhoods warm after sunset. Without air conditioning or another cool place to go, residents could not recover overnight. The lasting lesson of the heat wave is concrete: a hot home can remain hazardous long after the sun goes down, especially for people who are isolated or unable to reach reliable cooling.
