An adult should not remain in a parked hot car for more than 10 to 20 minutes, even with the windows cracked. The primary danger is heatstroke, which can occur when the body's core temperature rises to 104°F (40°C) or higher. A car's interior can become lethally hot in a shockingly short time, making it a serious health risk.
The science behind this is straightforward. On a mild 75°F (24°C) day, the temperature inside a closed vehicle can soar to over 100°F (38°C) within just 30 minutes. This rapid heating effect, known as the greenhouse effect, turns the car into an oven. The body cools itself primarily by sweating, but in a high-humidity environment inside a hot car, sweat cannot evaporate efficiently, causing core temperature to rise dangerously fast. Certain factors increase risk, including age, pre-existing health conditions (like heart or respiratory issues), and medication use.
The timeline to danger is not long. Early signs of heat exhaustion—dizziness, nausea, headache, and profuse sweating—can appear within minutes. If not addressed, this can quickly progress to heatstroke, which is a medical emergency characterized by confusion, a rapid pulse, and potentially loss of consciousness.
The following data illustrates how quickly interior temperatures can reach dangerous levels on a sunny day:
| Elapsed Time | Outside Temp: 75°F (24°C) | Outside Temp: 90°F (32°C) |
|---|
| 10 minutes | 94°F (34°C) | 109°F (43°C) |
| 20 minutes | 104°F (40°C) | 118°F (48°C) |
| 30 minutes | 111°F (44°C) | 124°F (51°C) |
| 60 minutes | 118°F (48°C) | 133°F (56°C) |
Source: Data adapted from studies by San Jose State University and the National Highway Traffic Safety Administration (NHTSA).
It is a common misconception that cracking the windows significantly helps. Research shows this only slightly slows the heating process and does not prevent the interior from reaching dangerous temperatures. The safest practice is to avoid staying in a parked car in warm weather altogether. If you must wait, ensure the engine and air conditioning are running.
