It's best not to leave bottled water in a hot car for more than a few hours, and certainly not for days or weeks. The primary concern isn't immediate poisoning, but the potential for chemical leaching from the plastic bottle into the water over time, especially when exposed to high heat. While the risk from a single bottle is low, repeated consumption over the long term may pose health concerns. High temperatures can also affect the taste and promote bacterial growth if the seal has been compromised.
The safety of the plastic is the main issue. Most single-use water bottles are made from PET (polyethylene terephthalate), which is generally considered safe for short-term use. However, heat acts as a catalyst, accelerating the transfer of chemicals like antimony and bisphenol A (BPA)—though most modern bottles are BPA-free. The rate of leaching depends heavily on the temperature inside the vehicle.
A car's interior can become an oven quickly. On an 85°F (29°C) day, the inside of a car can reach 100°F (38°C) in 10 minutes and soar to over 130°F (54°C) within an hour. At these temperatures, the process intensifies.
| Temperature Inside Car | Maximum Recommended Storage Time | Primary Concerns |
|---|
| Below 80°F (27°C) | Indefinite (for sealed bottles) | Minimal to no risk of leaching. |
| 80°F - 100°F (27°C - 38°C) | A few days | Very slow chemical migration; taste may be affected. |
| 100°F - 130°F (38°C - 54°C) | Less than 24 hours | Significant increase in leaching potential; "plastic" taste likely. |
| Above 130°F (54°C) | A few hours | Highest risk; avoid consumption if water is hot to the touch. |
For long-term storage, especially in a garage or car that gets hot, it's far safer to use containers designed for repeated use and high temperatures, such as those made from stainless steel or glass. If you find a forgotten case of water in your trunk after a week, it's probably fine for occasional drinking, but don't make a habit of consuming water that's been consistently heated and cooled. The older the bottle and the more heat cycles it has endured, the greater the potential for chemical migration. For your health, it's a simple practice to take your water with you when you leave the car.
