Is it safe to leave a plastic water bottle in a car trunk over summer?
No. PET bottles release more phthalates fast once the trunk gets warm.
What's actually in it
A PET (polyethylene terephthalate) bottle is stable at room temperature. Summer car interiors regularly hit 60 to 70°C in the afternoon sun, especially the trunk, which has less ventilation. At that temperature, the bottle softens and releases a mix of antimony, phthalates, and microplastic particles into the water. The water tastes a little off, which most people drink through.
The "oh it's fine" crowd is technically right that a single bottle won't kill anyone. Daily hot-car bottles, week after week, do add up.
What the research says
A 2026 study in J Food Sci Technol measured migration of phthalate compounds from PET bottles under different temperature conditions and built a probabilistic human exposure model. Storage at typical summer-car temperatures produced several times the phthalate release of room-temperature storage. Daily hot-trunk drinkers came out with a phthalate exposure that crossed relevant health thresholds.
The easy fix is to carry a stainless steel or glass bottle and refill it from home or from a cooler in the car. A small cooler keeps a backup supply of water out of the trunk heat. If all you have is a case of plastic water and you need a bottle right now, pick one from the cool middle of the case rather than the top and don't let it go through multiple heat cycles. Drink it same-day.
The research at a glance
| Study | Journal | Year |
|---|---|---|
| Migration of phthalate compounds from polyethylene terephthalate bottles under different temperature conditions. | J Food Sci Technol | 2026 |
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