Does leaving plastic water bottles in a hot car increase microplastic exposure?

What's actually in it

Most single-use water bottles are made from PET (polyethylene terephthalate). PET is considered food-safe under normal conditions, but it's not indestructible. Heat, sunlight, squeezing, and repeated handling all break down the plastic at a microscopic level, releasing nano- and microplastic particles into the water inside.

What the research says

A 2026 study in Water Res tested how normal, everyday handling of PET bottled water changes microplastic levels. The researchers simulated real-world conditions: storing bottles in warm places, exposing them to sunlight, and physical handling like squeezing and shaking.

The results showed that heat and sunlight were the biggest factors. Bottles stored in warm conditions released far more nano- and microplastic particles than bottles kept cool. The particles were tiny, many in the nanometer range, which means they're small enough to cross cell membranes and enter your bloodstream.

The study also found that socio-economic factors played a role. People in lower-income communities were more likely to store water in conditions that increase microplastic release, like keeping cases of water in garages or cars without climate control.

A single bottle stored in a hot car for a few hours contained far more plastic particles than one kept in a refrigerator. The longer the bottle sat in heat, the worse it got. Reusing single-use bottles made the problem even bigger, since each squeeze and refill damages the plastic further.