Does storing plastic water bottles in heat make them more dangerous to drink from?

What's actually in it

PET (polyethylene terephthalate) is the plastic in most single-use water bottles. PET is stable at room temperature but degrades when exposed to heat: in a hot car, in direct sunlight, or in a warehouse without climate control before the bottle reaches the store. As PET ages, it breaks down into smaller, chemically altered fragments.

These aged particles behave differently from fresh PET particles. The breakdown process changes the surface chemistry, making the fragments stickier and more reactive inside the body. They travel from the gut to the liver through the portal circulation.

What the research says

A 2026 study in Environ Sci Technol compared fresh PET microplastics to thermally aged PET particles in terms of their effects on the gut and liver. Aged particles caused more severe gut barrier disruption and significantly worse liver inflammation and fat accumulation (hepatic steatosis) than unaged particles.

The mechanism runs through the gut-liver axis: aged PET microplastics disrupt the intestinal barrier more severely, allowing more inflammatory signals through to the liver. The liver responds with inflammation and abnormal fat storage. The aged particles also disrupted the gut microbiome more severely than fresh ones.

The practical implication: a bottle that has sat in a hot car or warehouse has already degraded. Drinking from it delivers more biologically active microplastic particles than a properly stored bottle would. Switching to stainless steel or glass water bottles for everyday use eliminates this variable entirely.